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Seasonal energy storage.

Most buildings meet thermal loads using equipment and systems that generate or remove heat when building loads exist. Thermal energy storage (TES) enables buildings to meet heating and cooling loads using energy produced at other points in time.

TES can be designed for storing and providing energy on three basic timescales: diurnal, weekly, and seasonally. (1) Cold storage tank systems using ice or chilled water are examples of diurnal storage systems, i.e., they produce ice or chilled water in anticipation of cooling loads within the next 24 hours.

In contrast, seasonal thermal energy storage (STES) enables a building to use heat collected during the summer to heat the building in the winter, or to use snow collected during the winter to cool the building in the summer. Relative to diurnal storage systems, STES requires a much larger total size of the TES system, while the rate of charge and discharge varies much less with timescale.

STES systems consist of several components, including a heat (or coolness) source, heat exchange system, thermal distribution system, thermal storage medium, and thermal loads (Figure 1).

Ideally, an STES saves low-cost heat that would otherwise not be used. Thermal energy sources used by STES systems include solar thermal (typically low-temperature collectors), industrial waste heat, excess heat from district energy systems, snow and ice, and seawater. (2,3)

Examples of thermal loads met by STES are commercial and multifamily residential building space and water heating, space heating for greenhouses, roadway deicing/snow melting, and building space cooling. (1,2,4) In general, STES tends to be most attractive for applications with significant heating or cooling loads that are offset by several months from the peak availability of thermal resources.

For example, solar thermal collectors in Northern climates collect much larger quantities of heat during the summer than in the winter due to the longer solar days in summer, while space heating loads peak in winter. Similarly, cool STES also works best in climates where large quantities of snow and ice can be harvested during the winter to provide space cooling during the summer. Consequently, STES energy savings and economics tend to be more favorable in colder, Northern climates.

Several types of STES are used, and the most common systems, namely aquifer thermal energy storage (ATES) and borehole thermal energy storage (BTES), store heat in the ground. Both take advantage of the fact that deeper than 10 m to 20 m (33 ft to 66 ft), the ground and groundwater temperatures vary little over the course of the year (4) to reduce thermal losses.

ATES systems transfer heat to and from groundwater in aquifers via wells drilled from the surface into the aquifer. Often, the wells are grouped separately, as warm and cold, to provide heating and cooling. (5) Favorable environmental characteristics for ATES installations include high ground porosity levels and significant water content around the wells to enable effective heat transfer between the wells and the aquifer, and low ground water flow through the aquifer to avoid convection of the stored thermal energy away from the wells. (4,6) * Because these qualities fundamentally impact the viability of ATES, the aquifer must be characterized before initiating ATES projects. (4,7) Both lower (10[degrees]C-40[degrees]C [50[degrees]F-104[degrees]F]) and higher (40[degrees]C-150[degrees]C [104[degrees]F-302[degrees]F]) temperature systems exist. (4) Although higher temperature systems have a greater storage capacity per volume, they have greater thermal losses and may also experience more problems with mineral precipitation. (1)

BTES systems consist of many boreholes (0.15 - 0.2 m [0.5 ft to 0.66 ft] wide) (6) drilled into the ground at depths ranging from 35 m (8) to 200 m (6) (115 ft to 656 ft) deep. A pump circulates a fluid (typically water or a water-glycol mixture) through pipes buried in the boreholes that transfer heat to and from the boreholes. After drilling the boreholes and installing the pipes in the boreholes, the borehole is back-filled, often with a material to enhance thermal conductivity, such as water, sand, or bentonite clay. Many BTES installations use closed systems, i.e., with a continuous pipe loop (U-pipe), while some use open systems that inject water at the bottom of the borehole and extract it near the top (but below the local water table). Desirable ground characteristics for BTES include high specific heat and thermal conductivity, as well as low groundwater flow. (5,6)

Both ATES and BTES require suitable ground conditions that do not always exist. Consequently, people have worked to develop other STES concepts.

Pit TES transfers heat to and from water (with or without gravel) stored in an excavated pit, with the top surface usually near or at the ground surface to reduce excavation costs.

Usually, all sides of the pit are insulated to mitigate thermal losses, particularly from the top, and the sides are made of concrete with liner to prevent water (both liquid and vapor) migration from the tank. (6,9) As with BTES, plastic pipes throughout the pit transfer heat to and from the pit. The typical sizes of storage tanks range from 100 [m.sup.3] to 10 000 [m.sup.3] (3,500 [ft.sup.3] to 353,000 [ft.sup.3]) for underground and partly buried tanks to 1000 [m.sup.3] to 1 million [m.sup.3] (35,000 [ft.sup.3] to 35.3 million [ft.sup.3]) for pit storage. (3)

In addition, cold storage pits exist, including a snow storage pit used to cool a hospital in Sweden. During winter, natural and man-made snow fills the pit to create a frozen reservoir that is covered with wood chips to insulate the reservoir. During the cooling season, pumps extract the melt water from the pit and use it to cool the hospital. (3,6)

Many systems also use a diurnal storage component (typically a water tank) to complement the STES. This second, smaller tank acts as a thermal buffer between the STES and the heat sources and sinks, i.e., it can accept heat from the thermal resource--and transfer heat to the heating loads--at higher rates than a BTES can achieve. For example, the STES often cannot accept the peak output of solar thermal collectors during the summer. Instead, the hot water generated by the collectors primarily flows to a sizeable water storage tank during the middle of the day while the STES charges at a slower rate from the diurnal storage and the collectors throughout the day. (8,9)

Other concepts considered and, to varying degrees deployed include caverns, in-soil ducts, above-ground water tanks, rock storage with air circulation, latent heat storage (using phase change materials) and thermochemical heat. (3,4,6)

STES systems also include backup thermal energy sources, such as gas boilers and district heating systems.

STES systems may or may not have a heat pump to augment the quality of the thermal energy harvested. Each approach has its pros and cons. On one hand, a heat pump enables use of lower quality (e.g., for heating, lower storage temperature) resources, increasing the storage capacity ([dagger]) and decreasing the first cost of the storage itself. On the other hand, it incurs the first cost of the heat pump, which can be significant. Designing a system without the need for a heat pump avoids its first cost,1 but also requires that the systems have stored energy of sufficient quantity and quality throughout demand periods to avoid significant use of backup heating or cooling sources. This usually increases the storage temperature and/or size of the STES; the former increases thermal losses, while the latter increases first cost.

Although in theory STES can be applied at any scale ranging from a single home to a sizeable community, (10) its economics--and its efficiency--improve appreciably with scale. (5,7) As a result, most systems are built for applications with higher levels of heating demand, typically several hundred to thousands of kW of maximum thermal output. (5) Consequently, the rest of this article focuses on larger-scale STES.


Energy Savings Potential

STES saves energy by storing thermal energy that would otherwise be wasted and using it to meet a significant portion of building space heating, water heating, and/or cooling loads. As such, the energy savings potential of STES largely depends on the portion of these loads that the stored energy can supplant. In turn, this depends on the capacity of the STES relative to the loads and the efficiency of the STES, both of which are part of the economic optimization of a specific application.

For example, in solar thermal applications, the solar fraction equals the portion of the heating loads that the STES can provide. Actual and projected values for deployed solar thermal systems range from around 30% to more than 90%. (1,3,7,8) One application of snow storage, for cooling a Swedish hospital, achieves a STES fraction of between 77% and 93%. (3)

The efficiency of STES, defined as the portion of heat transferred into the STES that remains available to meet loads, also varies significantly. Important factors include effective insulation levels (be it from the ground in ATES or BTES or installed insulation in pit storage), STES size, storage temperature, and storage type (warm versus cold). In general, larger systems achieve higher efficiencies because thermal losses scale approximately with surface area while storage capacity scales with volume. Furthermore, cold storage systems generally have higher efficiencies than warm storage systems. For example, cold ATES can achieve efficiencies of 70% or greater, while warm ATES tends to have somewhat higher losses and, therefore, lower efficiencies (50%-80%) (1,4) due to natural convection. (4)

STES systems do not realize their full energy savings potential immediately. As a warm STES is "charged" with thermal energy, its temperature rises and heat diffuses away through the ground. When the STES discharges, its temperature falls and the losses decrease. Nonetheless, it can take several years for the thermal energy to diffuse through the ground surrounding the STES to the point where the time-varying annual temperature profile of the soil in the vicinity of the STES approaches a consistent time-varying profile from year-to-year. (4,8)

Market Factors

STES has been under investigation since at least the solar boom in the 1970s, but relatively few systems exist. To date, the majority of systems have been built in Europe. The Netherlands has the largest number of STES installations: approximately 700 ATES systems in operation and annual construction volumes of 50 to 100 ATES systems over the last several years. In the Netherlands, offices account for the largest fraction of STES (~40%), with multifamily residences the next most common application. (5)

First cost and its impact on the cost of stored energy is the foremost barrier to greater use of STES. Fundamentally, STES has a low cycling rate, i.e., it cycles once a year, amortizing its cost over a limited number of cycles. (1) For comparison, a service hot water tank cycles approximately daily. Furthermore, since most STES deployments cannot meet all of the thermal loads, they still require backup heating (or cooling) sources and the associated costs.

Solar thermal STES systems have particularly high estimated and projected costs per unit of thermal energy provided. For example, projected (not demonstrated) costs of STES for two new projects in Germany equal $0.24 to $0.51 ([double dagger]) per kWh of thermal energy. (7,9) Estimates for a 52-home STES project in Alberta, Canada, project a cost of about $0.08 [section] per kWh (thermal). (11) For comparison, natural gas at $10/MMBtu equals $0.034 per kWh (thermal).

Cold STES may have more favorable economics than warm STES, (1) presumably because it displaces more costly electricity (relative to thermal energy) while also achieving significant electric demand reductions.

In addition, groundwater chemistry can create several problems in ATES, including the fouling of heat exchangers and pipe clogging from the precipitation of minerals, heat exchanger and pipe corrosion, and aquifer clogging from precipitated minerals. To a significant extent, careful ATES design, operating at moderate temperatures (e.g., <60[degrees]C [140[degrees]F]), (1) or water treatment can help reduce or eliminate these problems. (4,5)

Other barriers include the perception of high risk for STES, unfamiliarity with STES, and concerns about regulatory/environmental issues (e.g., leaking of glycol into groundwater, imbalances in ATES groundwater volumes). (4,6) Furthermore, successful design and implementation of STES requires close integration between hydrologists, geologists, engineers, and architects. (4) Ground conditions may not be favorable for ATES or BTES, or the needed space for creating pit storage (3) is just not always available. However, particularly in urban areas, access to district and industrial waste heat may be available.

Ultimately, greater use of STES will require improved economics, primarily from decreasing storage cost. This suggests deploying larger STES systems, while also taking advantage of standardization to reduce costs. (5) In addition, future decreases in the cost of solar thermal collectors would improve the economics of solar-based STES systems.


(1.) Handorn, J.C. 2008. "Underground solar energy storage." Seminar at the 2008 ASHRAE Winter Meeting.

(2.) Hellstrom, G., 2008. "International development and application borehole geothermal energy technology." Seminar at the 2008 ASHRAE Winter Meeting.

(3.) Reuss, M., 2008. "Water pits and snow store: technologies for seasonal heat and cold storage." Seminar at the 2008 ASHRAE Winter Meeting.

(4.) Bridger, D.W., and D.M. Allen. 2005. "designing aquifer thermal energy storage systems." ASHRAE Journal 47(9):S32-S38.

(5.) Snijder, A.L. 2008. "Aquifer thermal energy storage (ATES): technology development and major applications." Seminar at the 2008 ASHRAE Winter Meeting.

(6.) Nielsen, K., H. Gether, and J. Gether. 2004. "Thermal Energy Storage--A State-of-the-Art." Report within the Smart Energy-Efficient Buildings Research Program at the Norwegian University of Science and Technology and SINTEF.

(7.) Schmidt, T., D. Mangold, and H. Muller-Steinhagen. 2004. "Central solar heating plants with seasonal storage in Germany." Solar Energy 76(1-3):165-74.

(8.) Sibbitt, B., et al. 2007. "The Drake Landing Solar Community Project--Early Results."

(9.) Schmidt, T. and D. Mangold. 2006. "New steps in seasonal thermal energy storage in Germany." Proceedings of Ecostock 2006: The Tenth International Conference on Thermal Energy Storage. http://

(10.) IEA. 2008. "Task 32: Advanced Storage Concepts for Solar and Low Energy Buildings." International Energy Agency (IEA) Solar Heating & Cooling Programme.

(11.) McClenahan, D., et al. 2006. "Okotoks: seasonal storage of solar energy for space heat in a new community." Proceedings of the ACEEE Summer Study for Energy Efficiency in Buildings. mclenahan.

By Kurt Roth, Ph.D., Associate Member ASHRAE; and James Brodrick, Ph.D., Member ASHRAE

Kurt Roth, Ph.D., is a principal with TIAX LLC, Cambridge, Mass. James Brodrick, Ph.D., is a project manager with the Building Technologies Program, U.S. Department of Energy, Washington, D.C.

* Bridger and Allen (2005) provide more detail about favorable aquifer characteristics and characterization methods. (4)

([dagger]) Because the lowest temperature that can provide useful heat decreases.

([double dagger]) Using 1 Euro = USD$0.79; Schmidt et al. (2004) used a 6% interest rate.

([section]) This value should be understood to be very rough, as the economic assumptions behind these costs were not presented in the citation.
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Title Annotation:Emerging Technologies
Author:Roth, Kurt; Brodrick, James
Publication:ASHRAE Journal
Geographic Code:1USA
Date:Jan 1, 2009
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