Applications of customized absorption heat pumps with heating capacities above 500 kW.

ABSTRACT

There are numerous potential applications for suitably tailored absorption heat pumps, especially in the range of medium and large heating capacities of 500 kW (1.7 x [10.sup.6] Btu/h) and above. In many cases, this is the most suitable technology to utilize the available heat sources. This is demonstrated in this paper with three examples of current installations in southern Germany.

At a municipal composting plant, waste heat is generated at a temperature around 45[degrees]C (113[degrees]F). Previously, this waste heat had to be rejected to the ambient by means of a cooling tower. Now, a direct-fired single-stage absorption heat pump lifts the waste heat to a temperature of 82[degrees]C (180[degrees]F), enabling its utilization in the local heating network of a commercial area.

At a spa with various swimming pools located next to a thermal spring, a combined heating and power (CHP) engine plant was installed. The reject heat of the gas engine drives a novel two-stage absorption heat pump that utilizes the spring water as a renewable heat source to provide heating of the pools and the building.

In Munich, a solar-assisted local district heating system was installed in a new housing development with about 300 units. At this site, a seasonal hot water storage for the solar system of about 5700 [m.sup.3] (2.0 x [10.sup.5] [ft.sup.3]) was erected. At the beginning of the heating season, it serves the local heating network directly, and afterwards--at a lower temperature level--it is utilized as a heat source for an absorption heat pump that is driven by the municipal district heating network. By that concept, two effects are accomplished: the available temperature change of the hot water storage is increased and the mean temperature of the solar system is decreased. Thus, an increase of the annual efficiency of the solar collectors and finally an increase of the annual solar gain is accomplished.

INTRODUCTION

An absorption heat pump utilizes driving heat [Q.sub.G] to lift a heat flux [Q.sub.V] from an ambient heat source--i.e., waste heat, solar heat, geothermal heat--to a useful temperature level. The driving heat [Q.sub.G] that enters the heat pump at a high temperature [T.sub.G] is released again at an intermediate temperature [T.sub.A] [approximately equal to] [T.sub.K] together with the ambient heat [Q.sub.V] that is absorbed at temperature [T.sub.V] (Figure 1).

The absorption heat pumps that are applied in the projects presented in this paper are customized single-stage and double-stage machines. Single-stage plants based on the working pair water/aqueous lithium bromide solution reach a thermal efficiency (coefficient of performance [COP] = quotient of useful heat output and driving heat input) of about 1.7. Due to the higher temperature of the driving heat, the thermal efficiency of double-stage plants reaches about 2.2. Thus, the application of an absorption heat pump can save about 50% of the primary energy required for the provision of useful heat (Ziegler 1997; Ziegler et al. 2000a). This results in even higher savings of primary energy than in applications of vapor compression heat pumps, where additionally the conversion efficiency of the electric power plant has to be taken into account to obtain the primary energy consumption for the provision of useful heat.

[FIGURE 1 OMITTED]

A more detailed comparison of an electrically driven compression heat pump versus a fossil-fuel-fired absorption heat pump is shown in Figure 2 for the provision of 10 arbitrary units (kJ or Btu) of heating energy. In each case, the upper bars reflect the useful heat and the contributions of ambient heat and driving energy. In the case of an absorption heat pump, the heat input into the generator heat exchanger serves as driving energy, while electricity or mechanical energy in general represent the driving energy input into a compression heat pump. For both heat pump principles, i.e., absorption and compression, the contribution of ambient heat and driving energy varies as a function of the cycle COP, as indicated in Figure 2. With growing COP, ambient heat utilization increases while driving energy input is reduced accordingly. The lower bars describe the energetic situation with regard to the primary energy demand for operation of the competing heat pump systems. For that purpose, the conversion of primary energy into driving energy is taken into account, i.e., combustion of fossil fuels to provide driving heat for the absorption heat pump or electricity generation for operation of the vapor compression heat pump. Due to the limited efficiency of electricity generation in thermal power plants, a rather high primary energy demand is found for the operation of the compression heat pump system, whereas in the case of a fossil fuel-fired absorption heat pump, only minor losses occur during the conversion of primary energy into driving heat. Assuming efficient thermal design for both systems--i.e., COP = 2.2 for the absorption heat pump and CO[P.sub.el] = 4.5 for the compression heat pump--about five units of primary energy are required for the operation of the thermally driven heat pump, while about 6.5 units of primary energy are required in the case of the electrically driven heat pump. Hereafter, the ratio of useful heat output and primary energy input will be referred to as the primary energy ratio (PER).

[FIGURE 2 OMITTED]

Another practical advantage of water/lithium bromide-based absorption heat pumps is that the same working fluid and the same basic design of all main components can be used for any useful heat temperature from about 20[degrees]C to 150[degrees]C (68[degrees]F to 302[degrees]F) and for heating capacities from about 50 to 5,000 kW (1.7 x [10.sup.5] to 1.7 x [10.sup.7] Btu/h). This simplifies the development of customized machines for special applications compared to vapor compression heat pumps, where such a customization is always subject to market availability of the suitable refrigerant and the appropriate compressor with regard to temperature levels, pressure ratio, capacity, etc.

UTILIZATION OF WASTE HEAT FROM A COMPOSTING PLANT

Application Overview

Within the rotting hall of the compost plant at a municipal waste processing plant in southern Germany, organic waste is composted by the so-called heap method. Inside the hall there are ten heaps in which the biological aerobe process takes place. Caused by the decomposition process, the temperatures rise up to 75[degrees]C (167[degrees]F). The first three heaps, the so-called intensive heaps containing the most recent base material, develop the highest temperatures. During the progress of the composting process, the internal temperatures of the heaps decrease; therefore, the last three heaps reach a temperature of only 30[degrees]C (86[degrees]F).

Every four to six days, the heaps are moved by a paddle wheel onto the next heap; hence, each batch passes through all heaps in about forty days. Due to the high process temperatures, the compost leaves the plant sanitized and well decomposed at the end of the process; hence, it can be applied for agricultural uses.

Between the moving procedures, the heaps have to be forced-air ventilated in order to maintain an appropriate oxygen supply as well as to stabilize their temperatures. Therefore, air is pumped through the heaps via two ventilators powered by gas engines. After passing the heaps, the air is moist and warm. As odor nuisance has to be avoided, the air has to pass a biological filter. Since this filter is sensitive to high temperatures, the warm air from the first three heaps has to be cooled before entering the filter. The cooling is performed by an exhaust air heat exchanger. After being cooled, the exhaust airflow from the first three heaps is mixed with the airflow from the other heaps within a mixing chamber. Particulate material included in this exhaust air is removed by an air washer. Afterward, the air passes the biological filter and then it is released to the environment. The exhaust air heat exchanger transfers heat from the exhaust air to a cold water cycle. This cold water cycle is utilized as a heat source for the evaporator of a specially designed absorption heat pump.

[FIGURE 3 OMITTED]

Design and Integration of the Absorption Heat Pump

The system scheme in Figure 3 shows the integration of the gas-fired absorption heat pump into the energy supply system at the waste processing plant.

In the machinery hall at the site, there is a natural-gas-powered cogeneration unit with an electrical power output of 130 kW (444 x [10.sup.3] Btu/h) and a thermal power output of 210 kW (717 x [10.sup.3] Btu/h). Additionally there are two gas engines driving ventilators in the composting plant. These engines deliver a thermal output of 180 kW (615 x [10.sup.3] Btu/h). Furthermore, the diesel engine of a hydraulic baling press for bundling waste supplies about 90 kW (307 x [10.sup.3] Btu/h) of thermal energy. The administration building of the waste processing plant is supplied with heat from this internal heating network. Within the building, there is an additional oil boiler, with a maximum thermal output of 64 kW (218 x [10.sup.3] Btu/h).

At a heat transfer station, the heat from the internal network is transferred to a district heating network to supply a nearby commercial area. In addition, two modulating oil-fired boilers are installed in order to satisfy the peak heat demand of the district heating system. The thermal output of each boiler is 500 kW (1707 x [10.sup.3] Btu/h).

[FIGURE 4 OMITTED]

Due to the intermittent processes of forced-air ventilation and of moving the compost heaps, the exhaust air temperature and the heat load on the cold water cycle show significant variations over time. Figure 4 shows the exhaust air temperature of the intensive heaps as well as the capacity transferred to the cold water cycle, which serves as heat source for the absorption heat pump. The fluctuations of the thermal output of the compost heaps have to be considered for the integration of the absorption heat pump for thermal utilization of the available waste heat. Hence, a buffer storage with a volume of 3.5 x [10.sup.3] L has been integrated into the cold water cycle to maintain a constant evaporator capacity. This buffer storage also works as a hydraulic separator. The evaporator of the heat pump is fed from this storage tank by means of a frequency-controlled pump.

The return from the cold-water storage feeds an air heating system in the composting hall before it returns to the exhaust air heat exchanger. To ensure the cooling of the exhaust air and the proper functioning of the biological filter when the heat pump is not in operation (e.g., during summertime), an additional cooling tower (shown in Figure 3) was installed at the site. During operation of the heat pump, however, the cooling tower is switched off and bypassed by the cold water cycle.

The graphs in Figure 5 show the design data of the heat pump. The system has to provide a constant outlet temperature at the condenser of the heat pump (useful heat) of 82[degrees]C (180[degrees]F) irrespective of the current operating condition. In order to achieve the large temperature lift from the heat source to useful heat of about 50[degrees]C (122[degrees]F), the heat pump is equipped with a gas-fired generator.

The gas burner is operated modulating, depending on the operating condition of the heat pump. Hence, a heating capacity from 325 to 600 kW (1110 x [10.sup.3] to 2049 x [10.sup.3] Btu/h) can be provided. The COP is expected to be about 1.6 at 50% part load, increasing up to 1.65 at full load. Taking into account a burner efficiency of about 88% in part load and about 85% in full load, a PER of about 1.45 is accomplished.

Optionally, a further increase of the PER could be achieved by utilization of the exhaust gas waste heat in an additional recuperator heat exchanger. Transfer of this waste heat to the cold water cycle could provide an increase as well as a stabilization of the evaporator capacity. For this plant, an increase in evaporator capacity, and therefore an increase in fuel utilization, of about 35 kW (120 x [10.sup.3] Btu/h) is to be expected.

[FIGURE 5 OMITTED]

First Operational Results

The heat pump was installed at the site in spring 2005; commissioning was completed in the heating season, winter 2005-06. In the beginning, detailed measuring instrumentation was installed, and to this date all operational data are recorded for evaluation. Some refinements in the hot water cycle (useful heat) will be performed during summer 2006; in winter 2006-07, the measurements will continue. Some recently recorded data from winter 2005-06 are shown in Figures 6 and 7.

Figure 6 shows the inlet and outlet temperatures of the external water cycles during 90 minutes of operation after completion of the start-up procedure of the heat pump. It shows that the setpoint temperature of 82[degrees]C (180[degrees]F) in the hot water cycle (condenser outlet) is maintained after a short swing at 11:05 h (end of start-up phase). The evaporator temperatures decrease gradually during operation but stay always above their design values of 42[degrees]C/34[degrees]C (108[degrees]F/93[degrees]F) because of the limited heating demand during the presented 90 minutes operating period. The significant fluctuations in the absorber inlet temperature result from an improper control of a mixer valve; this issue was fixed after the measurement data shown in Figure 6 had been evaluated.

Figure 7 shows the corresponding heat loads in the main components of the absorption heat pump that were calculated from the measured temperatures and flow rates. The generator capacity displayed in this figure is calculated from the capacities of the evaporator and absorber/condenser via an enthalpy balance.

Primary Energy Savings and Economics

Based on an average evaporator capacity of 195 kW (665 x [10.sup.3] Btu/h)--resulting in an average useful heat output of 500 kW (1706 x [10.sup.3] Btu/h)--and a runtime of 3500 hours per year, energy cost savings of about 26,800 euro per year, compared to a gas boiler with a 90% efficiency, can be calculated. The additional investment (including planning cost) of the installed heat pump, compared to a standard boiler system, amounts to about 137,000 euro. Hence, a return on investment is achieved in only 5.4 years when calculating with a 0% interest rate. An annuity calculation with a 7.5% interest rate yields annual savings of roughly 10,000 euro in comparison to the operation of a standard boiler system.

[FIGURE 6 OMITTED]

Compared to a gas boiler, about 37% of the fossil fuel is saved by the heat pump, resulting in a decrease of carbon dioxide emission of about 145 tons per year.

All detailed data for the energetic and economic analysis of the heat pump system are summarized in Table 1.

HEAT SUPPLY FOR A SWIMMING FACILITY WITH COMBINED COOLING, HEATING, AND POWER GENERATION

Application Overview: Energy System of the Swimming Facility

The municipal utilities of Konstanz in southern Germany are currently reconstructing one of their swimming facilities, including all buildings and in particular the central energy supply. In parallel to space heating of the building, the new energy system will supply heat to four swimming pools at temperatures between 24[degrees]C and 34[degrees]C (75[degrees]F and 93[degrees]F) with an average heating capacity of 600 to 800 kW (2.05 x [10.sup.6] to 2.73 x [10.sup.6] Btu/h). The peak heating demand of all heat consumers amounts to 2 MW (6.86 x [10.sup.6] Btu/h).

On site, a thermal spring delivers 24 [m.sup.3]/h (847.5 [ft.sup.3]/h) of warm thermal water at a temperature of 25.5[degrees]C (77[degrees]F). The availability of a geothermal heat source initiated the design of an innovative energy system comprising a double-effect absorption heat pump, which is driven by heat from a cogeneration engine. The electricity produced will either be used in the swimming pool or will be fed to the public grid. Such installations are called trigeneration or combined cooling, heating, and power (CCHP) systems; typically, they serve the supply of electricity for heating and cooling. In this case, the absorption chiller serves as a heat pump, yet the system configuration remains unchanged. According to the state of the art, single-effect chillers are applied to absorb all cogenerated heat of the motor engine, which is transferred via a conventional hot water cycle. Heading for increased energetic efficiency, several innovative CCHP installations recently have been carried out with two-stage absorption chillers (Radermacher 2003; Zhang 2003). As discussed later, in terms of efficiency and flexibility, the current project goes beyond that status.

[FIGURE 7 OMITTED]

In order to allow for efficient heat pump operation, all heat consumers have been designed for the lowest possible heating circuit temperatures. Floor heating and heating of the swimming pools have been fixed at 45[degrees]C/35[degrees]C (113[degrees]F/95[degrees]F) supply/return temperatures. Conventional radiator heating and ventilation operate at 70[degrees]C/50[degrees]C (158[degrees]F/122[degrees]F). Tap water heating--mainly serving the showers--has been designed for 70[degrees]C/30[degrees]C (158[degrees]F/86[degrees]F).

Low-temperature heat demand in the temperature range of 30[degrees]C to 45[degrees]C (86[degrees]F to 113[degrees]F) will be covered by heat output from the absorber and condenser of the two-stage absorption heat pump. Therefore, the double-effect absorption heat pump has been designed for 710 kW (2.42 x [10.sup.6] Btu/h) heating capacity to cover the base load heat demand of the bath, thus contributing the major part of the annual heating energy. The warm water from the geothermal well entering the evaporator of the heat pump at a temperature of 25[degrees]C (77[degrees]F) will be cooled down to 8[degrees]C (46[degrees]F) and then discharged to the nearby Lake of Konstanz. With respect to the high temperature lift from the chilled water outlet at 8[degrees]C (46[degrees]F) to 45[degrees]C (113[degrees]F) useful heat supply temperature, a so-called twin design of the low-pressure stage of the heat pump is applied. By that means, the chilled water is cooled by two evaporators in series. Analogously, absorption takes place in two absorbers located at slightly different pressure levels. In addition, the application of a double-effect cycle concept supports a high-temperature lift, given the thermodynamic limitations arising from the limited temperatures of the hot water loop linking the motor engine and the low-temperature generator of the heat pump.

During the maximum heat demand of 2 MW (6.86 x [10.sup.6] Btu/h), two natural gas condensing boilers will supplement the heat supply by the trigeneration system. Via a multi-chamber diverter, low- and high-temperature heat is supplied to the different heating systems, i.e., radiator space heating, air ventilation, floor heating, pool heating, and tap water heating. For additional energy savings, the thermal hot water is directly used to feed the showers.

Conceptual design of the energy system started in fall 2004, followed by a detailed planning phase. Major components and the erection of the hydraulic installation was contracted during winter 2005-06. Start-up of the trigeneration system is scheduled for fall/winter 2006.

Innovative System with Double-Effect Absorption Cycle

The objective of the project is the implementation of an innovative CCHP concept with a cogeneration engine and a double-effect absorption chiller/heat pump. The high-temperature stage of the chiller is directly driven by hot flue gas released by the motor engine.

Current trigeneration systems use standard single-stage absorption chillers to convert cogenerated heat into useful cold. Typically, cold is supplied to air-conditioning facilities via a chilled water loop with supply/return temperatures at about 6[degrees]C/12[degrees]C (German standard) or 44[degrees]F/54[degrees]F (ANSI/ARI standard); motor engines, gas turbines, or fuel cell systems are applied as cogenerators. For the heat transfer from the cogeneration unit to the absorption chiller, the chiller is coupled to the conventional hot water heating loop of the cogeneration system, as shown schematically in Figure 8.

Cogeneration systems release a major fraction of the cogenerated heat via a hot flue gas flow with exhaust temperatures about 450[degrees]C (842[degrees]F). Utilization of this flue gas heat is commonly characterized by a drastic reduction of the exergy content of the cogenerated heat when transferred to a hot water heating loop with temperatures below 100[degrees]C (212[degrees]F). This reduction in heat exergy, i.e., lowering of the temperature level in comparison to the initial flue gas temperature, is tolerable in the case of direct heating applications. Yet, in the case of utilizing this heat as driving heat for a thermally operated chiller or heat pump, i.e., an absorption heat pump, the reduction of the temperature level causes a significant loss in cooling or heat pumping potential.

For best utilization of the exergy content of the cogenerated heat, an innovative multi-effect absorption chiller/heat pump concept will be implemented. Hot flue gas (about 450[degrees]C [842[degrees]F]) leaving the cogeneration engine will first be fed to a double-effect absorption cycle and subsequently to a single-effect cycle. By this means, the major portion of the flue gas heat (down to about 200[degrees]C [392[degrees]F]) is used by the double-effect cycle with a heating COP of about 2.25 (cooling COP of about 1.25 if operated as chiller). The remaining part (down to about 120[degrees]C [248[degrees]F]) is used by the single-effect cycle (heating COP = 1.75 or 0.75 for cooling). The low-temperature heat of the cogeneration engine released by the mixture cooler, the lube oil cooling, and the engine (jacket) cooling is supplied to the single-effect chiller in addition to the low-temperature flue gas heat. As in conventional CHP systems, a proper design of the flue-gas-heated components, i.e., high-temperature generator and low-temperature flue gas heat exchanger, is required in order to avoid a negative impact on the performance of the CHP engine due to an increase of the back pressure at the flue gas outlet of the engine.

[FIGURE 8 OMITTED]

The above plant concept with integration of single-stage and double-stage absorption is called a double-effect/single-effect (DE/SE) chiller or heat pump. By integrating double-effect and single-effect absorption cycles, an optimum adaptation of the heat utilization by the absorption heat pump to the gliding heat output of the cogeneration system is accomplished. The loss in system performance due to the degradation of the cogenerated heat before entering the chiller at low hot-water temperature is eliminated (Ziegler et al. 2000b; Kren et al. 2001, 2002a, 2002b; Plura et al. 2004, 2005; Keil et al. 2005).

In comparison to alternative system concepts (Radermacher 2003; Zhang 2003), the current development is heading for a DE/SE integration with increased efficiency and flexibility: the implementation of a DE/SE absorption heat pump requires using high-temperature heat directly to drive the high-temperature generator of the heat pump and to merge different low-temperature heat flows as driving heat for the low-temperature generator. The major difference between the different system concepts is to be attributed to the form of coupling between the heat rejection of the cogenerator and the driving heat input into the absorption chiller. In general, two different concepts have to be taken into account:

* internal coupling

* external coupling (implemented within the current project in Konstanz)

In both cases, the high-temperature generator, belonging to the double-effect subcycle, is directly heated by the hot flue gas. The concepts differ with respect to the low-temperature heat input into the low-temperature generator G1 comprising

* internal heat transfer from the high-temperature stage to the low-temperature stage of the chiller,

* second-stage flue gas, leaving the high-temperature generator G2, and

* low-temperature hot water (jacket cooler, lube oil cooler, mixture cooler) of the cogenerator.

In the case of the internal coupling, for utilization of the different low-temperature driving heat sources, the chiller is equipped with three low-temperature generators. In the case of the external coupling (Kren et al. 2004), all low-temperature heat flows are transferred to the hot water loop. Thus, only one low-temperature generator is required, as shown in Figure 9.

An analysis (Plura et al. 2004, 2005) of the system concepts given above shows that the "external coupling" option offers substantial advantages in comparison to the alternative concepts in terms of

* optimum flexibility of the system in operation for heating and cooling and

* less complex absorption cycle and system integration.

By superimposing double-effect and single-effect cycles, an overall heating COP of about 1.95 will be accomplished, allowing for a 30% increase in heat intake from the thermal heat source as compared to standard single-effect absorption technology with heating COP = 1.75. Thus, by use of the thermal heat source via the DE/SE heat pump, the heat output of the CHP unit is approximately doubled. Useful heat is supplied at a low temperature level with 45[degrees]C/35[degrees]C (113[degrees]F/95[degrees]F) supply/return temperatures. Figure 10 shows the system configuration for the case of the swimming facility in Konstanz, Germany.

Primary Energy Reduction and Economics

While in conventional systems fossil fuel is consumed by a central heating boiler and for generation of electricity, the driving energy demand of the trigeneration system with an absorption heat pump is dominated by the CHP unit. With respect to optimum energetic performance and economics, the system has to be designed for continuous operation of the trigeneration system and only minor contribution of the peak boiler and external electricity supply. As a result, the combination of the DE/SE heat pump with a CHP unit will allow for a remarkable reduction of the primary energy consumption to supply heating and electrical power for the swimming pool. Compared to the application of a fossil-fuel-fired conventional central heating boiler and purchasing electricity from the public grid, primary energy use can be reduced to roughly 50%. In comparison to a sole cogeneration engine providing electricity and heating, still almost one-third of the fuel demand is saved. These data result from a detailed energetic and economic analysis of the integrated energy system.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

General technical and economic data for the comparison of the three system options are listed in Table 2. Table 3 gives an overview of the sizing of the different system components and their resulting contributions in order to provide the same amounts of useful heat and electricity. According to the size of the cogeneration engine under consideration, a very high annual utilization of more than 8,000 full-load operating hours is to be expected for the "Cogeneration Plant" without heat pump. Simultaneously, the utilization of the fossil-fuel-fired central heating boiler is reduced from 2,795 h/yr to 1,535 h/yr. Due to its higher heat output, the system "Cogeneration Plant + Absorption Heat Pump" reaches approximately 6,500 h/yr annual full-load operation with very little remaining utilization of the fossil-fuel-fired boiler during only 600 h/yr. Thus, both systems with a CHP unit allow for a substantial reduction of the external energy supply and of the respective energy cost. For comparison reasons, all three systems have been adjusted to the same annual heat production and the same amount of electricity (1729 MWh/yr) as set by the innovative system with CHP unit and absorption heat pump. Compared to this reference, the CHP system without a heat pump provides 531 MWh/yr surplus electricity that can be sold to the public grid, while the system without a CHP unit ("Fossil Boiler + Electric Grid") requires an external electric supply of 1878 MWh/yr. Taking into account the energy rates as given in Table 2, the two systems with a CHP unit achieve annual savings of energy cost and operating cost of about 88,000 euro (system without heat pump) and about 177,000 euro (system with absorption heat pump), respectively. Having in mind the increase of investment cost in comparison to the standard system without a CHP unit, payback periods of 2.7 and 2.6 years are obtained for the two energy-saving system concepts. Due to the rather short payback period, the interest rate has been set to 0% for this general economic analysis. On the basis of an annuity calculation with a 7.5% interest rate, the advantage of the innovative system with a CHP unit and absorption heat pump becomes evident. The total annual cost of this system is about 126,000 euro lower compared to the standard system without a CHP unit. During a 15-year modeling period, these savings amount to 1,900,000 euro, equal to approximately three times the initial investment. For the CHP system without an absorption heat pump, the total savings are expected to be about 50% lower.

The primary energy demand based on the described system modeling is listed in the bottom section of Table 3. As outlined at the beginning of this section, substantial energy savings is accomplished by the application of the integrated system with a CHP unit and absorption heat pump, resulting from the high overall efficiency of the CHP unit of 90% (36% electric + 54% thermal) and the heat pumping effect with a COP close to 2.

SOLAR-ASSISTED DISTRICT HEATING SYSTEM WITH SEASONAL STORAGE

Application Overview: Solar Housing Development at Ackermannbogen in Munich

In Munich, a solar-assisted local district heating system is presently installed in a new housing development area with about 300 units (Scholkopf et al. 2002; Mangold and Peuser 2003; Mangold et al. 2003). In all roofs with south orientation, solar flat plate collectors are integrated; a total collector area of 3600 m (3.9 x [10.sup.4] [ft.sup.2]) is installed. At the site, a seasonal hot water storage tank with a capacity of about 5,700 [m.sup.3] (2.0 x [10.sup.5] [ft.sup.3]) and a total height of about 16m (52 ft) is erected and will be covered by an artificial hill after completion. This storage will not only buffer daily peaks in the heating demand or solar irradiation, but it also will serve a part of the heating demand during the winter months by solar heat that was stored during the summer. Additional heating demand that exceeds the capacity of the solar system is served from the municipal district heat network.

A number of such large-scale solar-assisted heating systems with seasonal storage have been installed in various European cities in recent years (Schmidt et al. 2004), and an overview of demonstration plants in Germany is provided in Mangold and Peuser (2003), Schmidt et al. (2003), and Mangold et al. (2003). However, one general issue in conventional systems like this is that the seasonal hot water storage can only be utilized to serve the local heating network if hot water temperature in the storage is larger than the network return temperature. In this way, the storage tank can be cooled down to approximately 40[degrees]C (104[degrees]F). Afterward, the heating system has to switch to another heat source, such as the municipal district heating network. The remaining heat capacity in the storage tank is lost; the water cools down further during the rest of the winter due to thermal losses and has to be heated up again the next spring, consuming valuable solar heat. It has already been reported (Mangold et al. 2003; Schmidt et al. 2003, 2004; Raab et al. 2004) that the performance of solar heating plants with seasonal storage is very sensitive to the mean return temperature in the heating network. The annual solar fraction of supplied thermal energy drops significantly below its design value if the return temperatures exceed the foreseen limits. The integration of an additional heat pump has been identified as a promising option to resolve this issue in refined plant concepts (Scholkopf et al. 2002; Schmidt et al. 2004; Raab et al. 2004).

Therefore, an absorption heat pump will be integrated into the novel solar-assisted district heating system currently installed at Ackermannbogen in Munich. After the direct utilization of the hot water storage for heating is over, the heat pump will use the storage tank as a heat source for the evaporator at temperatures below approximately 40[degrees]C (104[degrees]F). Thus, the amount of additionally required non-solar heat from an alternative source can be reduced and the annual share of utilized solar heat in the local heating system is increased--without increasing the size of the most costly components, solar field and storage tank. The absorption heat pump will be driven by hot water from the supply line of the municipal district heating system.

Figure 11 shows a simplified scheme of the solar-assisted district heating system. Next to the seasonal storage tank, a district heat station is built; both are covered by an artificial hill. All technical installations, such as heat exchangers, pumps, and controls of the solar system, will reside there, together with the absorption heat pump and the connection to the municipal district heating network. A full backup system with an additional heat exchanger (shown in Figure 11) will ensure that the heating demand of the housing area can completely be served by the municipal district heat when the storage tank has been cooled down completely at the end of the winter. Under heat pump operation, this backup heat exchanger will be employed for additional heat recovery from the hot water leaving the regenerator of the heat pump. The district heat station is connected with all houses in the Ackermannbogen area via the local district heating network and the primary collector cycle of the solar system.

[FIGURE 11 OMITTED]

The local low-temperature district heating network at the housing development area will be operated all year with a constant supply temperature of 60[degrees]C (140[degrees]F) and a variable mass flow. In the return line, the design temperature is 30[degrees]C (86[degrees]F) with expected variations of about [+ or -]10[degrees]C ([+ or -]18[degrees]F). During summer, the water in the storage tank will be heated by the solar system, reaching temperatures around 95[degrees]C (203[degrees]F). As long as temperatures in the top section of the stratified tank are at least 65[degrees]C (149[degrees]F) or higher, the local heating network is completely served from the storage via a plate heat exchanger. At lower temperatures in the top section of the hot water tank, the backup system will be switched on, using some additional heat from the alternative source (the municipal district heating network) to maintain a 60[degrees]C (140[degrees]F) supply temperature in the local network. If temperatures at the top of the stratified storage fall below about 45[degrees]C (113[degrees]F), the heat pump will be switched on.

Boundary Conditions for the Heat Pump Application

During winter, the municipal district heating network is operated with the supply temperature gliding between approximately 90[degrees]C (194[degrees]F) and 130[degrees]C (266[degrees]F). In order to maximize the transport capacity of the district heating network, the return temperature has to be kept at no more than 50[degrees]C (122[degrees]F), as fixed by utility regulations. Therefore, the absorption heat pump has to cope with a variation of the entering temperature of the driving heat ("hot water cycle") in the range of 90[degrees]C to 130[degrees]C (194[degrees]F to 266[degrees]F). On the other hand, the hot water outlet temperature at the regenerator has to be as low as possible because all the hot water leaving the regenerator of the heat pump has to be cooled down in the heat exchangers of the backup system to 50[degrees]C (122[degrees]F) before entering the return line of the municipal network. This second share of the heat from the municipal network is directly transferred to the local heating network and cannot be utilized as driving heat for the heat pump. According to simulation calculations that have been performed, hot water outlet temperatures at the regenerator in the range of 60[degrees]C to 95[degrees]C (140[degrees]F to 203[degrees]F) during the heating season are required to achieve suitable capacities of the heat pump.

In the cooling water cycle (useful heat), the heat pump has to accept the return temperature of the local heating network in the range of 20[degrees]C to 40[degrees]C (68[degrees]F to 104[degrees]F) as the absorber inlet temperature, while a condenser outlet temperature of about 55[degrees]C (131[degrees]F) is required. In the chilled-water cycle (heat source/seasonal storage), during the heating season the evaporator inlet temperature drops from 45[degrees]C to 10[degrees]C (113[degrees]F to 50[degrees]F). The evaporator outlet temperature is expected to be in the range of 5[degrees]C to 10[degrees]C (41F to 50[degrees]F) at the end of the heating season to ensure a complete utilization of the heat capacity of the seasonal storage.

Customized Absorption Heat Pump

Such a large temperature lift of approximately 45[degrees]C (81[degrees]F) between evaporator outlet and condenser outlet temperature and such large temperature differences between inlet and outlet temperatures in the three cycles (about 10[degrees]C [18[degrees]F] in chilled water, 25[degrees]C [45[degrees]F] in cooling water, and 30[degrees]C [54[degrees]F] in hot water) are hardly feasible with a simple single-stage absorption cycle (Figure 11). Simulation calculations, however, showed that a serial coupling of two single-stage cycles (Figure 12) is suitable to fulfill all the requirements. This arrangement has a total of eight main heat exchangers and four pressure levels (two different evaporator pressures plus two different condenser pressures) with each external water loop passing consecutively to both absorption cycles. Thus, e.g., the hot water is utilized first in "Regenerator 2" at a higher temperature level and consecutively in "Regenerator 1" at a lower temperature level to achieve a total temperature glide of 30[degrees]C (54[degrees]F). It was shown that such a coupling scheme results in a reasonable size of the total surface area of the eight main heat exchangers and in a good COP of about 1.7 of the heat pump.

[FIGURE 12 OMITTED]

The detailed planning of the absorption heat pump has been completed in cooperation with an experienced manufacturer of standard absorption chillers and heat pumps. It was possible to integrate the two absorption cycles shown in Figure 12 into a single machine with only two main shells on a single base frame.

Project Status and Project Aims

Construction work at Ackermannbogen was started in 2005; during summer 2006 construction of the seasonal hot water storage tank and of some of the housing units was scheduled to be finished. The absorption heat pump has been ordered for manufacture and was to be delivered in September/October 2006. Commissioning of the heat pump is scheduled for winter 2006/2007, followed by a measuring campaign and an optimization of the operation of the novel system for several years.

For the housing development area at Ackermannbogen in Munich, a peak heating demand of approximately 1500 kW (5.1 x [10.sup.6] Btu/h) and a total annual heat demand of approximately 2300 MWh (7.85 x [10.sup.9] Btu) were estimated. The absorption heat pump will contribute a maximum heating capacity of 850 kW (2.9 x [10.sup.6] Btu/h). According to numerical simulations of the annual operation of the whole plant, it is estimated that the heat pump will recover approximately 160 MWh (5.48 x [10.sup.8] Btu) of low-temperature heat from storage each winter and increase the solar fraction of the supplied thermal energy in the heating network from about 43% to 50%.

One major objective of this project is to demonstrate that the proper application of a heat pump in solar heating systems with seasonal storage makes it easier to reach a solar fraction of 50% of the total annual heat demand (heating plus hot water) in Central Europe. Due to the application of the heat pump and the cooling of the storage tank in winter, the available storage capacity per unit volume of the tank is increased. Additionally, due to the lower annual mean temperature in the storage tank, the efficiency of the solar collector system increases. Thus, in comparison to conventional large-scale solar heating systems without heat pumps, for a given solar share, a smaller solar field and smaller hot water storage are required.

In this context, it has to be mentioned that at the development area, Ackermannbogen, which is situated in an urban environment with limited free space, all suitable roofs with south orientation are already fully covered with solar collectors. Thus, an increase of the solar field would not have been an alternative to the application of the heat pump.

Economic Considerations

A proper calculation of the economics of the heat pump is hard to provide, as all large-scale solar heating plants with seasonal heat storage are designed individually for the boundary conditions of their respective locations. Therefore, the specific costs of the different systems with individual sizes of storage tanks and solar fields are not directly comparable.

For evaluation of demonstration plants in Germany, conventionally a simplified definition of the solar heat cost in euro/kWh is employed, based on the mere capital cost for the investment, including planning but excluding financial subsidies and value added tax (VAT), assuming a standardized annuity of 8.72%. According to Schmidt et al. (2003, 2004) and Mangold et al. (2003), previous installations of large-scale plants with solar fractions of 40% to 60% achieved solar heat cost in the range of 0.17 to 0.42 euro/kWh. For the system at Ackermannbogen in Munich, solar heat cost of 0.20 to 0.25 euro/kWh is to be expected. The additional investment cost for the heat pump, including system integration, is about 300,000 euro; based on the standardized annuity of 8.72%, a specific price of only 0.16 euro/kWh for the recovered solar heat is obtained. Thus, the specific cost of the additionally recovered heat by application of the heat pump is expected to be lower than the average cost of useful solar heat in the total system at Ackermannbogen--and also lower than the cost of solar heat achieved in various previous installations without heat pumps.

SUMMARY AND DISCUSSION

Based on established and proven technology of water/lithium bromide absorption chillers, various customized single-stage and double-stage heat pump cycles adapted to specific applications can be designed. These heat pumps can either be fossil-fuel-fired or driven by heat from CHP systems or other sources.

This paper reports on three recent applications of customized absorption heat pumps in southern Germany:

A direct fired single-stage absorption heat pump lifts waste heat in a compost plant from about 40[degrees]C (104[degrees]F) to 82[degrees]C (180[degrees]F) useful heat temperature for supply of a district heating network. The unit combines the evaporator/absorber shell of a conventional absorption chiller with the high-temperature part of a double-effect chiller.

To supply a spa with various swimming pools at a thermal spring with low-temperature heat at 45[degrees]C/35[degrees]C (113[degrees]F/95[degrees]F), a combined single/double-stage absorption heat pump in combination with a CHP unit is applied. Thermal water being cooled down from 25[degrees]C to 8[degrees]C (77[degrees]F to 46[degrees]F) is employed as an ambient heat source for the heat pump. The double-stage part is fired by exhaust gas from a gas engine in the range of 450[degrees]C to 180[degrees]C (842[degrees]F to 356[degrees]F). The single-stage part utilizes cooling water from the CHP engine and exhaust gas in the temperature range of 180[degrees]C to 120[degrees]C (356[degrees]F to 248[degrees]F) as driving heat. This results in a significantly increased energy efficiency compared to conventional CHP systems with single-stage absorption cycles.

In a solar-assisted local district heating system, a special single-stage absorption heat pump is to be installed that realizes large temperature differences in the external cycles due to four internal pressure levels. A seasonal hot water storage is cooled from about 45[degrees]C to 10[degrees]C (113[degrees]F to 50[degrees]F) and useful heat is provided at about 55[degrees]C (131[degrees]F). Hot water from the municipal district heating network at temperatures of 90[degrees]C to 130[degrees]C (194[degrees]F to 266[degrees]F) is utilized as driving heat of the heat pump.

Single-stage heat pumps reach thermal efficiencies around 1.7, double-stage cycles reach efficiencies above 2.0; i.e., about 40% to 50% of the supplied useful heat originates from the ambient heat source instead of the driving heat. For direct-fired machines, however, the boiler efficiency also has to be taken into account to determine the PER.

As has been shown by detailed economic analyses, in cases where the heat pump replaces a fossil-fuel-fired boiler, a payback period of only a few years can be achieved, strongly depending on the annual utilization of the heat pump system. In the case of the waste heat utilization of a composting plant, where the utilization of the heat pump is limited to the heating period, a payback period of 5.4 years has been found. At the thermal swimming pool, due to continuous operation of the heat pump system, return of the investment is reached after 2.6 years. In both cases, the specific cost for the supply of useful heat is reduced substantially in comparison to the application of a fossil boiler (see Tables 1 and 3). In the composting plant, the cost for the useful heat provided by the heat pump is 0.041 euro/kWh and in the thermal bath, 0.028 euro/kWh are to be expected, whereas heat from a fossil fuel boiler is available for about 0.45 euro/kWh. The system with seasonal heat storage, of course, yields a higher specific cost for the useful heat output due to the limited availability of ambient heat input from the seasonal storage and an accordingly short annual utilization. Yet, the specific cost for the additional solar heat (0.16 euro/kWh) gained by integrating an absorption heat pump is lower than the cost of solar heat achieved in earlier installations without an absorption heat pump.

REFERENCES

Keil, C., S. Plura, C. Kren, and C. Schweigler. 2005. Einsatzbeispiele angepasster Absorptions-Warmepumpen mit Heizleistungen uber 500 kW. Proceedings of the DKV-Tagungsbericht 2005, 32. Deutsche Kalte-Klima-Tagung, Wurzburg, November 16-18, 4:293-308.

Kren, C., T. Dantele, C. Schweigler, F. Ziegler, J. Sadler, R.J. Tucker, J. Scharfe, C. Larger, and J. Sahun. 2001. An efficient LiBr absorption chiller for the European air conditioning market. Proceedings of the 2001 International Gas Research Conference (IGRC), November 5-8, Amsterdam, The Netherlands, paper no. RCO-07.

Kren, C., C. Schweigler, and F. Storkenmaier. 2004. Multistage absorption refrigerating apparatus or heat pump and the use of the apparatus in an energy conversion system. European Patent Application, Pub. No. EP 1 391 665 A2. Retrieved from http://v3.espacenet.com/origdoc?DB=EPODOC&IDX=EP1391665.

Kren, C., C. Schweigler, and F. Ziegler. 2002a. Efficient LiBr absorption chillers for the European air conditioning market. ISHPC '02, Proceedings of the International Sorption Heat Pump Conference, Shanghai, China, September 24-27, pp. 76-83.

Kren, C., F. Storkenmaier, and C. Schweigler. 2002b. Sorptionskalteanlagen fur effiziente Kraft-Warme-Kalte-Kopplung. Proceedings of the Energie Innovativ 2002, Kongress, Nurnberg, June 4-5, pp. 71-82.

Mangold, D., and F.A. Peuser. 2003. Solarthermie-2000: 10 years of research and development in large solar heating systems in Germany. Proceedings of the ISES Solar World Congress 2003, Gateborg, Sweden, 14.-19.06.03. Retrieved from http://www.itw.uni-stuttgart.de/ITW Homepage/Sun/englisch/public/pdfDateien/03-07.pdf.

Mangold, D., T. Schmidt, and V. Lottner. 2003. Seasonal thermal energy storage in Germany. Proceedings of Futurestock, Warsaw, 01.-04.09.03. Retrieved from http://www.itw.uni-stuttgart.de/ITWHomepage/Sun/englisch/public/pdfDateien/03-09.pdf.

Plura, S., C. Kren, and C. Schweigler. 2005. System concept for efficient and flexible tri-generation. Proceedings of the International Sorption Heat Pump Conference, (ISHPC), June 22-24, Denver, CO, paper no. ISHPC-071-2005. Retrieved from http://www.enme.umd.edu/ceee/ishpc/content/ISHPC2005Proceedings.pdf.

Plura, S., U. Paul, C. Kren, and C. Schweigler. 2004. Absorptionskalteanlagen fur effiziente Kraft-Warme-Kalte-Kopplung, Proceedings of DKV-Tagungsbericht 2004, 31. Deutsche Kalte-Klima-Tagung, Bremen, November 17-19, II(1):127-44.

Raab, S., D. Mangold, W. Heidemann, and H. Muller-Steinhagen. 2004. Solar assisted district heating system with seasonal hot water heat store in Friedrichshafen (Germany). Proceedings of EuroSun 2004--The 5th ISES Europe Solar Conference, June 20-23, Freiburg, Germany. Retrieved from http://www.itw.uni-stuttgart.de/ITWHomepage/Sun/englisch/public/pdfDateien/04-13.pdf.

Radermacher, R. 2003. Short course: Integration of refrigeration and air-conditioning with distributed generation, University of Maryland. Oral presentations of different authors and paper handout. 21st International Congress of Refrigeration, August 17-22, Washington, DC, USA.

Schmidt, T., D. Mangold, and H. Muller-Steinhagen. 2003. Seasonal thermal energy storage in Germany. Proceedings of ISES Solar World Congress 2003, Gateborg, Sweden, June 14-19. Retrieved from http://www.itw.uni-stuttgart.de/ITWHomepage/Sun/englisch/public/pdfDateien/03-06.pdf.

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. Accessible via digital object identifier http://dx.doi.org/doi:10.1016/j.solener.2003.07.025.

Scholkopf, W., M. Reuss, and C. Oberdorf. 2002. Solar gestutzte Nahwarmeanlagen mit saisonalem Warmespeicher und integrierten Warmepumpen. Proceedings of Energie Innovativ 2002, Kongress, Nurnberg, June 4-5, pp. 37-48.

Zhang, Y. 2003. BCHP System--The hope of summer. In R. Radermacher, Short course: Integration of refrigeration and air-conditioning with distributed generation, University of Maryland. Oral presentation and CD-ROM handout. 21st International Congress of Refrigeration, August 17-22, Washington, DC, USA.

Ziegler, F. 1997. Sorptionswarmepumpen. Professorial dissertation, Forschungsberichte des Deutschen Kalte-und Klimatechnischen Vereins e.V. (DKV) Nr. 57, Stuttgart: Deutscher Kalte-und Klimatechnischer Verein e.V. (DKV).

Ziegler, F., C. Kren, and C. Schweigler. 2000a. Sorptions-Warmepumpen fur Gewerbe und Industrie. Proceedings of Tagungsbericht der Jahrestagung des DKV, Bremen, Stuttgart: Deutscher Kalte-und Klimatechnischer Verein e.V. (DKV).

Ziegler, F., C. Schweigler, M. Hagenauer, and M. Rzepka. 2000b. Kraft-Warme-Kalte-Kopplung: Integrationsmaglichkeiten durch angepasste Sorptionskalteanlagen. Proceedings of Fachtagung Integrierte Energiesysteme, VDI-GET, Bayern Innovativ, Nurnberg, April 4-5, 1539:97-107.

Christoph Kren

Christian Keil

Stefan Plura

Michael Radspieler

Matthias Schicktanz

Christian Schweigler, PhD

Christoph Kren and Matthias Schicktanz are scientists, Christian Keil and Michael Radspieler are engineers, Stefan Plura is a mechanical engineer, and Christian Schweigler is head of the Chillers and Heat Pumps Group at the Bavarian Center for Applied Energy Research, Garching, Germany.

Table 1. Energetic and Economic Assessment in Comparison to a Fossil-
Fuel-Fired Boiler

Fossil fuel    [eta] thermal     [-]
  boiler
               Heating capacity  [kW]
Absorption     COP               [-]
  heat pump
               PER               [-]
               Heating capacity  [kW]
                 (full load)
               Heating capacity  [kW]
                 (average)
               Fuel demand       [kW]
                 (full load)
Hours of       Operating hours   [h/yr]
  operation      (full load)
               Operating hours   [h/yr]
                 (at average
                 capacity)
Heat                             [kWh/yr]
  production
Fuel           Natural gas       [kWh/yr]
  consumption    demand
Investment     Planning          [euro]
  cost
               Central-heating   [euro]
                 boiler
               Absorption heat   [euro]
                 pump
               Housing           [euro]
               Piping and        [euro]
                 peripheral
                 equipment
               Total             [euro]
               Extra cost vs.    [euro]
                 reference
Fuel cost      Gas demand rate   [euro/kW x month]
               Natural gas rate  [euro/kWh]
               Fuel demand cost  [euro/yr]
               Fuel energy cost  [euro/yr]
               Total fuel cost   [euro/yr]
Operating      Total operating   [euro/yr]
  cost           cost
               Annual saving     [euro/yr]
                 vs. reference

Payback (with                    [Years]
  Interest
  rate 0%)
Annuity        Annuity of        [euro/yr]
                 investment
               Energy cost plus  [euro/yr]
                 operating cost
               Total annual      [euro/yr]
                 cost
               Annual saving     [euro/yr]
                 vs. reference
Specific                         [euro/kWh]
  useful heat
  cost
Primary        Fuel consumption  [kWh/yr]
  energy
               C[O.sub.2]        [ton/yr]
                 emission
                                                     Absorption Heat
                                                     Pump
                                                     Gas-Fired
                                 Fossil Fuel Boiler  Single-Effect
                                 Reference System    Absorption Heat
                                 Without Heat Pump   Pump

Fossil fuel    [eta] thermal             0.9
  boiler
               Heating capacity        600
Absorption     COP                       -                   1.65
  heat pump
               PER                                           1.45
               Heating capacity                            600
                 (full load)
               Heating capacity                            500
                 (average)
               Fuel demand             667                 417
                 (full load)
Hours of       Operating hours                2917
  Operation      (full load)
               Operating hours                3500
                 (at average
                 capacity)
Heat                                     1,750,000
  production
Fuel           Natural gas       1,944,444           1,215,278
 consumption     demand
Investment     Planning               5250              28,200
  cost
               Central-heating      39,000

                 boiler
               Absorption heat                         115,000
                 pump
               Housing              23,000              23,000
               Piping and           12,000              50,000
                 peripheral
                 equipment
               Total                79,250             216,200
               Extra cost vs.            -             136,950
                 reference
Fuel cost      Gas demand rate                   0.562
               Natural gas rate                  0.035
               Fuel demand cost       3374                2109
               Fuel energy cost     68,056              42,535
               Total fuel cost      71,430              44,644
Operating      Total operating        1238                2475
  cost           cost
               Annual saving             -              25,549
                 vs. reference
Payback (with                            -                   5.4
  Interest
  rate 0%)
Annuity        Annuity of             8978              24,493
                 investment
               Energy cost plus     72,667              47,119
                 operating cost
               Total annual         81,645              71,611
                 cost
               Annual saving             -              10,034
                 vs. reference
Specific                             0.047                   0.041
  useful heat
  cost
Primary        Fuel consumption  1,944,444           1,215,278
  energy
               C[O.sub.2]              389                 243
                 emission

Table 2. General Data for Energetic and Economic Assessment of Different
System Concepts

Technical Data

Cogeneration plant    [eta] electric          [-]                 0.36
                      [eta] thermal           [-]                 0.54
Fossil fuel boiler    [eta] thermal           [-]                 0.9
Absorption heat pump  COP heat pump           [-]                 1.94
Electrical power      [eta] electricity       [-]                 0.38
  generation
Energy rates          Electrical energy rate  [euro/kWh]          0.0725
                      Gas energy rate         [euro/kWh]          0.0305
                      Gas demand rate         [euro/kW x month]   0.5624
Economic data         Interest rate           [%]                 7.5
                      Model period            [Years]            15
                      Annuity factor          -                   0.11

Table 3. Energetic and Economic Assessment of Different System Concepts

                                                        Fossil Boiler +
                                                        Electrical
                                                        Grid
                                                        System
                                                        Without Heat
                                                        Pump

Fossil fuel        Heating capacity        [kW]              2387
  boiler
Cogeneration       Gas input               [kW]                 -
  plant
                   Directly used heating   [kW]                 -
                     capacity
Absorption heat    Heating capacity        [kW]                 -
  pump
Hours of           Cogeneration plant +    [h/a]                -
  operation          heat pump
                   Only fossil boiler      [h/a]             2795
Heat production    Absorption heat pump    [kWh/yr]             -
                   Cogeneration plant      [kWh/yr]             -
                   Fossil boiler           [kWh/yr]     6,671,725
                   Total                   [kWh/yr]
Electricity        Cogeneration plant      [kWh/yr]             -
  production
Electricity        Active trade balance    [kWh/yr]      -149,379
  balance
External supply    Gas supply for          [kWh/yr]             0
                     cogeneration plant
                   Gas supply for fossil   [kWh/yr]     7,413,028
                     boiler
                   Electrical energy       [kWh/yr]     1,878,069
                     supply
Investment cost    Total                   [euro]         161,145
                   Extra cost vs.          [euro]               -
                     reference
Energy cost: feul  Total                   [euro/yr]      419,304
  electricity
Operating cost     Total                   [euro/yr]         1611
                   Annual saving vs.       [euro/yr]            -
                     reference
Payback (with                              [Years]              -
  interest
  rate 0%)
Annuity            Annuity of investment   [euro/yr]       18,256
                   Energy cost +           [euro/yr]      420,915
                     operating cost
                   Total annual cost       [euro/yr]      439,171
                   Annual saving vs.       [euro/yr]            -
                     reference
Specific useful                            [euro/kWh]           0.045
  heat cost
Primary energy     Fuel for cogeneration   [kWh/yr]             0
                   Fuel for fossil boiler  [kWh/yr]     7,413,028
                   Fuel for power          [kWh/yr]     4,942,287
                     generation (grid
                     electricity)
                   Total                   [kWh/yr]    12,355,315

                                                           Cogeneration
                                                           Plant +
                                                           Absorption
                                                           Heat Pump
                                           Cogeneration    Double-
                                           Plant           Effect/
                                           System          Single-Effect
                                           Without Heat    Absorption
                                           Pump            Heat Pump

Fossil fuel        Heating capacity              1927           1495
  boiler
Cogeneration       Gas input                      844            844
  plant
                   Directly used heating          460              0
                     capacity
Absorption heat    Heating capacity                 -            892
  pump
Hours of           Cogeneration plant +          8073           6474
  operation          heat pump
                   Only fossil boiler            1535            598
Heat production    Absorption heat pump             -      5,777,597
                   Cogeneration plant       3,713,626              -
                   Fossil boiler            2,958,099        894,127
                   Total                    6,671,725
Electricity        Cogeneration plant       2,421,930      1,942,268
  production
Electricity        Active trade balance     2,259,759      1,728,690
  balance
External supply    Gas supply for           6,817,285      5,467,123
                     cogeneration plant
                   Gas supply for fossil    3,286,777        993,475
                     boiler
                   Electrical energy         -531,069              0
                     supply
Investment cost    Total                      403,095        613,908
                   Extra cost vs.             241,950        452,763
                     reference
Energy cost: feul  Total                      306,677        218,442
  electricity
Operating cost     Total                       26,150         25,481
                   Annual saving vs.           88,088        176,991
                     reference
Payback (with                                       2.7            2.6
  Interest
  rate 0%)
Annuity            Annuity of investment       45,666         69,548
                   Energy cost +              332,827        243,924
                     operating cost
                   Total annual cost          378,493        313,472
                   Annual saving vs.           60,678        125,699
                     reference
Specific useful                                     0.032          0.028
  heat cost
Primary energy     Fuel for cogeneration    6,817,285      5,467,123
                   Fuel for fossil boiler   3,286,777        993,475
                   Fuel for power          -1,397,550              0
                     generation (grid
                     electricity)
                   Total                    8,706,512      6,460,598