Thermal energy storage--a review of concepts and systems for heating and cooling applications in buildings: Part 1--seasonal storage in the ground.
Energy demand in buildings varies on a daily, weekly, and seasonal basis. This demand can be matched with the help of thermal energy storage (TES) systems that operate synergistically and are carefully matched to each specific application. TES systems have the potential of making the use of HVAC systems more effective, and they are an important means of offsetting the mismatch between thermal energy availability and demand. Well-designed systems can reduce initial and maintenance costs and improve energy efficiency (Dincer and Dost, 1996; Dincer et al. 1997).
A variety of TES techniques for heating and cooling applications have been developed over the past decades (Dincer and Rosen 2011). It is favorable to characterize the different types of TES depending on the storage duration: short-term (diurnal) storage or long-term (seasonal) storage. Nordell (2000) specified additional parameters for classification of TES systems, according to storage purpose (heating and/or cooling), storage temperature (low <40-50[degrees]C [~100-120[degrees]F] or high >50[degrees]C [~120[degrees]F]), storage technology (borehole TES [BTES], pit-tank TES, phase change materials [PCMs], etc.), and storage application (residential or commercial). The different storage concepts have very different characteristics, possible applications, strengths, and weaknesses. The selection of a TES system mainly depends on the storage period required, economic viability, and operating conditions.
In continental climates, it is possible to store heat from the warm summer months for use in winter, while the cold ambient temperatures of winter can charge a cold store to provide cooling in summer. This example of seasonal storage can meet the energy needs caused by seasonal fluctuations in temperature.
Due to the intermittent nature of the energy source, achieving the full potential of solar thermal technologies, for space heating and domestic hot water (DHW) applications, is dependent on the availability of efficient and effective energy storage systems. This is particularly true at high latitude locations, where seasonal variations of solar radiation are significant, and in cold climates, where seasonally varying space heating loads dominate energy consumption.
This literature review article attempts to summarize developments during the last four decades in seasonal TES in the ground, using different storage concepts and utilizing different natural and renewable energy sources. The aim is to provide the basis for development of new intelligent seasonal TES possibilities for use in combination with space heating, space cooling, and DHW applications.
Underground TES (UTES) concepts
The principle methods available for seasonal storage of thermal energy mostly store energy in the form of sensible heat. Storage of sensible heat is influenced by energy losses during the storage time. These losses are function of storage time, storage temperature, storage volume, storage geometry, and thermal properties of the storage medium. Since seasonal TES requires large inexpensive storage volumes, due to the large storage timescales, the most promising technologies were found in the ground, where the ground temperatures vary much less than the ambient temperature. Such systems are called UTES systems (Nordell 2000). Among the UTES systems developed since the 1970s, the ongoing engineering research focused mainly on four types of storages: water tank, gravel-water pit, aquifer TES (ATES), and BTES; see Figure 1. In Table 1, are summarized the characteristics of the main underground seasonal storage concepts.
Water-tank TES usually consists of a reinforced concrete tank partially or fully buried in the ground, which can be built nearly independently of geological conditions. It is thermally insulated at least on the roof area and on the vertical walls. Furthermore, steel liners are introduced in the structure to guarantee water tightness and to reduce thermal losses caused by vapor transport through the walls (Schmidt et al. 2004). Due to the high specific heat of water, and the possibility for high capacity rates for charging and discharging, this technology seems to be the most favorable from a thermodynamic point of view.
Gravel-water pits consist of a mix of gravel and water and are normally buried in the ground. They need to be waterproofed and insulated on at least at the side walls and on the top (Schmidt et al. 2004). Thermal energy is charged into and discharged out of the storage either by direct water exchange or by a heat exchanger based on plastic piping installed in different layers inside the storage. The gravel-water mixture has lower specific heat capacity than water alone, and for this reason, the volume of the whole basin has to be higher compared to water-tank storage to obtain the same thermal storage capacity.
Aquifers are below-ground widely distributed sand, gravel, sandstone, or limestone layers with high hydraulic conductivity that are filled with groundwater (Schmidt et al. 2004). If there are impervious layers above and below, and no or low natural groundwater flow, they can be used for thermal storage. In this case, two wells or groups of wells are drilled into the aquifer and serve for extraction or injection of groundwater. During charging periods, cold groundwater is extracted from the cold well, heated up by the heat source, and injected into the hot well. In discharging periods, the flow direction is reversed: warm water is extracted from the warm well, cooled down by the heat sink and injected into the cold well. Due to the different flow directions, both wells are equipped with pumps and production and injection pipes. Because the storage volume of ATES calmot be thermally insulated against the surroundings, heat storage at high temperatures (above 50[degrees]C [122[degrees] F]) is normally only efficient for large storage volumes (more than 20,000 [m.sup.3] [706,300 [ft.sup.3]]) with a favorable surface-tovolume ratio. For low-temperature or cooling applications also, smaller storage can be feasible. Especially for high-temperature thermal storage, a good knowledge of the mineralogy, geochemistry, and microbiology in the underground is necessary to prevent damage to the system caused by well-clogging, scaling, etc.
[FIGURE 1 OMITTED]
In BTES, thermal energy is directly stored in the ground. In suitable geological formations (e.g., rock- or water-saturated soils [Schmidt et al. 2004)], U-pipes, or so-called ground heat exchangers, are inserted into vertical boreholes to a depth of 30-200 m (98-656 ft) to build a huge heat exchanger. The boreholes are usually filled with groundwater (Northern Europe), or with bentonite, quartz sand, or thermally enhanced grouts (North America, Central Europe). While water is running in the U-pipes, heat can be fed into or out of the ground. The heated ground volume comprises the volume of the storage. At the top of the storage, there is usually a heat insulation layer to reduce thermal losses to the surface.
Depending on the application and the heating/cooling demand on the storage, different de sign requirements for borehole configuration and groundwater flow may apply. For solar thermal applications, due to the need for seasonal storage of solar heat, minimized heat exchange between the BTES and surrounding ground is desired. Therefore, rectangular or circular configuration (with high volume-to-surface area ratio) of the storage and low natural groundwater flow are prerequisites for design. For ground-source heat pump (GSHP) applications, the requirements for the configuration of the boreholes are set by the share of the heating and cooling of the total energy need of the building. For example, if there is a cooling-dominated situation, the heat rejection to the ground in summertime will be higher than the heat extraction from the ground in winter. Due to that imbalance in the ground loop, the ground temperature will increase. The increasing of the ground temperature will reduce the efficiency of the GSHP system in the summer. The opposite situation will be observed in a heating-dominated situation. In these cases, the heat exchange area between the borehole system and surrounding ground should be maximized in order to avoid heat accumulation or depression in the borehole field. That would make the linear borehole configuration more beneficial. Additionally, high groundwater flow would enhance the dissipation of thermal energy in the surrounding ground, thus improving system performance. For balanced annual heating and cooling loads, the GSHP-BTES system would benefit from using the ground system for seasonal storage of heat or cold, and the design prerequisites for a solar thermal system will apply.
BTES has a horizontal rather than vertical temperature stratification from the center to the borders. This is because the heat transfer is driven by heat conduction and not by convection. At the boundaries, there is a temperature decrease as a result of the heat losses to the surroundings. When rectangular or circular storage is designed, the horizontal stratification is supported by connecting the supply pipes in the center of the storage and the return pipes at the boundaries. During charging, the flow direction is from the center to the boundaries of the storage to obtain high temperatures in the center and lower ones at the boundaries of the storage. During discharging, the flow direction is reversed.
One advantage of this type of storage is the possibility for a modular design. Additional boreholes can be connected easily, and the store can grow, e.g., with the size of a housing district. The size of the storage should be three to five times higher than that of water-tank storage to obtain the same thermal capacity. Table 2 shows typical general values for BTES systems.
The interest in large-scale seasonal TES started with the oil crisis in the early 1970s. The objectives of such systems are either to store solar heat collected in summer for space heating in winter, or to provide heating and cooling by storing heat underground in summer and removing heat in winter.
In winter, a GSHP extracts heat from the thermal storage; in summer, it extracts heat from the building to store it in the ground. These systems contribute significantly to improving the energy efficiency and reducing the greenhouse gas emissions to the atmosphere.
Design guidelines for UTES
For the construction of ground-buried TES, like water tanks and gravel-water pit storages, there are no standard procedures available regarding wall construction, charging device, geometry (e.g., surface-to-volume ratio), etc. Due to the size and geometry, and also due to the requirements in terms of leakage detection and lifetime, most techniques and materials have their origin in landfill construction. However, with respect to high operation temperature, materials and techniques cannot be simply transferred. Design recommendations for construction of water-tank and gravel-water pit storages are given in the HIGH-COMBI Report (2008).
General demands and recommendations for the design of ATES and BTES systems can be found in the Verein Deutscher Ingenieure's (VDI [Association of German Engineers] n.d.) "Guideline VDI 4640: Thermal use of the Underground, Parts 1-4." The guidelines concern the thermal use of ground to a depth of about 400 m (1300 ft). Systems for heating only, cooling only, and both heating and cooling are treated in Part 1. Environmental aspects, such as primary energy use, C[O.sub.2] emissions, thermal impacts on the ground and groundwater, hydraulic impacts, and possible consequences of leakage of heat carrier fluids, are included as well. Part 2 includes design guidelines for the possible specific heat extraction rate for ground-coupled heat pumps with vertical boreholes or shallow horizontal pipes. In Part 3, storage-specific aspects, e.g., water treatment methods to prevent precipitation caused by chemical changes and suitable materials for different applications (temperatures), are mentioned. ATES and BTES systems are described in detail, including hydrogeological prerequisites and recommendations for design. Part 4 focuses on the direct usage of underground cold or heat without any additional equipment, such as groundwater cooling or heating.
Additional information for ground-source cooling systems with TES, utilizing ATES and BTES concepts, is given in a pre-design guide developed by Hummelshoj (2004).
Seasonal storage of solar thermal energy for heating applications
Seasonal heat storage for solar thermal applications needs large volumes of storage to supply the energy stored during summertime for winter. That large storage requires the development of technologies capable of minimizing heat losses in order to preserve the thermal performance and lifetime of the solar heating plant. These approaches must be coupled with low investment, at least lower than conventional heating and cooling systems.
Four different seasonal heat storage types for solar thermal applications have been investigated in this article: gravel-water storage, gravel-water pit storage, BTES, and ATES. The selection of a specific storage type depends on the geological and geo-hydrological situation in the ground at the respective construction site. A preliminary geological examination of the site is recommended, especially for ATES and BTES. Determining the geological and geo-hydrological conditions can be expensive and time consuming. Ford and Wong (2010) studied the above-mentioned phenomena and presented regional models for screening potential underground areas for ATES and BTES systems implementation. The findings are based both on geological data and output from a three-dimensional groundwater flow model, MODFLOW (McDonald and Harbaugh 1988).
[FIGURE 2 OMITTED]
Seasonal storage of solar thermal energy for space heating purposes has been under investigation in Europe since the mid 1970s within large-scale solar heating projects. Most large-scale solar systems have been built in Sweden, Denmark, The Netherlands, Germany, and Austria (Dalenback 2007). The first demonstration plants were developed in Sweden in 1978/1979, based on results from a national research program (Dalenback and Jilar
1985). The seasonal storage concept research work continued within the IEA "Solar Heating and Cooling" program. Experiences have been gained and exchanged in Task VII's central solar heating plants with seasonal storage (CSHPSS) since 1979 in many countries. In the past two decades, the main activities have been within the work initiated in the CSHPSS Working Group, IEA Solar Heating and Cooling program, the work carried out in Europe within the EU/APAS-project "Large-Scale Solar Heating Systems" (Fisch et al. 1998), as well as the German R&D programs Solarthermie-2000
and Solarthermie-2000 plus (Lottner et al. 2000; Schmidt et al. 2004; Bauer et al. 2010). Figure 2 shows a scheme of a CSHPSS (distributed rooftop solar collectors, central plant with heat pump, solar collectors, and heat distribution networks).
So far, the development of seasonal storage has been aimed at heating large district system stores part of CSHPSS in order to fulfill technical viability and cost effectiveness by using large storage volumes. Fisch et al. (1998) reviewed large-scale solar plant development in Europe during the 1990s. The work refers to two large-scale solar heating applications: systems with short-term (diurnal) storage designed to supply 10-20% of the annual heating demand or 50% of the DHW, and systems with long-term (seasonal) storage capable of supplying 50-70% of the annual heating demand. Within the findings of that work was that large-scale solar applications benefit from the effect of scale. Compared to small solar DHW systems, the solar heat cost can be cut at least in third. Among the main results of the evaluation of the existing projects was the need to reduce the cost-benefit ratio for CSHPSS.
Comparison of technologies and experiences from pilot projects for CSHPSS
Table 3 summarizes the technical characteristics of some demonstration plants in central solar heating systems with water tank, gravel-water pit, borehole, and aquifer storage. The experimental projects have been selected as they are large-scale pilot plants. An overview of the effectiveness of diverse configurations of these systems, including solar heat systems costs, is provided. The given numbers for the solar fraction of total heat delivered are simulated values for long-term operation. Depending on the type of seasonal heat store, the systems have start-up times of three to five years to reach normal operating conditions. Within this time, the underground around the seasonal storage has to be heated up, and hence, heat losses are higher than in the long-time operation. Because of this, the system efficiency is lower in the first years of plant operation (Schmidt et al. 2004).
Water-tank TES is technically feasible and works well. However, construction costs and thermal losses are still too high. Experiences from the plants built in Hamburg and Friedrichshafen have shown that the main cost for hot water storage tanks is caused by the concrete construction, ground works, insulation, and the use of steel liners to reduce water permeability (Kubler et al. 1997). Considerable cost reductions can be obtained with the development of high-density concrete materials, which would allow the omission of the use of expensive steel liners for the storage construction (Schmidt et al. 2004). The water-tank storage in Hannover is the first one utilizing that concept on a large scale. Another novelty in that project has been the introduction of stratification devices in the water-tank storage. Problems with high thermal losses due to wet thermal insulation have been experienced in Sweden in the past (Dalenback and Jilar 1985), and they have revealed the importance of water-tank insulation for the longterm performance of CSHPSS. Advances in stratification devices and heat insulation have resulted in significantly lower construction costs for the seasonal storage in Munich than for costs experienced in previous projects (Schmidt and Mangold 2006). Further research and development related to high-density concrete materials, prefabricated sandwich elements for water-tank wall construction, and simultaneous charging and discharging stratification devices would give the possibility of improving the thermal performance and decreasing the construction costs for water-tank seasonal storage technology.
Gravel-water pit TES
Experiences with gravel-water storage in Chemnitz have shown that sealing of the pit, insulation, and ground works account for a significant part of the costs (Schmidt et al. 2004). The seasonal storages in Steinfurt (Pfiel and Koch 2000) and Ottrupgaard (Heller 2000) have shown that the construction of the wall (liner, insulation) can barely be realized at the required low costs to be cost effective for seasonal storage. Moisture protection of the insulation is also important for the concept. In addition, the seasonal gravel-water pit storage in Ottrupgaard has shown difficulties in making it sufficiently tight and localizing and repairing leakages. The concept of a floating cover was investigated for the plants in Ottrupgaard and Eggenstein (Bauer et al. 2010), and it appears to be an expensive construction. Research for developing cost-effective solutions is needed. Further studies regarding system thermal performance relative to the use of direct and indirect heat exchangers are necessary.
For BTES, the experiences with CSHPSS built in Neckarsulm (Nussbicker et al. 2003; Bauer et al. 2010), Crailsheim (Mangold 2007), and Anneberg (Nordell and Hellstrom 2000; Lundh and Dalenback 2008) show that the major investment for a solar plant is the cost of building the storage, e.g., drilling boreholes, constructing heat exchangers, refilling boreholes. As drilling costs increase with the depth of the borehole, the length and the number of boreholes are important. Thermal properties (heat capacity, thermal conductivity) of the ground determine the spacing of the heat exchangers. Number, length, and spacing of boreholes taken together allow the storage volume to be calculated. In addition to storage design, due to the low heat transfer rates between the circulating fluid and ground, these systems have shown dependence on the development of buffer storage techniques. Buffer storage hot water tanks are often added to the system in order to manage the high-capacity rates of the solar collectors during summertime and the high-demand rates for heating and DHW during wintertime.
Well construction is the predominant part of the costs for ATES. In reality, depending on site-specific conditions, several serious problems have to be solved, e.g., clogging of wells, scaling of the external heat exchangers, necessity of water treatment, and high heat losses, especially in small aquifer storage projects like the one in Rostock (Lottner et al. 2000; Schmidt et al. 2004; Bauer et al. 2010).
Operational experiences and design considerations for CSHPSS
The operational characteristics of the different CSHPSS considered in this review article are based on simulated values for long-term performance of the solar plants. In Bauer et al. (2010), three different types of seasonal TES have been tested and monitored under realistic operating conditions: Friedrichshafen (water tank), Neckarsulm (boreholes), and Rostock (aquifer). Their operational characteristics are compared using measured data from an extensive monitoring program. The longterm operational experiences are shown.
The solar fraction based on total heat demand for the plant in Friedrichshafen for the period 1997-2007 varied between 21% and 33%, where the design value has been 47%. One reason the targeted value was not met was that the heat demand for the buildings was 10% higher than expected, due to the increased living area compared to initial design. As a result, the additional heat demand was covered by the two gas condensing boilers installed in the system, which decreased the demand covered by solar energy. In addition, the design return temperatures of the heat distribution network have been assumed to be lower (40[degrees]C [104[degrees]F] yearly average weighted by volumetric flow) than measured values of up to 55.4[degrees]C (132[degrees]F) in 2006. The reason for the high return temperatures of the heat distribution network has been poorly performing heat exchangers at the demand (buildings) side. Because of the high net return temperatures, the TES has been operating at higher than design temperatures, which has resulted in increased heat losses of the thermal storage between 322 MWh/a and 482 MWh/a (1100 MBtu/a and 1646 MBtu/a), in contrast to design values of 220 MWh/a (751 MBtu/a). The higher operating temperature of the thermal storage has also caused higher temperatures in the solar collector circuit and thus reduced collector efficiency.
The CSHPSS in Neckarsulm has been monitored in the period 1999-2007. The solar fractions achieved (based on total heat demand) have been between 17% and 44.8%, where the design value has been 50%. Reasons for not achieving the desired solar fractions have been the 10% smaller-than-designed solar collectors' area and the higher net return temperatures of the heat distribution net (47[degrees]C-50[degrees]C [116[degrees]F-122[degrees]F] instead of 40[degrees]C [104[degrees]F]). In addition, the highest achieved solar fraction of 44.8% has been obtained during the last year of monitoring, when the maximum borehole seasonal storage temperature reached 65[degrees]C (149[degrees]F), 20[degrees]C (68[degrees]F) lower than planned. The smaller solar collector area and the heat up of the surrounding ground have contributed to that effect.
Monitoring results from the solar heating plant with aquifer seasonal storage in Rostock have shown solar fractions from 32% to 57%. The maximum temperature of the storage has been limited to 50[degrees]C (122[degrees]F) due to local hydrogeological conditions. The heat distribution net has been operating at 45/30[degrees]C (113/86[degrees]F) supply/return temperatures, which required the use of a heat pump for utilization of the stored heat. Due to the use of the heat pump and the high efficiency of the aquifer storage, the system has managed to reach the high solar fraction values.
The results from the monitoring campaigns at the different solar plants have shown that in order to achieve high solar energy efficiency, the solar plants have to be operated at low temperatures. Low storage temperature limits heat losses and improves solar collector efficiencies. Suitable techniques for fully benefitting from such low-temperature systems are to use low-temperature heating systems (typical range of 25[degrees]C-35[degrees]C [77[degrees]F-95[degrees]F]), like floor and wall heating in the buildings. In contrast, high-temperature systems must be built on a much bigger scale than low-temperature systems because of the higher storage heat losses.
For seasonal storage, low-temperature concepts with the use of heat pumps to raise the temperature of the water used for space heating to a suitable level is an appropriate option. This technology, conceptually and practically implemented in the plants in Rostock, Eggenstein, and Crailsheim (Lottner et al. 2000; Schmidt et al. 2004; Bauer et al. 2010), and in conceptual phase for the plant in Okotoks (Chapuis and Bernier 2009), enables the utilization of the full potential of solar heating plants with seasonal storage. Using a heat pump to discharge the seasonal storage to lower temperatures allows higher usability and increased storage capacity and storage efficiency. The solar plant becomes more robust against high return temperatures of the heat distribution net and solar collectors net, which allows reduction of the solar collector area required, increase of the solar collectors' efficiency, and obtaining high solar fractions (based on total heating demand).
The solar fraction of the delivered heat is not the only parameter that can be used for performance assessment of CSHPSS. In addition, the efficiency of solar-assisted district heating systems can be evaluated by the amount of solar heat per [m.sup.2] collector area delivered into the district heating net. Even though this parameter is dependent on local site conditions, like irradiation on the collector pane, it could give insight into any advantages or disadvantages of using different storage concepts. The monitoring results from the different plants shown in Bauer et al. (2010) do not show any clear tendencies in favor of or against a certain storage concept.
In addition, the above-discussed parameter could give some design prerequisites regarding solar collector area and storage volume. Different methods for determining the optimal size of collector area and storage volume for seasonal storage of solar heat have been developed. Braun et al. (1981) described a methodology for the design of these systems using the simulation program TRNSYS (Klein 2004). Significant reduction in the collector area has been achieved by use of seasonal storage. This effect is more pronounced for higher solar fractions. It has been shown that the trade-offs between collector area and storage volume requirements for a fixed system performance are location dependent. Greater reductions in collector area requirements with increasing storage capacity occur in northern latitudes (valid for the northern hemisphere). Similar results have been confirmed from the demonstration plants studied by Lottner et al. (2000), Schmidt et al. (2004), and Bauer et al. (2010). However, no clear guidelines or design recommendations have been developed.
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The seasonal energy storage technologies for solar energy applications are characterized by many factors, such as solar collectors, annual sun exposure, heat distribution networks, heat demand and insulation of the buildings, and the seasonal thermal storage requirements. Once these technologies have been well developed, the main effort consists of reducing costs in order to make them market competitive against conventional energy sources.
As some authors suggest, the specific storage costs are related to water-equivalent storage volume. The water equivalent is the corresponding water volume to store the same amount of heat. Experiences carried out in demonstration plants have achieved cost reduction by increasing the storage volume in large-scale solar applications. Figure 3 presents the cost data of some pilot and demonstration plants reviewed in this study. The strong cost reduction with an increasing storage volume is obvious. Appropriate sizes for seasonal heat storage are located between 2,000-20,000 [m.sup.3] (70,600-706,000 [ft.sup.3]) water equivalent. Within this range, the investment costs vary between 40-250 [euro]/[m.sup.3] (1.50-9.30 US $/[ft.sup.3]). Generally, water-tank storage is the most expensive concept. On the other hand, it has some advantages concerning the thermodynamic behavior and it can be built almost independently of the geohydrological site conditions. The lowest costs can be reached with ATES and BTES. However, they often need additional equipment for operation, such as buffer storage or water treatment, and they have the highest requirements on the local ground conditions.
The economy of CSHPSS depends not only on the storage costs, but also on the thermal performance of the storage and the connected system. Before starting the design of a new plant, geological conditions of the location, characteristics of the heat source, and demands of the consumers have to be analyzed thoroughly. Important parameters are maximum and minimum operating temperatures of the storage and heat distribution system. Optimal size of solar collector area and seasonal storage volume are of vital importance.
To determine the economy of a type of storage, the investment and maintenance costs of that storage have to be related to its thermal performance (the cost of the usable stored energy). If the geohydrological site conditions make different storage types feasible, an economic optimization via system simulations should be conducted by taking the construction costs of the different concepts into account.
The first attempts to develop dynamic simulation tools capable of determining the technical feasibility and cost effectiveness of CSHPSS were performed in Task VII's "CSHPSS" of the IEA "Solar Heating and Cooling" program (Chant et al. 1983). The applicability and the limitations of the computer simulation programs MINSUN (Mazzarella 1990) and TRNSYS (Klein 2004) in simulating a wide range of CSHPSS with different collector types, different seasonal storage types, with or without heat pumps, different load sizes and different operational strategies, have been evaluated. The results obtained from different case studies have shown the powerful capabilities of the two simulation programs to provide economic, cost, and sensitivity analyses for a variety of parameters and variables and to optimize parameter values to minimize overall system cost.
Another contribution in the field was made by Lund and Petolla (1992) and Lund (1997) with the development of SOLCHIPS computer simulation tool for optimization of solar heating systems with seasonal storage.
The pre-design tools for CSHPSS MINSUN and SOLCHIPS are fast and easy to use, but they do not provide information for design and optimization of additional system components, e.g., buffer storage tanks when seasonal ground storage is used (Pahud 2000). The TRNSYS program, on the other hand, is a detailed and versatile simulation tool capable of handling many subsystem, user-defined, and case-specific modules. The ability to simulate thermal storage behavior at a more detailed level, e.g., on a system and subsystem level, makes TRNSYS a state-of-the-art tool for detailed design, dimensioning, and optimization of CSHPSS.
Summarizing the findings from computer simulation studies and monitoring campaigns, it is evident that although well developed and also widely used in some countries, the concept of CSHPSS of solar energy requires further research in order to make it economically competitive with conventional energy sources. Research could include studies related to cost reductions for construction of the storage; heat insulation and reduction of storage heat losses; operating temperatures of the storage, solar collectors net, and heat distribution net in regards to efficiently utilizing the low-temperature concept with the use of heat pumps; efficiency of solar collectors; determining of optimal solar collector area and seasonal storage volume; coupling between solar plant and low-temperature heating systems in the buildings; etc.
UTES with heat pumps
UTES and GSHP systems use the underground for exchange of thermal energy for efficient heating and cooling of buildings. The application is based on the natural ground temperature. The GSHP extracts heat from the ground in winter and injects heat in summer. The GSHP technology offers higher energy efficiency for air conditioning compared to conventional air-conditioning systems, because the underground environment provides a lower temperature for cooling, a higher temperature for heating, and experiences less temperature fluctuation than ambient air temperature. These result in a high coefficient of performance (COP) of the heat pump in both heating and cooling modes.
In general, two types of UTES for combined heating and cooling applications can be distinguished: ATES and BTES (Nordell 2000). As discussed in the section for seasonal storage of solar energy, for geological or geo-hydrological reasons, it is not possible to construct these systems at any location.
[FIGURE 4 OMITTED]
An ATES system is a large open-loop system optimized and operated to realize seasonal TES; the principle is shown in Figure 4. In summer, groundwater is extracted from the cold well and used for cooling purposes. The warmed-up water is injected into the warm well. In winter the process is reversed; water is pumped from the warm well and applied as a heat source, e.g., as a low-temperature heat source for a heat pump. The chilled groundwater is then injected into the cold well again. With ATES, all the water extracted from one well is reinjected in another well. This means that there is no net extraction of groundwater from the soil, which minimizes negative impacts on the environment.
A BTES system consists of a number of closely spaced boreholes; the principle is shown in Figure 5. During the heating season, the borehole heat exchanger is used for extraction of heat from the ground, which serves as a heat source for the heat pump. While the circuit water passes through the heat pump, its temperature cools down. The chilled water is returned in the borehole heat exchanger and the "cold energy" is stored in the ground. During the cooling season, the flow in the BTES system is reversed. The stored cooling energy is extracted and
passed through a heat exchanger, providing direct cooling to the building. In periods of peak cooling demand, the (reversible) heat pump can be used. The circuit water will pick up energy from the building and thus be raised in temperature. It will be returned in the borehole heat exchanger where the "warm energy" is stored in the ground tbr the next heating season.
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The development of UTES is strongly supported within the framework of the International Energy Agency (IEA). The IEA Energy Storage Annex 8 is the focal point for all activities related to UTES. Underground heat storage in the temperature range below 40[degrees]C (104[degrees]F) is usually done to increase the heat-source temperature of heat pumps. High-temperature UTES systems have storage temperatures above 40[degrees]C to 50[degrees]C (104[degrees]F to 122[degrees]F). Heat pumps are either used at the end of the storage unloading period, when temperatures drop, or for achieving higher supply temperatures. With increasing temperatures, hydrochemical, biological, and geotechnical problems increase. Annex 12 of the IEA program on Energy Conservation through Energy Storage (IEA ECES) addresses the specific problems of high-temperature UTES.
Cold storage systems with heat pumps have been under investigation by the IEA for many years. Typical modes of operation of such systems are direct cooling in spring and during low-demand periods and cooling by heat pump in summer or during peak-demand periods. These systems substitute chillers, which, compared to thermal storage, have a relatively high energy demand. Seasonal cold storage is now commercialized in some countries. A database made under IEA ECES Annex 7 lists approximately 90 realized projects in the 4 participating countries (Canada, Germany, the Netherlands, and Sweden). Forty of these projects include heat pumps. The size and capacity of cold UTES varies widely. A trend toward very large systems can be seen.
GSHP and UTES systems are applied in various European countries and North America. While, in some countries, these systems are already considered a standard design option for heating and cooling, the technology is quite recent in others. Rybach and Sanner (2000) described the technologies, market situation, future trends, and questions related to GSHP systems development in Europe. Applications and experiences with UTES systems in combination with GSHPs in various European countries were given in Hendriks et al. (2008). A world overview of geothermal heat pump development and utilization was presented by Lund et al. (2004). Status development and applications in the United States and Europe are investigated. Insight into system's efficiency, particularly heat pump COPs in heating and cooling mode, is given.
Comparison of technologies and experiences from pilot projects utilizing the GSHP technology
UTES systems have been used for seasonal storage of large quantities of thermal energy to supply space heating and cooling. UTES systems are most commonly used in combination with GSHPs. Such systems have found broad applications in Europe and North America, with the most common technologies being aquifer storage and borehole storage. Table 4 summarizes some tcchnical data of pilot projects with GSHPs utilizing aquifer or borehole seasonal storage concepts.
ATES systems require that relatively high well yields can be obtained on site. Because of this, the applicability depends strongly on site-specific hydrogeological conditions. An advantage of such systems is the generally higher heat transfer capacity of a well compared to a borehole. This makes ATES usually the cheapest alternative if the subsurface is hydrogeologically and hydrochemically suited for the system. In recent years, a large number of ATES systems were built in North America and Europe (Bakema and Snijders 1998; Andersson 2007a, 2007b, 2007c; Lee 2010; Hendriks et al. 2008; Sorensen and Qvale 2007; Lund et al. 2004).
The ATES system at the campus of Eindhoven University, Netherlands, is supplying direct cooling in summer as well as low-temperature heating in winter (via heat pumps). It reduces the overall electricity consumption and, consequently, the C[O.sub.2] emissions by approximately 20% in comparison with a conventional chiller system (Snijders and Van Aarssen 2003).
The concept and operational experience of the ATES system, which is part of the space-conditioning system for the buildings of the German Parliament in Berlin, was given by Sanner et al. (2005). Simulation studies show heat storage efficiency of 77% and cold storage efficiency of 93% (storage efficiency is ratio of stored to retrieved thermal power). A performed monitoring program shows good agreement between measured and simulated system performance.
In Malmo, Sweden, a residential area uses ATES as part of the district heating and cooling system. The system is delivering free cooling to the district cooling system at a temperature level below +6[degrees]C-8[degrees]C (42.8[degrees]F-46.4[degrees]F). A specific variable cost of produced cold of 4 [euro]/MWh (1.54 US $/MBtu) has been evaluated from a monitoring campaign (Andersson 2007c).
ATES is used to serve the district cooling system of a residential area in Stockholm, Sweden. The system has been designed for 25 MW (85 MBtu/h) free cooling power at a storage temperature of +4[degrees]C to +14[degrees]C (+39.2[degrees]F to +57.2[degrees]F). During the first year of operation (1999), a low storage efficiency of 60% was monitored. In following years, the efficiency has gradually increased due to a better operational strategy (Andersson 2007b; Andersson and Rudling 2000).
The ATES system at Oslo's Gardermoen International Airport in Norway covers the total cooling needs of the airport, of which 25% (2.8 GWh/a [8536 MBtu/a]) is free cooling via direct heat exchange with cold groundwater and 75% (8.5 GWh/a [29,002 MBtu/a]) is active cooling via the use of heat pumps. The estimated payback time, compared to traditional heating and cooling systems, is less than four years (Eggen and Vangsnes 2005).
One of the largest ATES installations in the United States is in Louisville, Kentucky (Nordell and Sanner 1998). The GSHP system is providing 15.8 MW (53.95 MBtu/h) of cooling and 19.6 MW (66.92 MBtu/h) of heating capacity. The energy consumed is approximately 53% of an adjacent similar non-GSHP building, saving $25,000 per month.
The Pacific Agricultural Research Centre (PARC) in Agassiz, Canada, employs ATES to supply heating and cooling to a 7000 [m.sup.2] (75,350 [ft.sup.2]) facility. The system provides free cooling and heating via heat pump. Estimated peak cooling and heating capacities are 563 kW and 293 kW (1.92 MBtu/h and 1 MBtu/h), respectively. There is no sufficient operational data from system monitoring campaigns (Bridge and Allen 2010).
A pilot project for an ATES system combined with GSHP is currently under development for a commercial building of 12,000 [m.sup.2] (120,000 [ft.sup.2]) in Medicine Hat, Canada. The system is designed to provide 90% of the annual cooling demand by direct cooling and 57% of the annual heating demand via the GSHP. The system should provide a net greenhouse gas (GHG) emissions reduction of 480 tonnes per year compared to a conventional energy system (Wong et al. 2006).
Closed-loop BTES systems depend less on site-specific hydrogeologic conditions than ATES systems and are better suited for areas where relatively high well yields are not obtainable. In addition, since the systems are operated in a closed loop (i.e., there is no contact between natural ground water and the heat exchange fluid), they have the potential to find much wider applications compared to ATES. A disadvantage of this concept is the relatively high construction cost, mainly due to drilling.
The largest BTES system in Sweden so far is located in Stockholm, offering 220 kW (751,174 Btu/h) of cooling from 30 borehole heat exchangers (BHEs) (Rybach and Sanner 2000). During a ten-year period, a saving of 40% of costs compared to conventional alternatives is expected.
Monitoring data of a small BTES for heating and cooling of the astronomy house in Lund, Sweden, comprised of 20 boreholes, were given by Andersson (2007d). Data from 2002 shows that during summer, the system delivered free cooling of 150 MWh (512 MBtu) at a COP of approximately 50. During winter, the COP of the total system (ground store and heat pump) has been 4.8, approximately 300 MWh (1024 MBtu) of heat and 95 MWh (324 MBtu) of electricity. The estimated payback period for the system has been ten years.
Midttomme et al. (2008) provided information on GSHPs with BTES in Norway. A new BTES system has recently been completed for a museum in Levanger. The heating and cooling scheme comprises a 130-kW (0.444-MBtu/h) heat pump and nine 180 m (590.6 ft) deep boreholes. The payback time for the extra capital costs of the ground-source system, compared to a conventional heating and cooling system, is estimated to be 12 years.
Desmedt et al. (2008) and Desmedt and Van Bael (2010) presented the results from a feasibility study to the implementation phase of vertical ground heat exchangers (GHEs) in combination with a GSHP for a Belgian office building. The energy savings, optimal configuration, and environmental benefits offered by using this system were calculated. Simulation results show that primary energy savings and C[O.sub.2] emission reduction of 31% can be obtained compared to classic primary energy consuming technologies. The ground storage system supplementary investment is paid back in eight years.
A large scale BTES system of Ontario Institute of Technology (UOIT), Canada, was presented by Dincer and Rosen (2011). The system has 380 boreholes, each 213 m (700 ft) deep, and is a critical component of the university's heating and cooling system. Monitoring results show annual energy savings for heating and cooling of 40% and 16% respectively. A payback period of 7.5 years is expected.
The German Air Traffic Control headquarters in Langen has been conceived as a low-energy office (Sanner et al. 2003). The BTES field comprising 154 BHEs is integrated into the heating and cooling system of the building. The BTES supplies a total cooling capacity of 340 kW (1.16 MBtu/h) and heating capacity of 330 kW (1.13 M Btu/h), representing 80% of the annual cooling and 70% of the annual heating requirements.
Operational experiences and design considerations for GSHP with seasonal storage
The use of ground-coupled systems in buildings offers economic as well as environmental advantages. When both heating and cooling is required, a ground-coupled system can function both as a heat source and a heat sink. These double-effect storage projects are more likely to be economical.
Experiences from ATES pilot projects and demonstration plants show storage efficiencies of 60% to 90% (Sanner et al. 2005; Andersson 2007b, 2007c). Primary energy savings and C[O.sub.2] emission reductions vary from 20% to 50% in the different projects (Snijders and Van Aarssen 2003; Nordell and Sanner 1998). These systems show high potential for free cooling operation, the concept of which has been widely utilized in the projects discussed in this article. For heating purposes, temperature upgrade by heat pumps is needed.
Experiences from BTES pilot projects and demonstration plants have shown primary energy savings and C[O.sub.2] emission reductions from 16% to 40% in the different projects (Fellin and Sommer 2003; Rybach and Sanner 2000; Desmedt et al. 2008; Desmeth and Van Bael 2010; Dincer and Rosen 2011). Estimated payback periods for the different projects have resulted in 8 to 12 years. Compared to systems with ATES, BTES systems show less potential for free cooling operation, mainly for a period at the beginning of the cooling season. Due to the low heat transfer rates between borehole heat carrier fluid and the surrounding ground, and heat up of the storage, reverse heat pump operation mode is used to supplement cooling operation. For heating purposes, the ground storage supplies low-temperature heat to heat pump evaporators.
GSHP projects with ATES and BTES have high investment costs. Therefore, detailed system simulation models are needed for design and dimensioning. Comprehensive thermodynamic analyses, evaluating thermal storage in aquifers for space heating and cooling, were performed by Carotenuto et al. (1990). A procedure for a numerical evaluation of the system performance and optimization is presented in a convenient form for system development and application.
Kangas and Lund (1994) developed a computer model AQSYST for simulating energy systems employing ATES. The model has been used to study the application of different types of aquifers for seasonal storage of thermal energy. Simulation results suggest that high-temperature storage (60[degrees]C-90[degrees]C [140[degrees]F-194[degrees]F]) is feasible only in stagnant aquifers, whereas for low-grade heat (15[degrees]C-20[degrees]C [59-68[degrees]F]), aquifers with high natural flows can be used (500-600 m/y [1500-1800 ft/y]).
Various analytical and numerical solutions have been developed and used as design/research tools to predict the short- and long-term response of BTES systems. Table 5 summarizes some of the most significant contributions to modeling the short- and long-term response of borehole ground heat exchanger systems.
Single borehole systems can be designed by considering only the long-term thermal response of the borehole. For multiple borehole systems, used for energy storage, the short-term response of the borehole has significant impact on the efficiency of the whole GSHP system. When determining the shortterm response, the borehole thermal capacitance and both the filling material of the borehole and the heat carrier fluid inside the ground heat exchanger should be considered. Short-term response of the ground is critical during heat flux build-up stages and for cases with both heating and cooling demands. Determining the hourly thermal energy use and the electrical demands of GSHP systems also requires the shortterm response of the ground to be considered.
The approach of Eskilson (1987) for numerical modelling of the thermal response of borehole systems using non-dimensional thermal response functions (g-functions) is considered state-of-the-art and has been implemented in software like EED (Blomberg et al. 2008), TRNSYS (Claesson et al. 1981; Hellstrom 1989; Mazzarella 1989; Pahud 1996; Klein 2004), HVACSIM+ (Clark 1985), and GLHEPRO (Spitler 2000).
In order to design efficient ground-coupled systems for the heating and cooling of buildings, temperature levels, surface areas of room heaters/coolers, performance characteristics of heat pumps, heat exchangers, circulation pumps, borehole geometry (aquifer characteristics), and cooling/heating demand of the buildings must be taken into account to achieve an optimal system that works efficiently in economical and technical terms.
For UTES systems, one of the most important external factors is the required temperature level for the heating/cooling case involved. TES systems become more efficient if the temperature requirement for space heating is low, about 35[degrees]C (95[degrees]F), and if the temperature for cooling is high, about 15[degrees]C (59[degrees]F). In that case, low temperature difference between the store and demand side will be present and also the heat pump would operate at lower temperature difference. Proper design will result in high COP for the whole system. However, this would require the use of low-temperature heating and high-temperature cooling radiant system in the building. Thermo-active building systems (TABS) for office and commercial buildings, and floor heating/cooling systems for single- and double-family residential houses, have proven successful in practice.
In Fellin and Sommer (2003) simulation analysis of an office building equipped with a thermal slabs system was presented. Two different climatic zones, two different strategies of ventilation, and two possibilities of plant--a traditional plant (low-temperature gas boiler and air-condensed chiller) and an innovative plant based on a GSHP--were studied. The results show that, by using a ground-coupled heat pump, more than 40% of energy can be saved compared to the use of a conventional system. The utilized advantages here are that the heat pump is coupled with a low-temperature heating and high-temperature cooling system (TABS), and in this particular building simulation, the temperature required for slab heating in winter is only 35[degrees]C (95[degrees]F) and for slab cooling in summer is 16[degrees]C (61[degrees]F).
For sizing GHEs, such properties as undisturbed ground temperature, ground thermal conductivity, borehole thermal resistance, and specific heat capacity are needed to deliver thermal energy at a proper temperature (Signorelli et al. 2004). The thermal efficiency of the BTES depends on the soil properties, ground water movement, temperature, and characteristics of the thermal store itself (geometry, borehole spacing, grouting material, pipe thermal conductivity) (Gehlin and Nordell 1997; Pahud and Matthey 2001; Zeng et al. 2003; Hellstrom 1991; Kjellsson and Hellstrom 1997; Hellstrom et al. 1988; Lund 1985; Reuss et al. 1997).
The design of GSHP with GHEs is influenced by the heating and cooling load characteristics of the building (load pattern), the size of the ground system (depth and number of boreholes), and the geometry of the ground system (configuration, i.e., positioning of the boreholes). The maximum and minimum design water temperature from the ground loop and the annual heat rejection to and extraction from the ground loop are important parameters.
Naumov (2005) studied, through computer simulations with EED software, the influence of the heating and cooling load characteristics of the building (load pattern), the size of the ground system (depth and number of the boreholes), and the geometry of the ground system (configuration, i.e., positioning of the boreholes) on the overall efficiency of a GSHP system. Simulation results have shown that the brine temperature (mean brine temperature in the boreholes) differences for different borehole configurations using the same specific borehole load (ratio between building heating and cooling needs and total borehole length) are not very large, being within 1-2K. On the other hand, changes in the specific borehole load have a large influence on ground system performance, and a mean brine temperature difference of 4-8K is needed for a specific borehole load increased with 200%-400%. Obviously, the larger the energy need of the building, the larger demand there is for the ground system and vice versa. For example, larger demand means that the total borehole length should be larger in order to keep the mean brine temperature within required limits for heat rejection/extraction.
Furthermore, simulations have shown that a system with balanced heating and cooling and storage (rectangular borehole layout) is more sensitive to the accuracy of the assumed load pattern than is a system for heating or cooling only (linear borehole layout). That is explainable by the fact that the requirements for the configuration of the boreholes are set by the share of the heating and cooling of the total energy need of the building. For example, if there is a heating- or cooling-dominated situation, then the heat exchange area of the ground system should be maximized to enhance the rejection of thermal energy in the ground and avoid heat accumulation or depression during long-term operation. In that case, linear borehole geometry is preferable. With a balanced heating and cooling situation, however, a rectangular layout with storage capability is advantageous in terms of seasonal energy performance. However, any significant deviation from that balance (assumed load pattern) will have significant influence on the long-term performance of the system (time span of ten years considered), which has been confirmed by the simulation results. The models developed in the work of Naumov (2005) together with the design software were applied in a case study (Astronomihuset, Lund). Simulated and measured data agreed reasonably well. Detailed results of the simulation and monitoring study are presented in the given references.
Regardless of the high energy-saving potential and the intensive technological development in recent years, GSHP systems with BTES wide application has been obstructed by the high investment costs associated with installing a ground loop to meet peak cooling or heating load. From another side, GSHP systems with ATES have considerably lower investment costs, but these systems are dependent on the availability of suitable natural aquifers at the site. An alternative design for ground-source systems is the hybrid system. This approach, widely considered a variation of the ground-coupled to GHEs design, utilizes the use of a cooling towers (in cooling-dominated commercial buildings) or boilers (in heating-dominated residential buildings) in parallel with the GSHP system. The use of a cooling tower or a boiler allows the designer to size the ground loop for the smaller of the heating or the cooling loads and use the GSHP in combination with the cooling tower to meet the peak cooling demand or with the boiler to meet the peak heating demand. The hybrid equipment preserves some of the energy efficiency of the system but reduces the capital cost associated with the ground loop installation. In addition, such a hybrid system would allow balancing the seasonal heating and cooling loads for the ground loop. The excess of building heating or cooling loads in a heating- or cooling-dominated building will be handled by the hybrid part of the system, and heat accumulation or depression in the ground system will be avoided, thus allowing design of the borehole layout in a way to benefit from seasonal storage of thermal energy.
Rafferty (1995) evaluated through numerical calculations the capital costs associated with ATES, BTES, and hybrid-BTES ground-source designs for cooling-dominated commercial buildings. Specifically, the costs considered are those associated with the heat source/sink or the ground portion of the system. The heat rejection over the three designs has been standardized, assuming constant heat pump loop temperature conditions, permitting in that way a direct comparison of the three systems. Considering the same building load for the three system types, cost calculations were made for a wide variety of soil (or groundwater) temperatures, well depths (groundwater), loop lengths (ground coupled), and tower-loop ratios (hybrid system). Results show that at system capacities of 100-175 tons (351.7-615 kW) and above, the ATES system has a capital cost advantage over hybrid BTES and BTES systems. Below this range, the hybrid system is the most attractive. Only for systems with very low capacities (less than 100 tons [351.7 kW]) and very deep aquifer depths (more than 800 ft [244 m]) will the ATES system capital cost exceed that of the BTES. Detailed results are given in Rafferty (1995).
In a recent study, Hackel and Petzborn (2011) analyzed, through computer simulations with TRNSYS, the energetic and economic performance of three hybrid-BTES GSHP installations in the United States compared to a BTES and a conventional HVAC. Two of the installations were for a cooling-dominated commercial buildings located in Las Vegas (NV), and one was for a heating-dominated residential building in Madison (WI). The results show that all three hybrid installations were economically cost effective. The average rate of return for investing in hybrids in these three cases has been 10% compared to a conventional HVAC system. The additional investment for a full BTES ground-source system would result in an average rate of return of just 3%.
The study of Hackel and Petzborn (2011) has shown the importance of component sizing in a hybrid system. Care should be taken not oversize the load that drives the GHE size. In addition, optimizing algorithms for proper sizing and operational modes of hybrid equipment (cooling towers and boilers) should be used. Circulation pumps use a lot of energy in a hybrid system, and due to that fact, pump sizes should be minimized and the focus should be on part-load operation. Variable-speed pumps should be used whenever possible. For the heat pump operation, large peaks should be avoided.
Basic design information for hybrid GSHP systems was given in Kavanaugh (1998), while in Xu (2007), optimal control strategies for these systems were developed.
As some authors suggest, the development of a unified simulation model for the combination of ground system, heat pump, heat exchanger, and building installations would be a useful task for future research. Future research should also include more-detailed GSHP system designs, including storage pattern and geometry and coupling to low-temperature beating and high-temperature cooling building systems. Additionally, new designs validation through measurements in different types of buildings would boost technological development of the concept.
Future prospects--TES in sustainable buildings
Seasonal TES is a mature technology that has been applied successfully throughout the world in residential, commercial, and institutional building applications. Energy sources include electric heat pumps, solar energy, winter ambient air, and waste thermal energy like industrial process heat. The concept has often been applied in standard buildings with the objective to demonstrate that the energy storage techniques could be successfully applied rather than to optimize the building performance. Indeed the design of the building and the design of the energy storage were often not coordinated, and energy storage simply supplied the building demand, whatever it might be.
Sustainable buildings need to take advantage of renewable and waste energy to approach ultra-low energy and zero-emission buildings. Such buildings will need to apply TES techniques customized for smaller loads and community-based thermal sources. Lower exergy heating and cooling sources will be more common. Utilization of low-exergy heating and cooling sources requires that energy storage is intimately integrated into sustainable building design.
A coordinated set of actions for improved seasonal UTES design and sizing is needed if the potential benefits are to be fully realized. Well-designed UTES systems can reduce initial and maintenance costs and can significantly reduce energy use and demand. Increased flexibility of operation, improved indoor environmental quality, conservation of fossil fuels, and reduced pollutant emissions are other benefits.
At present, IEA ECES Annex 23 "Applying Energy Storage in Buildings of the Future" is dealing with integration of energy storage in ultra-low-energy buildings.
Seasonal UTES is advanced energy technology, and there has been increasing interest in using it for thermal applications, such as DHW and space heating and cooling. Within the contents of this literature review article, the current status in the development and implementation of UTES has been investigated. The different energy storage concepts have very different characteristics, possible applications, strengths, and weaknesses. This study aims to motivate and provide the basis for the development of new intelligent TES possibilities in buildings.
The selection of an UTES system mainly depends on the energy source, local geo-hydrological site conditions, economic viability, and operating conditions. Specific parameters that influence the viability of a UTES system include facility thermal loads, thermal load profiles, availability of waste or excess thermal energy, availability of natural and renewable energy sources, type of thermal generating equipment, and building type and occupancy. The economic justification for a UTES system usually requires that annual capital and operating costs are less than the cost for conventional systems and equipment supplying the same service loads and periods. Well-designed systems can reduce initial and maintenance costs and energy use and demand. Although the different seasonal UTES solutions have found many applications in practice, it is still not clear how these can best be integrated into ultra-low-energy and zero-emission buildings that are capable of being replicated in a variety of climates and technical capabilities. Detailed studies on the dynamic performance and control strategies of the energy storage systems for different building types, weather conditions, and user behavior should be performed. Advanced design strategies for UTES solutions should be developed.
Received October 13,2011 ; accepted February 2, 2012
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* Corresponding author e-mail: firstname.lastname@example.org
Georgi K. Pavlov * and Bjarne W. Olesen
International Centre for Indoor Enviromnent and Energy (ICIEE), Department of Civil Engineering, Technical University of Denmark. Nils Koppels Alle, Building 402, Room 227, 2800 Kgs. Lyngby, Denmark
Georgi K. Pavlov, Student Member ASHRAE, is PhD Student. Bjarne W. Olesen, PhD, Fellow ASHRAE, is Professor.
Table 1. Comparison of storage concepts (Schmidt et al. 2003, Novo et al. 2010). Storage concept Water tank Gravel-water pit Storage medium Water Gravel-water Thermal capacity, 60-80 (5.8-7.73) 30-50 (2.9-4.83) kWh/[m.sup.3] (kBtu/[ft.sup.3]) Storage volume for 1 [m.sup.3] (35.3 1.3-2 [m.sup.3] (46-71 1 [m.sup.3] (35.3 [ft.sup.3]) [ft.sup.3]) [ft.sup.3]) water equivalent Geological --Stable ground --Stable ground requirements conditions conditions --Preferably no --Preferably no ground water ground water --5-15 m (16-49 ft) --5-15 m (16-49 ft) deep deep Application Heating: CSHPSS Heating: CSHPSS Storage concept Aquifer Borehole Storage medium Sand/water-gravel Soil/rock Thermal capacity, 30-40 (2.9-3.9) 15-30 (1.45-3.9) kWh/[m.sup.3] (kBtu/[ft.sup.3]) Storage volume for 2-3 [m.sup.3] (71-106 3-5 [m.sup.3] (106-177 1 [m.sup.3] (35.3 [ft.sup.3]) [ft.sup.3]) [ft.sup.3]) water equivalent Geological --Natural aquifer --Drillable ground requirements layer, high hydraulic conductivity --Confining layers --High heat capacity on top and below --No or low natural --High thermal ground water conductivity flow --Suitable water --Low hydraulic chemistry at high conductivity temperatures --20-50 m (65-164 --No or low natural ft) deep ground water flow (<20 m/year [65 ft/year]) (a) --30-200 m (98-656 ft) deep Application Heating: CSHPSS Heating: CSHPSS (a) Heating and Heating and cooling: GSHP cooling: GSHP (a) Geological requirement for seasonal storage of solar thermal energy. Table 2. Typical values of BTES systems for heat storage applications. Borehole diameter 0.1-0.15 m (0.3-0.5 ft) Borehole depth 30-200 m (98-656 ft) Distance b/n 2-4 m (6.6-13.1 ft) boreholes Ground thermal cond. 2-4 W/(mK) (3-7 Btu/(h.ft.[degrees]F)) Borehole diameter Flowrate in U-pipes Borehole depth Average capacity per m (ft) borehole Distance b/n Minimum/maximum boreholes inlet temperature Ground thermal cond. Cost of BTES per m (ft) borehole Borehole diameter 0.5-1.0 m/s (1.6-3.3 ft/s) Borehole depth 20-30W/m (Btu/h.ft) Distance b/n -5/-1-90[degrees]C (23-194[degrees]F) boreholes Ground thermal cond. 50-80 [euro]/m (20-33 us $/ft) Source: http://www.highcombi.eu Table 3. Technical data of CSHPSS. Total heat demand, Solar CSHPSS with GJ/a collector area, storage type (GBtu/a) [m.sup.2] ([ft.sup.2]) Water tank Hamburg, DE 5796 (5.5) 3000 (32, 291) Friedricshafen, 14, 782 (14) 5600 (60, 277) DE Hannover, DE 2498 (2.4) 1350 (14, 531) Munich, DE 8280 (7.9) 2900 (31, 215) Ingelstad, SE 1320 (14, 208) Lambohov, SE 2700 (29, 062) Hoerby, DK Herlev, DK 4520 (4.3) 1025 (11, 033) Gravel-water pit Stuttgart, DE 360 (0.34) 211 (2271) Chemnitz, DE 4320 (4.1) 2000 (21, 527) Steinfurt, DE 1170(l.1) 510 (5489) Eggenstein, DE 3276 (3.1) 1600 (17, 222) Ottrupgaard, DK 1630 (1.55) 560 (6027) BTES Neckarsulm, DE 1663 (l.58) 5000 (53, 819) Crailsheim, DE 14, 760 (14) 7300 (78, 576) Attenkirchen, DE 1753 (l.7) 800 (8611) Anneberg, SE 3888 (3.7) 3000 (32, 291) Okotoks, CA 1900 (1.8) 2293 (24, 681) ATES Rostock, DE 1789 (1.7) 1000 (10, 763) Solar CSHPSS with Storage volume, fraction, storage type [m.sup.3] ([ft.sup.3]) % Water tank Hamburg, DE 4500(159 * [10.sup.3]) 49 (a) Friedricshafen, 12, 000 (424 * [10.sup.3]) 47 (a) DE Hannover, DE 2750 (97 * [10.sup.3]) 39 (a) Munich, DE 5700 (201 * [10.sup.3]) 47 (a) Ingelstad, SE 5000 (177 * [10.sup.3]) 14 (a) Lambohov, SE 10, 000 (353 [10.sup.3]) 37 (a) Hoerby, DK 500 (18 * [10.sup.3]) Herlev, DK 3000 (106 * [10.sup.3]) 35 (a) Gravel-water pit Stuttgart, DE 1050 (37 * [10.sup.3]) 60 (a) Chemnitz, DE 8000 (283 * [10.sup.3]) 42 (a) Steinfurt, DE 1500 (533 [10.sup.3]) 34 (a) Eggenstein, DE 4500 (159 * [10.sup.3]) 40 (a) Ottrupgaard, DK 1500 (53 * [10.sup.3]) 16 (a) BTES Neckarsulm, DE 63, 400 (2240 * [10.sup.3]) 50 (a) Crailsheim, DE 37, 500 (1324 * [10.sup.3]) 50 (a) Attenkirchen, DE 10, 000 (353 * [10.sup.3]) 55 (a) Anneberg, SE 60000 (2120 * [10.sup.3]) 60 (a) Okotoks, CA 35, 000 (l236 * [10.sup.3]) 90 (a) ATES Rostock, DE 20, 000 (706 * [10.sup.3]) 62 (a) Maximum design storage Solar heat temperature, cost analysis CSHPSS with [degrees]C date, MWh storage type ([degrees]F) (3.41 R MBtu) Water tank Hamburg, DE 95(203) 256 EUR Friedricshafen, 95(203) 158 EUR DE Hannover, DE 95(203) 414 EUR Munich, DE 95(203) 240 EUR Ingelstad, SE 1900 SEK Lambohov, SE 1100 SEK Hoerby, DK Herlev, DK Gravel-water pit Stuttgart, DE 85(185) Chemnitz, DE 85(185) 240 EUR Steinfurt, DE 90(194) 424 EUR Eggenstein, DE 80(176) Ottrupgaard, DK BTES Neckarsulm, DE 85(185) 172 EUR Crailsheim, DE 85(185) 190 EUR Attenkirchen, DE 85(185) 170 EUR Anneberg, SE 45(113) 1000 SEK Okotoks, CA 80(176) ATES Rostock, DE 50(122) 255 EUR CSHPSS with storage type References Water tank Hamburg, DE Kubler et al. (1997), Schmidt et al. (2004), Lottner (2000), Bauer (2010) Friedricshafen, Kubler et al. (1997), Schmidt DE et al. (2004), Loaner (2000), Bauer (2010) Hannover, DE Schmidt et al. (2004), Lottner (2000), Bauer (2010) Munich, DE Schmidt and Mangold (2006), Bauer (2010) Ingelstad, SE Dalenb5ck et al. (1985) Lambohov, SE Dalenb5ck et al. (1985) Hoerby, DK Heller (2000) Herlev, DK Heller (2000) Gravel-water pit Stuttgart, DE Hahne 2000 Chemnitz, DE Schmidt et al. (2004) Steinfurt, DE Pfiel et al. (2000) Eggenstein, DE Bauer et al. (2010) Ottrupgaard, DK Heller (2000) BTES Neckarsulm, DE NuBbicker et al. (2003), Schmidt et al. (2004), Bauer et al. (2010) Crailsheim, DE Mangold (2007) Attenkirchen, DE Schmidt et al. (2004) Anneberg, SE Nordell et al. (2000), Lundh et al. (2008) Okotoks, CA McDowell & Thornton (2008), Sibbit et al. (2007), Chapuis & Bernier (2009) ATES Rostock, DE Schmidt et al. (2000, 2004), Lottner (2000), Bauer (2010) Note: CA = Canada, DE = Germany, DK = Denmark, SE = Sweden. (a) Calculated values for long-time operation; simulations carried out using TRNSYS dynamic simulation software; weather data of the test reference year (TRY) used in the simulations. Table 4. Technical data of GSHP systems with aquifer seasonal TES. Total building System purpose area, [m.sup.2] ([ft.sup.2]) Heating Cooling GSHP with ATES Eindhoven (TUe), NL 250,000 + + (2,690,977) Berlin (Parliament), - + + DE Chr. Hansen A/S, DK - - + DBI Plast A/S, DK - - + Sky-Light A/S, DK - - + Billund Lufthavn A/S, - - + DK AKV Langholt A/S, - - + DK Malmo, SE - - + Stockholm, SE - - + Gardermoen Airport, - - + NO Louisville, Kentucky, 161,651 + + US (1,739,997) PARC, Agassiz, CA 7000 (75,347) + + Medicine Hat, CA (b) 12,000 + + (129,166) GSHP with BTES Stockholm, SE - - + Lund, SE 4200 (45,208) + + Falstadsenteret, NO 2850 (30,677) + + EANDIS, BE 16,363 + + (176,129) UOIT, CA 80,000 + + (861,112) Langen, DE 44,500 + + (478,994) System capacity, MW Energy delivery, (MBtu/h) GWh/a (GBtu/a) Heating Cooling Heating GSHP with ATES Eindhoven (TUe), NL 20(68) 20(68) 25-33 (85-113) Berlin (Parliament), - - 2.05 (7.0) DE Chr. Hansen A/S, DK - 3.7 (12.6) - DBI Plast A/S, DK - 0.175 (0.6) - Sky-Light A/S, DK - 0.45(l.53) - Billund Lufthavn A/S, - 2.4 (8.2) - DK AKV Langholt A/S, - 3.9 (13.3) - DK Malmo, SE - 1.3 (4.4) - Stockholm, SE - 25 (85.3) - Gardermoen Airport, - 8(27.3) - NO Louisville, Kentucky, 19.6 (66.9) 15.8 (53.9) - US PARC, Agassiz, CA 0.3(l.0) 0.56 (l .92) - Medicine Hat, CA (b) 1.8 (6.1) 1.1 (3.75) 2.7 (9.2) GSHP with BTES Stockholm, SE - 0.22 (0.75) - Lund, SE 0.3(l.0) 0.3(l.0) 0.395 (1.35) Falstadsenteret, NO 0.13 (0.44) 0.13 (0.44) - EANDIS, BE 1.9 (6.5) 1.2 (4.1) 0.9 (3.1) UOIT, CA 1.4 (4.8) 1.3(4.4) - Langen, DE 0.33(l.13) 0.34(l.16) 0.07 (0.24) Energy delivery, Savings GWh/a (GBtu/a) Electricity Cooling reduction GSHP with ATES Eindhoven (TUe), NL 25-30 (85-102) 20% (a) Berlin (Parliament), 3.95 (13.5) - DE Chr. Hansen A/S, DK 6.0 (20.5) - DBI Plast A/S, DK 0.875 (2.98) - Sky-Light A/S, DK 3.5 (11.94) - Billund Lufthavn A/S, 0.87 (2.97) - DK AKV Langholt A/S, 8.75 (29.8) - DK Malmo, SE 3.9 (13.3) - Stockholm, SE 0.9 (3.1) - Gardermoen Airport, 13 (44.4) - NO Louisville, Kentucky, - 47% (a) US PARC, Agassiz, CA - - Medicine Hat, CA (b) 1.1(3.75) - GSHP with BTES Stockholm, SE - 40% (a) Lund, SE 0.15 (0.51) - Falstadsenteret, NO - - EANDIS, BE 0.824 (2.81) 31% (a) UOIT, CA - 40%/oh (a) 16%c (a) Langen, DE 0.06 (0.21) 77% Savings C[0.sub.2] reduction References GSHP with ATES Eindhoven (TUe), NL 20% (a) Snijders et al. (2003) Berlin (Parliament), - Sanner et al. (2005) DE Chr. Hansen A/S, DK 468 ton/a Sorensen et al. (2007), Qvale et al. (1988), Schleisner et al. (1991) DBI Plast A/S, DK 202 ton/a Sky-Light A/S, DK 686 ton/a Billund Lufthavn A/S, 202 ton/a DK AKV Langholt A/S, 1290 ton/a DK Malmo, SE - Andersson et al. (2003) Andersson (2007c) Stockholm, SE - Andersson and Rudling (2000), Andersson (2007b) Gardermoen Airport, - Eggen and Vangsnes(2005) NO Louisville, Kentucky, 47% (a) Nordell and Sanner (1998) US PARC, Agassiz, CA - Bridger and Allen (2010) Medicine Hat, CA (b) 480 ton/a Wong et al. (2006) GSHP with BTES Stockholm, SE 40% (a) Rybach and Sanner (2000) Lund, SE - Andersson (2007d) Falstadsenteret, NO - Midttemme et al. (2008) EANDIS, BE 31% (a) Desmedt et al. (2008) Desmedt et al. (2010) UOIT, CA Dincer and Rosen (2011) Langen, DE 77% Sanner et al. (2003) Note: BE = Belgium, DE = Germany, DK = Denmark, NL = Nederlands, NO = Norway, SE = Sweden, US = United States. (a) Savings compared to conventional systems supplying the system loads and services. (b) pilot project under development. Table 5. Mathematical models for design and dimensioning of borehole heat exchangers. Model types Reference Long-term response of BTES systems --Line source theory Ingersoll et al. (1954) --Cylindrical source theory Ingersoll et al. (1954), Kavanaugh (1985), Bernier (2001), Bernier et al. (2004), Nagano et al. (2006) --Numerical non-dimensional Eskilson (1987) g-functions --Analytical g-functions Eskilson (1987), Zeng et al. (2002), Lamarche and Beauchamp (2007a), Bandos et al. (2009) --Capacity-resistance model Zarella et al. (2010) (CaRM) Short-term response of BTES systems --Short time-step g-functions Yavuzturk 1999; Yavuzturk and Spitler 1999, 2001; Xu and Spitler 2006 --Analytical Young (2001), solutions for short- Lamarche and term response of Beauchamp (2007b), borehole heat Bandyopadhyay et al. exchangers (2008), Javed 2010 --Capacity resisance model with Zarella et al. short-term response modelling (2011) (CaRM)
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|Author:||Pavlov, Georgi K.; Olesen, Bjarne W.|
|Publication:||HVAC & R Research|
|Date:||Jun 1, 2012|
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