Category Archives: EuroSun2008-10

Year round performance

Fig.6 shows the year-round performance of the solar DHW system. In Fig.6 the collector efficiency, the DHW heat load, the solar contribution, the CO2 emission and energy cost are compared. The performance in Fig.6 is expresses for a housing unit. The CO2 emission was calculated from the gas calorific value of 45MJ/m3 and the CO2 emission coefficient of 2.21kg/m3. The energy cost was calculated using the fee structure of the city gas company in Tokyo. The average unit price of the gas is 149JPY/m3. The collector efficiency for the collector area of 20m2 with Case A DHW supply is from 53.0% to 63.4%. It tends to increase according to the storage tank volume is large. The collector efficiency decreases by 3.6%-7.2%, when the collector area increases by 10m2. The considerable difference in the collector efficiency is found when increasing from 0.5m3 to 1.5m3 of the storage tank volume, however there is only few difference when the storage tank volume is more than 2.0m3. The collector efficiency for the collector area of 30m2 and the storage tank volume of 1.0m3 is 53.0% with Case A DHW supply, 49.5% with Case B DHW supply, and 41.6% with Case C DHW supply, respectively. The CO2 emission for collector area of 30m2 and the


Fig.6 Year-round performance from the simulation results expressed for a housing unit.


storage tank volume of 1.0m3 is 650kg-CO2/year with Case A DHW supply, 300kg-CO2/year with


Case B DHW supply, and 67kg-CO2/year with Case C DHW supply, respectively, for a housing unit.

Relationships of the collector area and the storage tank volume for the solar contribution is shown in Fig.7. The solar contribution increase by 4%-26% when collector area increases by 10m2 in all simulation cases. In addition, the solar contribution increases with larger collector area and larger storage tank volume. The solar contribution only increase


by 2.2% when the storage tank volume is increased from 1.0m3 to 1.5m3 in case of the collector area of 20m2 and Case A DHW supply. However, the solar contribution only increase by 6.7% when the storage tank volume is increased from 1.0m3 to 1.5m3 in case of the collector area of 50m2 and Case A DHW supply.


The comparison of the saved cost of the city gas for a housing unit was shown in Fig.8. The dark bars in Fig.8 show the suitable combinations of the collector area and the storage tank volume judged from the solar contribution. The saved energy cost for the collector area of 30m2 and the storage tank volume of 1.0m3 with Case A DHW supply is estimated to be 19,700JPY/year for a housing unit. The comparison of the installation cost for 10 housing unit is shown in Fig.9. The installation cost for the collector area of 30m2 and the storage tank volume of 1.0m3 with Case A DHW supply should be suppressed 2.0 million JPY when the pay back period of 10 years is assumed. The installation cost should be suppressed 3.0 million JPY when the pay back period of 15 years is assumed. The saved energy cost for the collector area of 20m2 and the storage tank volume of 0.5m3 with Case A DHW is estimated to be 80,000JPY/year, and the installation costs are 8.0 million JPY and 1.2 million JPY for pay back period of 10 and 15 years, respectively.

Fig.9 The installation cost for 10 housing units assuming the pay back periods of 10 and 15 years.

6. Conclusion

The central type of solar DHW supply system for 10 housing units was simulated to examine the relationships of the collector area and the storage tank volume with considering the DHW supply rate. The simulation results showed the followings.

1) The collector efficiency for the collector area of 30m2 varies from 38% to 59% depending on the storage tank volume and the DHW supply rate. The collector efficiency decrease by 3.6%-7.2%, when the collector area increases by 10m2.

2) The suitable storage tank volume is from 1.0m3 to 1.5m3, since the collector efficiency decreases extremely when the storage tank volume is 0.5m3. The efficiency for 2.0m3 is almost same as the case for 1.5m3.

3) The solar contribution for the collector area of 30m2 and the storage tank volume of 1.0m3 is 37.1% with Case A DHW supply, 52.5% with Case B DHW supply, and 74.6% with Case C DHW supply, respectively.

4) The boiler load for the collector area of 30m2 and the storage tank volume of 1.0m3 is 105.9GJ/year with Case A DHW supply, 48.6GJ/year with Case B DHW supply, and 10.8GJ/year with Case C DHW supply, respectively.

5) The installation cost for the collector area of 30m2 and the storage tank volume of 1.0m3 should be suppressed 2.0 million JPY with Case A DHW supply, 1.7 million JPY with Case B DHW supply and 1.0 million JPY with Case C when the pay back period of 10 years. The installation cost for the collector area of 30m2 and the storage tank volume of 1.0m3 should be suppressed 3.0 million JPY with Case A DHW supply, 2.5 million JPY with Case B DHW supply and 1.5 million JPY with Case C when the pay back period of 15 years.


[1] T. Kusunoki and M. Udagawa, Planning of Solar Collector Arrangement for Solar DHW Heating Apartment House, Proceedings of JSES/JWEA Joint Conference 2006, pp. 125-128. (In Japanese)

[2] H. Roh and M. Udagawa, Study on Standardization of Solar DHW Heating System for Apartment Houses, Proceedings of JSES/JWEA Joint Conference 2005, pp.79-82. (In Japanese)

[3] M. Udagawa, H. Roh and M. Satoh, Design of Solar DHW System for Apartment Houses, Proceedings of ISES Solar World Congress 2005

[4] M. Udagawa, and M. Satoh, Energy Simulation of Residential Houses Using EESLISM, Proceedings of Building Simulation ‘99, pp.91-98. (In Japanese)

[5] Architectural Institute of Japan, Expended AMeDAS Weather Data, 2005. (In Japanese)

[6] S. Kaneko, M. Udagawa and T. Kusunoki, Design of Solar DHW Heating System for Small Apartment House, Proceedings of JSES/JWEA Joint Conference 2007, pp.213-216. (In Japanese)

Principles of action

The aims of the work are as follows:

— to develop a simple self-acting device that will transfer heat downward up to a depth above 10 m. The temperature difference between the warm and cold branches would be only a few degrees, and

— to test the performance of the device under laboratory conditions and when integrated with a solar installation.

The proposed device operates cyclically, is self-controlled, has a simple design and has no

mechanical moving elements except flaps of check valves.

The principles of action are shown in Fig. 1. The cycle of action include two stages [2], [3], [4]:

— heating the upper part of warm branch and pushing warm liquid downward. It will cause rising of level of liquid in the cold branch (see Fig. 1a),

Подпись: a b Fig. 1. Scheme and stages of action of the device: 1 - warm descending pipe, 6 -control valve (liquid seal), 2 -cold up-flow pipe, 7 - upper way of the circuit, 3 - intermediate canal, 8 - tank-accumulator, 4 - check valve, a - stage of pushing warm liquid downward, 5 -source of heat, b - stage of pouring off cold liquid to warm branch.

opening the upper path in the flow circuit, equalising vapour pressure in both branches, pouring off the surplus of cold liquid through the intermediate canal to upper part of warm branch gravity (see Fig. 1b).

The upper parts of each branch have to be made wider. It will cause the pumping of a greater portion of liquid during each cycle. The directions of liquid flow in the main part of the circuit and intermediate canal are maintained by placing non-return valves. A special control valve has to be used for periodical opening and closing of the upper path of the flow circuit.

Standard vs. weather forecast HWSH

In a standard system the auxiliary electric heater run identically every day, no matter the season. During night, it brings the upper volume of the tank at the set-up temperature (here 70°C). This one is usually preset. Solar energy is recovered in the lower part of the tank, and, if the temperature rises above the set-up temperature in the upper volume, than the solar loop will heat up the entire tank. Due to a large variation of the solar radiation between winter and summer, the heat recuperation varies also. During winter the heat input of the solar loop is very poor and the tap hot water is produced mainly by the auxiliary heater. Contrary, during summer, the solar contribution to the tap hot water is important, even complete. Thus, the tank can contain an important thermal energy at a sufficiently high temperature to provide tap hot water without running the auxiliary electric heater.

Introducing a control device able to take into account the weather forecast for the next day, we try to diminish the electricity consumption during warm seasons keeping the customer’s comfort at a good level. For this, we are able to control the set-up temperature of the auxiliary heater depending on the solar radiation of the next day. We decided to introduce three temperature levels:

• If the maximum solar radiation of the next day is above 800 W/m2 the set-up temperature is Tsetup = 50°C, representing mostly the summer operation;

• If the maximum solar radiation of the next day is between 450 W/m2 and 800 W/m2 the set-up temperature is raised at Tsetup = 60°C, usually during intermediary seasons;

• And finally, for solar radiation lower than 450 W/m2 the set-up temperature is pushed up to the standard temperature applied during winter Tsetup = 70°C.

Next graphic presents the comparison between the standard system and the HWSH with weather forecast. The temperature variation at 10 levels[12] in the tank and the operation of the electric heater are shown for both cases. The tap hot water pouring, solar radiation and pump operation are alike for both cases.


Figure 3. Comparison between standard system and HWSH with weather forecast.

One can observe that on April 1st the electrical heater change its behaviour due to the lower set-up temperature. This small electric economy is multiplied by an important number of days leading to a considerably annual economy. This one is presented in the next table computed for Paris and Grenoble.

Table 2. Comparison between standard system and HWSH with weather forecast in different locations.















T (%)







COP (-)







Eelec (kWh/y)









Esolar (kWh/y)







Qloss (kWh/y)







One notice the annual electricity economy given by the weather forecast control of around 60 kWh in both cases, meaning 7% of the total electric energy consumption.

5. Conclusions and perspectives

The numerical model presented in this paper is able to simulate a HWSH with an advanced control system taking into account the weather forecast for the next day. Using a simple modification of the electrical heater algorithm we are able to cut down 7% of the annual electrical consumption and to increase with 3% the solar cover coefficient.

The electricity gain can be optimised by a fine setting of the two solar radiation limits used to determine the set-up temperature of the heater, depending on the system location and tap hot water pouring.


[1] F. A. Peuser, K-H. Remmers, M. Schnauss, (2003) Installations solaires thermiques. Conception et mis en oeuvre, Systemes Solaires, Paris.

[2] T. Prud’homme, D. Gillet, Energy and buildings, 33 (2001) 463-475.

[3] M. LeBreux, M. Lacroix, G. Lachiver, Energy and buildings, 38 (2006) 1149-1155.

Experimental Evaluation of an Indirect Solar Assisted. Heat Pump System for Domestic Water Heating

A. Bridgeman* and S. J. Harrison

Solar Calorimetry Laboratory, Queen’s University, Department of Mechanical and Materials Engineering,

130 Stuart Street, Kingston, ON, K7L 3N6, Canada
Corresponding Author, Bridgeman@me. queensu. ca


An indirect solar assisted heat pump (ISAHP) system for heating domestic hot water has shown promise as an alternative to conventional electric or natural gas water heaters. In a previously conducted theoretical study, it was concluded that an ISAHP could operate with a lower life-cycle cost than a conventional solar domestic hot water (SDHW) system. Therefore, to further investigate the feasibility of the proposed system, an experimental study was conducted on a prototype (ISAHP) system. To undertake the study, a fully instrumented heat pump water heater was assembled in a laboratory environment and connected to a simulated “solar heat” input. The “solar” input was provided by an electrically heated circulation loop that delivered temperature-controlled fluid to the heat pump evaporator. This allowed repeatable test sequences to be performed in the laboratory regardless of weather conditions. A simulated solar profile ranging from 750 — 1500 W was delivered to the heater throughout the test. The corresponding fluid temperature ranged from 15 — 37°C, and the results indicated coefficient of performance (COP) values ranging from 2.4 to 3.2. These results, while in close agreement, are approximately 12% lower than those predicted from previous theoretical values.

Keywords: Solar assisted heat pumps, Heat pumps, Domestic water heating, Canada

1. Introduction

In Canada, water heating is the second most energy intensive end use in the residential sector, accounting for 22% of the consumed energy [1]. Due to growing concern for depleting fuel supplies, higher fuel prices and greenhouse gas emissions, alternatives to the conventional water heating methods such as electric and natural gas water heaters are being investigated. Two systems currently receiving considerable attention worldwide are Solar Domestic Water Heaters (SDWHs) and heat pump systems that source energy from the ambient air, or geothermal energy.

While each of these systems may operate with lower energy consumption than a typical electric water heater, both systems have performance limitations. Air-source heat pump water heaters are attractive in temperate regions, but lack popularity in Canada due to the warm temperatures needed for their proper function [2]. Geothermal heat pumps demonstrate improved performance over air source heat pumps because the heat is drawn from the earth, which is much warmer than ambient in the winter. However, due to the ground loops necessary for these types of heat pumps to function, property alterations and high initial costs have made them less practical for existing homes [2]. Solar Domestic Water Heaters have been increasing in popularity in Canada, and can decrease the energy consumption of an electric water heater by up to 90% in the summer [3], but large temperature differences between the collector and ambient air during the winter months lower the collector efficiency significantly, limiting the seasonal performance.

A combined system, known as a Solar Assisted Heat Pump (SAHP) could be used to alleviate many of the disadvantages of either system operating independently. The advantage to the heat pump cycle, by coupling it with a solar thermal collector, is an increase in evaporator temperature over either air-source or ground-source heat pumps. This increase in temperature results in an improved heat pump coefficient of performance (COP). From the solar collector point of view, the use of the heat pump lowers the fluid temperature returning to the collector near or below ambient. This lower temperature increases the collector efficiency, and allows for substantial heat gains with low cost unglazed solar absorber panels, even under marginal conditions [4, 5, 6]. The combined system allows for efficient operation over a wider range of seasons and weather conditions, and for more hours throughout the day.

The concept of a SAHP dates back to 1955 when it was first proposed by Sporn and Ambrose [7]. Numerous studies took place in the 1980s and early 90s examining the feasibility of SAHP systems for either space or water heating. Most of these systems were classified as Direct Expansion Solar Assisted Heat Pumps (DX-SAHP), in which the refrigerant would flow through the solar collector directly, which doubled as the evaporator for the heat pump. Chaturvedi [5, 8] found that collector efficiencies between 40 — 70% were feasible with bare collectors operating under ambient condition in winter, and found heat pump COPs ranging from 2 — 3, which was 30 — 50% higher than air source heat pumps. In the mid 90s Morrison [6] stated that the majority of previous systems proposed had not achieved commercial success due to the complexity of the combination of heat pump and solar collector components, and high installation costs due to the need for plumbing, electrical and refrigeration connections during installation. He then proposed an integral design, in which the collector and heat pump unit were incorporated as an integral part of the storage tank, which must be installed outside. Morrison found only a slight decrease in performance compared to a typical system in Sydney, Australia, but predicted a substantial reduction in cost, and simplification of installation. Huang and Chyng [9, 10] have recently investigated similar integral DX-SAHP systems in Taiwan. They found COPs reaching up to 3.83 during a long term performance test, in which the system was run for 13,000 hours continuously.

Although the integral DX-SAHP overcame installation complexities of SAHP systems and achieved commercial success in some parts of the world, installing the water storage tank outdoors introduces another problem in the Canadian environment. The cold conditions in the winter months increase the heat loss from the tank decreasing the system’s performance. To avoid this problem, an Indirect Solar Assisted Heat Pump (ISAHP) is under investigation at the Queen’s Solar Laboratory in Ontario, Canada. A schematic of an ISAHP is shown in Figure 1. This system differs from a direct solar assisted heat pump in that the heat pump collects energy via a heat exchanger connected to the collector anti-freeze loop, rather than flowing through the collector itself. This eliminates the need for long refrigeration lines and costly refrigeration fittings on the collector, but allows for the heat pump unit and storage tank to be installed inside the residence. Another feature of this system is the external side-arm natural convection heat exchanger, which acts as the heat pump’s condenser. As the heat exchanger transfers energy from the refrigerant to the potable water, the water increases in temperature causing its density to decrease. This induces buoyancy driven natural convection, circulating the water through the heat exchanger and eliminating the need for a pump. Due to the relatively low flow of the natural convection loop, this configuration has the potential for increasing thermal stratification in the storage tank. The benefit of stratification is that it delivers cool fluid from the bottom of the tank to the heat pump condenser, while maintaining hot water at the top of the storage for distribution to the load. This improves the overall system performance [11].

Conception of the MaxLean System

Подпись:The heating system investigated in this work was designed as a simple yet competitive solar combisystem. The scope was to generate a system concept that can be easily integrated into the heating system of an existing house with water-based central heating and which, involving some further steps, is suitable for thorough cost reduction while maintaining a good thermal energetic performance. As such, drainback technology in combination with a pressure-less storage tank is foreseen, whereby the latter would possibly be made of plastics instead of steel, leading to a reduction of both material and production costs. The solar collectors are driven with water taken directly from the tank that also serves as a drainback vessel (see Fig. 1). The various peripheral components are connected to the storage tank by independent hydraulic circuits without any heat exchangers built into the store. The auxiliary heater loop is connected directly to the tank. All components included in this theoretical approach are readily available on the market, however none of the existing system concepts includes all of the features mentioned. A further conceptual modification has been developed regarding the

flat-plate collector circuit with stratifier, DHW plate

space heating system. Here a thermostatic heat exchanger, flow rate controlled heating loop

mixing valve is applied with a fixed set with stratifier.

temperature (50°C) such that a variation of the

volume flow rate (through the radiators or the heating floor) modulates the space heating power instead of the more commonly used flow temperature modulation with a mixing valve and a variable temperature set according to a heating curve. A comprehensive description of the system concept and the advantages of the improvements can be found in [1].

Solar thermal systems combined with heat pumps — investigation of different combisystem concepts

S. Bachmann, H. Drtick, H. Mtiller-Steinhagen

University of Stuttgart, Institute for Thermodynamics and Thermal Engineering (ITW) Pfaffenwaldring 6, 70550 Stuttgart, Germany Tel.: +49 711 / 685-63553, Fax: +49 711 / 685-63503

Corresponding Author, email: bachmann@itw. uni-stuttgart. de


Two solar combisystem concepts have been investigated where a compression heat pump is used as the only auxiliary heater. The investigations have shown that the combination of a solar combisystem with a heat pump can result in a promising approach for saving primary energy. This especially holds true compared with a monovalent heat pump system (without a solar thermal system). However, the investigated systems show a great difference concerning the electric energy consumption of the heat pump. Especially systems where the brine is solar preheated can offer a great reduction of electric energy consumption for the heat pump.

Keywords: compression heat pump, solar combisystem, seasonal performance factor, component testing

1. Introduction

Heat pumps are presently enjoying a wide popularity in Europe. In 2007, the market for heat pumps showed a growth rate of about 100% in Germany. In recent years heat pumps were predominantly used as monovalent systems or in combination with a fossil fuel fired auxiliary heating system. At present, however, a number of new heating systems are being launched on the market where the heat pump is used as the only auxiliary heater for a solar combisystem. In addition to biomass heating systems, heat pumps offer the possibility of supporting the solar thermal system by, at least partly, renewable energy. Depending on the individual system design, there is a particular charm in combining heat pumps with solar thermal systems because this combination can provide an additional heat sink with a low temperature level so that the solar collector energy yield can be increased. For the heat pump on the other hand this approach offers a heat source with a relatively high temperature level which is beneficial for the coefficient of performance of the heat pump.

At the Research and Testing Centre for Thermal Solar Systems (TZS) in the Institute for Thermodynamics and Thermal Engineering (ITW), University of Stuttgart, two solar combisystems combined with a compression heat pump were investigated based on the component testing approach as described in the European Standard ENV 12977-2. One system consists of a compression heat pump where the condenser is located in the auxiliary section of the combistore. The other system uses an external compression heat pump in combination with an additional water store as heat source for the heat pump, which is exclusively charged by the solar collectors.

This paper presents the different system concepts. The results of the investigations such as the electric energy consumption of the heat pumps required to cover the total heat demand of the building, will be presented and discussed.

2. The investigated systems

Experimental testing

The schematic diagram of the laboratory installation and its photo are shown in Figs. 2 and 3. The warm and cold work vessels are made of cylinders and mounted one above another. The diameter and the height of the cylinders are 110 mm and 140 mm, respectively. Both cylinders are transparent and made of Plexiglas for visual observation. An electrical resistance heater with a capacity of 300 W was used as a source of heat. The heater had a temperature control for maintaining a few set-point temperatures. The control valve was made as a liquid seal. The heat exchanger (cooler) used for cooling the heat-carrier was located one metre below. The heat exchanger was made of a copper coil with a surface area of 0.15 m2 and placed into the cylinder vessel with a capacity of 7 litres filled with water. The vessel of the cooler was filled with water that was not running, but only accumulating heat leading to a temperature rise during the experiment.

The recorded data consisted of measurement of pressure inside the cold vessel and temperature measurements: at the top and the bottom of the inside warm vessel, at the descending pipe, at the


up-flow pipe and at the vessel of cooler. Processes in the model are variable. It was decided to take measurements every 10 s since the duration of cycles was foreseen about 1 min and more.

Подпись:diagram of the passive heat transfer downward model:

4 — electrical heater, 7 — warm work vessel

5 — intermediate canal, 8 — cold work vessel,

6 — control valve (liquid seal), t — points of temperature


Experiments were done at a temperature of heat source of 33, 41, 49, 57 and 60°C.

Подпись: Q mc C (/desc.pipe image168

The amount of heat carried by liquid heat-carrier from warm vessel during one cycle was calculated by using average temperature difference, tdescpipe measured at lower pipe and average minimum value of temperature, tup_flow pipe measured at up-flow pipe:

Measurements on a New Developed Compact. Solar Combisystem in Practice

A. ТЬйг and S. Furbo2

1 AEE INTEC, Feldgasse 19, 8200 Gleisdorf, Austria
2 Department of Civil Engineering, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark

* Corresponding Author, a. thuer@aee. at


Based on elaborated knowledge in international research projects within IEA-SHC Task 26 and the ALTENER project “Solar Combisystems”, a new solar combisystem concept was developed. Therefore the focus was concentrated on minimizing the temperature in the system with the goal to reduce system heat losses and to increase the efficiency of the condensing natural gas boiler and the solar collector. After development and test of the first prototype in the laboratory, a demonstration system was built which replaced an old conventional natural gas heating system in a one-family house. Measurements in practice showed how this new natural gas — solar heating concept performs in comparison with the old one. A Solar gain of 370 kWh and energy savings of 536 kWh per m2 collector area were achieved for the new solar combisystem. Domestic hot water consumption reduced by 20 % and hot water circulation losses of only 7% of hot water consumption was achieved. The average space heating temperature difference could be increased by 50% leading to lowest possible return temperatures. In spite of more installed pumps and valves the electricity consumption of the heating system could be decreased slightly.

Keywords: solar combisystem, energy savings, measurements

1. Introduction

The project REBUS — „Competitive solar heating systems for residential buildings” was carried out from 2003 to 2006 with the goal to develop new concepts for solar combisystems in co­operation with industry partners [1]. Prototypes were first built and tested in the laboratory and further on installed in one family demonstration houses for in-situ long time testing and measurements. According to national boundary conditions PhD projects in Sweden and Latvia worked on solar combisystems in combination with pellet boiler and in Norway and Denmark solar combisystems in combination with natural gas boiler were investigated.

This paper summarizes the final results of the Danish PhD project where a new compact solar combisystem concept with direct integration of a condensing natural gas boiler was developed, tested as a laboratory prototype and finally installed as a second generation prototype in a demonstration one family house [2, 3, 4, 5].

2. Demonstration house

The demonstration one family house with 3 occupants is situated in a small city about 40 km north of Copenhagen/Denmark. The old heating system was a non-condensing natural gas boiler (construction year 1990) with 22 kW nominal power in combination with a 50 Liter domestic hot water tank. The heating system consists of old cast iron radiators with thermostatic valves. This old


Подпись: Fig. 1. Left: Demonstration house with collectors on the roof; Middle: installed solar combisystem in the basement; right: prefabricated technical unit before installation

heating system was monitored in the period August 2004 to April 2006. In spring 2006 the new solar combisystem was installed: collector area of 6.75 m2 and 360 litre solar tank in combination with a condensing natural gas boiler (Milton Smart Line HR24). Beside the installation of the new solar heating system also two additional rooms were prepared to be used in the basement: one living room and a bath room, both equipped with floor heating. Monitoring of this new system started in October 2006.

. Previous Studies

Two previous studies have been undertaken investigating ISAHP systems. The first study involved developing a model in TRNSYS [12], a transient simulation program, to investigate the feasibility of the system, performing both a performance and cost analysis. The second study involved building a prototype of the system, and performing a range of constant temperature input tests, comparing the results with the simulated results. A brief description of each previous study is provided below.

1.1. Numerical Analysis

The first study was conducted in the Solar Calorimetry Lab by Freeman [3] and simulated the performance of indirect solar assisted heat pump using TRNSYS. The program used mostly component models developed with the TRNSYS software, but models were created for the heat pump, the natural convection heat exchanger, and the heat pump controller. A detailed description of the TRNSYS model, and a theoretical analysis and derivation of the steady-state vapour compression heat pump model is given by Freeman [3], and is briefly summarized in a previous paper by the authors [13].

Results of these previous studies predicted higher seasonal solar fractions than conventional Solar Domestic Hot Water (SDHW) systems. It was concluded that the ISAHP gathered more energy from the environment during marginal weather conditions, as well as during the winter when compared to either an SDHW or air-to-water heat pump systems. The study also found that the life cycle cost of the ISAHP system showed up to 29% savings over the SDHW system for major cities across Canada.

Dimensioning for the best cost benefit ratio

The conventional dimensioning of a solar heating system for one — or multi-family houses is commonly based on a trade-off between the solar fraction and the level of utilisation [2], whereas the customer and user might rather be interested in elements like additional cost and primary energy savings. To meet these requirements a different dimensioning directive was introduced [1][3]. The simulation program TRNSYS was coupled with the optimisation program GenOpt for the dimensioning in response to this directive. A short description and the resulting optimal dimensions of the MaxLean system concept are given in the following and serve as basis for the subsequent sensitivity analysis.

3.1 Dimensioning directive

The objective function minimised by the optimisation algorithm is the cost/benefit ratio described in Eq. (1):

a*I0 + BMaxLean Bref

a annuity factor

I0 total investment costs of the solar thermal system

BMaxLean annual operation costs of the MaxLean system concept (including the heating circuit)

Bref annual operation costs of the conventional reference system

Eprim, sav primary energy savings

As long as a solar heating system is not economically rewarding, the primary energy saving is the preferential merit on which value is laid. The additional cost for these primary energy savings consists of the additional investment cost minus the difference in operation costs between the conventional and the solar assisted heating system. The investment costs of a solar heating system are calculated by Eq. (2), (cf. [2]):

A ( V 6536

I0 = 2559€ + *368€ + 3983€I I Eq 2

m2 t m3 I 4′

Acoll flat-plate collector area

Vstor storage device capacity

The annual primary energy savings are calculated from the difference between the energy consumption of the solar and non-solar heating system, as well as the embodied energy of the solar system. (Thus, the embodied energy of the non-solar heating system —i. e. a water heater store— is not accounted for). The gas and electricity consumption is converted to primary energy consumptions. To obtain annual values of the embodied energy the total amount is divided by the service lifetime.