SOLAR COMBISYSTEMS IN DENMARK - THE MOST COMMON SYSTEM DESIGNS Klaus Ellehauge SolEnergi Center Danmark, Danish Technological Institute, Energy, Teknologiparken, DK-8 Aarhus C, Tel.: +45 722 137, Fax: +45 722 119, E-mail: klaus.ellehauge@teknologisk.dk Louise Jivan Shah Department of Buildings and Energy, Technical University of Denmark DK-28 Kgs. Lyngby, Denmark Phone +45 45 1888, Fax +45 45 93443, Email: ljs@ibe.dtu.dk Abstract About 33-5% of all solar heating systems in Denmark are combined systems, which supply heat for domestic hot water as well as for space heating. Especially one design is the most popular and it is used for more than 9% of all combisystems in Denmark and can be delivered by nearly all (1-13) Danish manufacturers. The system is characterised by using a normal solar hot water tank for domestic hot water, while the solar heat for space heating is supplied directly from the solar collector loop to the central heating loop via a heat exchanger. In this way, solar heat for space heating is not stored and can only be supplied when the collector loop is in operation. The system has the advantage of being inexpensive, but may have problematic performances that are very dependent on the thermal performance of the house and on the users. Another design is an enlarged hot water tank with an extra heat exchanger for withdrawal of heat for the central heating loop. This design and the first mentioned are the Danish designs, which are being evaluated in IEA task 26 Solar Combisystems. Other designs are systems with drain-back and domestic hot water production via the central heating loop, and tank in tank designs, which are experienced especially in combination with biomass fired boilers. This paper gives an overview of the most common solar combisystems in Denmark. Furthermore, the two above-mentioned systems are studied in detail. 1. INTRODUCTION In Denmark, solar heating systems for combined space heating and hot water heating are becoming more and more ordinary compared to solar heating systems only for domestic hot water. Some years ago, a specification showed that app. 1/3 of all installed systems were combined systems. A lot of work has previously not been carried out in Denmark concerning the evaluation and development of combined systems. Recently however, a project stating the principles of combined systems in Denmark has been finished. In addition, Denmark is participating in IEA task 26, concerning solar combisystems. The Danish Energy Agency financed the finalised project and finances the participation in the IEA task. 1 SYSTEM DESIGNS Most of the combined systems that are installed in Denmark correspond to the system illustrated in Figure 1 (Ellehauge K et al). The control operates the three-way valve in the solar collector circuit so solar heat is supplied either to the storage tank or to the heat exchanger between the collector loop and the space heating loop. Figure 1: Combisystem 1: Heat Exchanger between Collector Loop and Space Heating Loop. There are different variations of the control. However they all have a sensor in the solar collector, a sensor at the bottom of the tank and a sensor in the space heating loop before the heat exchanger. The control principle is usually that solar heating is supplied to the tank till the tank has reached a temperature that e.g. can be fixed at 5 C. When that temperature has been reached heat is supplied to the heat exchanger. Further, other system designs have started to appear. For instance, a solar heating company installs combisystems as shown in Figure 2.
A Stoker T1H T3H M3 16 l M T1L M1 T3L T4H 5 l M2 T2H M S DHW T4L M4 ENERGY SUPPLY TRANSFER, STORAGE, CONTROL AND DISTRIBUTION LOAD Figure 2: Combisystem 2: DHW Tank as Space Heating Storage Device. In combination with a solid fuel furnace a number of systems with tank in tank storage have appeared in the market and they are often installed together with an additonal storage in parallel as shown in Figure 3. M1 T1H ENERGY SUPPLY TRANSFER, STORAGE, CONTROL AND DISTRIBUTION LOAD Figure 4: Combisystem 4: Drain Back Storage in Existing Space Heating Loop. 2 EVALUATION OF COMBISYSTEM 1: HEAT EXCHANGER BETWEEN COLLECTOR LOOP AND SPACE HEATING LOOP T1L T4H M4 T4L M3 T3H T3L 75 l 1 l T2H M2 Characteristic of system 1 is that it only can supply solar heating for space heating when the collector loop is in operation (that is, when the sun is shining). That means that the system itself cannot store the solar heat that is used for space heating. A frequently appearing space heating demand on days when the sun is shining could be similar to figure 5. There is only a limited or no space heating demand during the day time when the sun is shining through the windows and when a solar collector performance can be obtained. ENERGY SUPPLY TRANSFER, STORAGE, CONTROL AND DISTRIBUTION LOAD Figure 3: Combisystem 3: "Tank in Tank". Another company has developed a system as shown in Figure 4. Space heating demand Solar radiation System 1 and system 2 are Danish systems that will be evaluated in detail in IEA task 26 Combisystems, and which are described in more details in this paper. 1 3 5 7 9 11 13 17 19 21 23 Hour Figure 5: Space heating demand and solar irradiation on a typical Danish April day. In order to evaluate the system more closely simulation calculations have been carried out on a system connected to a single-family house with a total energy consumption of app. 17, kwh/year of which 2,7 kwh are for domestic hot water. There is a little space heating demand in the cool periods during summer. The simulations have been carried out with the simulation programme EMGP3 (Dutré, W L. (1991)), but will be redone with TRNSYS (Klein S.A et al. (1996)).
TANK DESIGN Hot water volume.3 m³ Store heat loss coefficient Height Diameter Coil heat transfer capacity rate 2.4 W/K 1.22 m.56 m 5 W/K/m² collector COLLECTOR Area 5/12/18 m² Start efficiency.77- Heat loss coefficient 5.1 W/m²K 2 nd degree of heat loss coefficient.22 W/m²K² - Tilt 45 Orientation South HEAT EXCHANGER BETWEEN COLLECTOR AND SPACE HEATING LOOP Heat transfer capacity rate Table 1: System data Combisystem 1. 5 W/K/m² collector In Figure 6 the performance of the combined system has been compared with the performance of a plant only for domestic hot water. In connection with the calculations it has been taken into account that the furnace is in operation all year round. As it appears from the figure the performance of such a system is not greater than the performance of a system solely for domestic hot water. Yearly solar performance (kwh/year) 25 2 1 5 Combisystem 1 and DHW-system 5 12 18 Solar collector area (m²) DHW-System System design A thermal mass in a concrete floor with floor heating. The storage capacity is in this case not utilised. In order to examine the performance of the system when it is allowed to operate with varying temperatures in the house a control system has been constructed (in the simulations) that can switch the furnace off when the temperature in the house is exactly above 2 C. By placing the radiator thermostats at more than 2 C it is still possible to supply solar heat to the house when the furnace is not in operation. That means that the house has an excess temperature by means of solar heating and the furnace only starts operating when the desired minimum temperature no longer can be maintained. When calculating the various possibilities a system with a solar collector of 12 m 2 was used although the system normally is installed with a smaller solar collector area. The storage tank is 3 liter. The calculation results appear from figure 7. Yearly performance (kwh/year Combisystem 1: Improvement of performance 4 35 3 25 2 1 5 BV A B C D Simulation (see text) Figure 7:Combisystem 1: Improvement of Performance. The various calculations are described below: BV the result of calculations carried out on a system supplying heat for domestic hot water. Figure 6: Performance of Combisystem 1 and DHWsystem. However, the utilisation of sun for space heating could be larger if solar heating for space heating could be stored in the thermal mass of the house, so the house would become a bit warmer when solar heat was supplied and so this heat could be utilised later on. With ordinary control this strategy is only possible in the periods when the furnace is not in operation and if the radiator thermostats are turned to a rather higher point than the desired temperature. When the furnace is in operation the space heating loop will supply exactly the amount of heat that is required whether there is solar heating or not. That is also the case even if there is a large A B C the result of calculations carried out on a normally controlled space heating system where no solar heating can be stored in the house. the result of calculations carried out on a system where the furnace is switched off when the house has a temperature of or above the desired minimum temperature (2 C), but where the radiator thermostates still operate at 2 C ± 1 C. as B, but the radiator thermostates operate at 2,7 C ± 1 C.
D as B, but the radiator thermostates operate at 21,7 C ± 1 C. 8 As it appears, the possibility to create (merely a modest) excess temperature in the house by means of solar heating is very decisive for the performance of the solar heating system as the performance in the last case is 7% larger than in the first case. In practice, the system is not controlled as described and the system s performance is therefore dependent on the length of the periods during which the furnace is turned off manually and to which extent the thermostatic valves are set at excess temperature. These are conditions that the ordinary consumer hardly knows about or is aware of, and even if only a few measurements have been carried out on the system this is probably one of the reasons why the measurements that have been carried out have shown very different performances. Luckily, this type of system is usually sold for the purpose of supplying a minor amount of additional solar heating to a bathroom floor during summer when the furnace is turned off, but if a greater amount of solar heating is required, then this type of system is not appropriate. App. 8 systems have already been installed in Denmark. 3 EVALUATION OF SYSTEM 2: DHW TANK AS SPACE HEATING STORAGE DEVICE The characteristic of the Danish combisystem 2 is that the domestic hot water storage also is used as solar energy storage for the space heating demand. This simplifies the system as only one storage is needed. However, it also implies some limitations as the storage cannot be too large due to the risk of decreased water quality [ref.]. The principles of the system are illustrated in Figure 2. The hot water storage contains three internal heat exchanger coils. The bottom coil is for the collector loop, the middle coil takes out heat for the space heating loop and the top coil is connected to an auxiliary energy supply and heats up the top volume to a certain temperature to assure hot water comfort. For evaluation of the system, a simulation model of the combisystem is built in TrnSys [ref]. Furthermore, a building model, also from TrnSys, is used to provide a realistic space heating load to the combisystem model. The space heating load and the hot water load are similar to the loads used for the evaluation of the Danish combisystem 1. Thus, there is a yearly hot water load of approximately 27 kwh and a space heating load of approximately kwh. The hot water load is evenly distributed over the year and the pattern of the space heating load can be seen in Figure 8. Hourly space heating load [kwh] 7 6 5 4 3 2 1 Hour of the year [-] Figure 8: Distribution of the space heating load over the year. Table 2 summarises the different system configurations that have been taken into calculation. The solar collector is connected to the storage tank through a 1 m foaminsulated outlet pipe and a 1 m foam-insulated return pipe. The solar collector loop is equipped with a circulation pump which is controlled by a differential thermostat that measures the temperature difference between the outlet from the solar collector and the bottom of the storage tank. The differential thermostat has a start/stop set point at 6/2 K. The auxiliary energy supplies are treated in a simple way in the model. An electric heating element is placed inside the tank to heat up the consumption water if the required hot water temperature is not reached with the solar heating alone. Also, if the required forward temperature for the space heating loop is not reached with the solar heating alone, the fluid in the loop is afterheated in an electrical boiler. This means that no-load losses and burner efficiencies are NOT taken into account when the system is evaluated. Or said in another way: in real life the system performances will be higher. TANK DESIGN Hot water volume.3 /.72 / 1.8 m³ Store heat loss coefficient Height Diameter Coil heat transfer capacity rate 2.5 / 4.4 / 5.6 W/K 1.23 / 1.65 / 1.85 m.56 /.75 /.86 m 6 W/K/m² collector COLLECTOR Area 5/12/18 m² Start efficiency.77- Heat loss coefficient 5.1 W/m²K 2 nd degree of heat loss coefficient.22 W/m²K² - Tilt 45 Orientation South Table 2: System data.
Figure 9 shows the net utilized solar energy for the combisystem compared with the net utilized solar energy for a system that only supplies hot water. The net utilized solar energy is defined as: [hot water load and space heating load] minus [auxiliary energy supplied to the storage and auxiliary energy supplied to the space heating loop]. The collector area varies from 5 m² to 18 m² but the store volume is 3 l for all simulations. Thus the figure is comparable with Figure 3 that shows the same results for the Danish combisystem 1. From the figures it is seen that combisystem 2 has a better performance than combisystem 1, when larger collector areas are used. The differences are due to the fact that combisystem 2 is actually able to store the energy even though the total store volume is only 3 l. 35 3 25 2 1 5 DWH Combisystem 5 12 18 Collector area [m²] Figure 9: for the combisystem compared with a system that only supplies hot water. However, the results in Figure 9 are not so relevant for combisystem 2, because, unlike combisystem 1, it has a storage possibility. Figure 1 shows the same set of parameter variations, but now also the storage volume is changed, so it fits the collector size. From the figure it appears that the performance of the 12 m² system is increased with 12% and the 18 m² system is increased by 2% when a proper storage size is used. In the figure the solar fraction, defined as net utilized solar energy divided by the total load, is also shown. 4 35 3 25 2 1 5 Solar fraction 162 2913 3755 DHW, 3L/5m² 3L/5m² 72L/12m² 18L/18m² Storage volume [L] / Collector area [m²] Figure 1: and solar fractions for different sizes of the combisystem. 25 2 1 5 Solar fraction [%] constant yearly load of kwh and a constant forward temperature, three different return temperature profiles are examined. The return temperatures are changed by varying the nominal power of the radiator system and thus also the flow is changed. Figure 11 shows the temperature profiles over the year. Temperature [ C] 9 8 7 6 5 4 3 2 1 Hour of the year [-] T(forward) T(return, high) T(return, medium) T(return, low) Figure 11: Variation of the return temperature from the heating system. In Figure 12 net utilized solar energy and solar fractions for the 18m²/18L storage with high, medium and low return temperatures from the space heating loop. As expected the figure shows that the lowest return temperature gives the best performance of the combisystem. When changing from the highest to the lowest return temperature an increase of 1% on the net utilized solar energy is achieved. 45 4 35 3 25 2 1 5 352 3755 3839 High Medium Low Solar fraction Figure 12: and solar fractions for the 18m²/18L storage with high, medium and low return temperatures from the space heating loop. The calculations above are all made for a traditional heating system with a traditional radiator system. As a last example, a low temperature heating system (e.g. floor heating) is discussed. Figure 13 shows typical forward and return temperatures for a low temperature heating system. 25 24 23 22 21 2 19 18 17 16 Solar fraction [%] Another interesting point when discussing combisystems is the return temperature from the heating system. For a
Temperature [ C] 9 8 7 6 5 4 3 2 1 T(forward) T(return) 73 146 219 292 365 438 511 584 657 73 83 876 Hour of the year [-] Figure 13: Typical forward and return temperature in a low temperature heating system. With this low temperature profile from the heating system the solar performance is further increased as can be seen in Figure 14. Here, the first three bars are similar to the ones in Figure 12 and the last bar shows the net utilized solar energy for the combisystem connected to a low temperature heating system. So, by changing the heating system from a traditional heating system with a low return temperature to a low temperature heating system an additional 4% on the net utilized solar energy can be achieved. 45 4 35 3 25 2 1 5 352 3755 3839 45 High Medium Low Low temperature system Solar fraction 25 24 23 22 21 2 19 18 17 16 Figure 14: and solar fractions for the 18m²/18L storage with high, medium and low return temperatures in a traditional heating system compared to the net utilized solar energy with a low temperature heating system. Less than 5 plants have presumably been installed. 4 EVALUATION OF SYSTEM 3: "TANK IN TANK" The system shown in Figure 3 is becoming more and more widespread in connection with solar heating plants combined with a biofuel boiler. The storage tank with the submerged hot water tank is typically 5 or 75 liters, and especially 2 types are used in the Danish market. The system is mainly used to store heat from the furnace at the top of the storage tank. In connection with the fuel boiler the storage volume is often not sufficient and Solar fraction [%] therefore several tanks are often placed parallel as shown in the figure. Measurements have been carried out on 2 systems according to the system principle. These are described in the paper: Solar Combisystems in Denmark Solar & Biomass Systems. It is decisive for the system that it has been carefully insulated as there might be many pipe connections etc. that could increase the heat loss. In addition, the irregular operation of a solid fuel stove and radiators due to manual firing has shown that it is difficult to keep the return temperature from the radiators down, which is decisive for a good solar collector performance. Less than systems have presumably been installed. 5 EVALUATION OF SYSTEM 4: DRAIN BACK IN EXISTING SPACE HEATING LOOP The system shown in figure 4 is designed and marketed by a single company. The storage tank has an incorporated drain back tank for the collector loop that functions according to the drain back principle. The storage tank is placed in the space heating loop and often the existing hot water tank is maintained. No special efforts are made to utilise the cold temperatures of the domestic water in the system. The idea is that if the system is cheap enough an increased solar collector area can compensate for the higher temperature level of the system. Measurements have been carried out on 1 system according to the system principle. These have also been described in the paper called: Solar Combisystems in Denmark Solar & Biomass Systems. Less than 5 systems have been installed. 6 CONCLUSIONS None of the combisystem designs experienced in Denmark are sophisticated with respect to temperature stratification devices or temperature managing. The philosophy has been not to make expensive design. The far most common combisystem design in Denmark only has extra costs compared to DHW systems with respect to extra collector area and with respect to an extra heat exchanger between collector loop and central heating loop and an extra sensor in the control. However, this design has disadvantages as it only gives additional performances if the customer makes use of the storage capacity of the building construction by allowing the temperature in the house to vary. This is not very wellknown and is also very dependent of customer habits.
Perhaps more sophisticated controls which have not yet been developed could change this. Besides considering the specific solar heating system design it is also important to consider flow and especially return temperatures in the central heating loop. Other designs are appearing in the market. These have storages that also store the heat used for space heating then allowing a much better control of the indoor temperature. Some of the designs are especially suited to be combined with biomass boilers. It is expected that the ongoing work in IEA task 26 will result in guidelines and evaluations about which system designs are the best suited for Danish conditions with respect to performance and economy. However, it is too early to arrive at any conclusions to this matter. 7 REFERENCES Ellehauge K et al (1999). Aktive solvarmeanlæg med større dækning af husets samlede varmebehov, Udredning og skitseprojekter. Udarbejdet for Energistyrelsen J.nr. 51181/97-4. Klein S.A et al. (1996). TRNSYS 14.1, User Manual. University of Wisconsin Solar Energy Laboratory. Dutré, W L. (1991). Simulation of Thermal Systems. A modular program withan interactive preprocessor (EMGP3). Kluwer Academic.