TESTIN

The basis for system development is accurate and reproducible system testing. Since more than 10 years, the solar energy laboratory SPF is involved in system testing. Many systems have been tested according to different approaches. Well proved to investigate new concepts is the component based method, where dynamic fitting in conjunction with TRNSYS is applied to each of the different components. Variation of components in the systems (e.g. heat exchanger, solar collector) demonstrate further possibilities for improvements. According to this component based method, more than 30 systems have been tested in the last 5 years at SPF. Comparison of different concepts are discussed in order to see advantages and disadvantages of low-flow to normal flow concepts. Also the pros and cons of helix heat exchangers to mantle tanks are discussed by comparing the most efficient storage tanks coupled to identical collectors.


INTRODUCTION
Solar technologies, such as solar thermal, possess a huge future potential. This is not denied, even by the people from the lobby of the conventional energies, e.g. the oil or gas industries. But not in all countries the market is growing like it used to in the last years (Austria, Switzerland, Denmark)! Reasons are different but it seems that the small marked segment of the idealistic ecologists connected with ideal constraints (funding etc.) are now equipped with solar systems. To augment the market, new market segments have to be found. In the area of domestic hot water systems this means, the so called "early buyers" must be persuaded to invest in such a system. The only way to do this, is to improve the systems performance while lowering their costs to bring their energy prices near conventional systems. Simultaneous the technical status as well as the appearance must be highly professional. The current developments go towards advanced smart systems of which all components are designed and optimized to fit to the overall system. This approach leads to the so called "kit"-systems where in the scope of delivery all necessary components are included (collectors, storage tank, pump and so forth). In Switzerland this approach has been introduced into the market with success since 1996. Figure 1 shows the market evolution. The idea of kit systems is penetrating also into the area of combisystems (combination of solar domestic hot water and solar heating system) and the solar domestic hot water preheating systems for large multifamily houses. An important advantage of the kit-system is, that they can be tested and optimized in the laboratory. If this task is done consequently, system specific components have to be developed. Especially for the main components such as the storage tank and the solar collectors this is a difficult and expensive exercise! Therefore in a number of kit systems available on the market, standard components are used. The remaining question is, whether these systems including standard components have an acceptable performance compared to the kit systems including especially designed components.  1996 1997 1998 1999 Year Numbers of systems DHW < 10 m^2 Figure 1: Marked trend of small domestic hot water systems Important design features in these systems are the operating conditions in the solar loop (Low-flow versus High-flow), the heat exchanger principal (heat exchanger spirals contra mantle heat exchangers), the tubing (integral, all in one envelope contra traditional tubing) or the solar collectors (flat plate contra evacuated tubular collectors).

SYSTEM TESTING
At SPF, Rapperswil the CTSS (Component Test and System Simulation) method is used for solar domestic hot water system testing /1/. The systems are installed completely and with their solar collectors outdoors. A variety of sensors are fitted to assess the performance of the different components, such as: storage tank, collectors, pump, collector loop piping. A typical simplified measurement set-up is illustrated in figure 2. With a series of short dynamic test sequences and subsequent parameter identification a performance model of each component can be derived. Using TRNSYS /2/ the performance of the system can be calculated. Simulations with measured boundary conditions may be used for validation purposes. Arbitrary boundary conditions are used for standardized annual performance predictions.

The boundary conditions for simulation
• The collectors plane is at a tilt angle of 45 degrees and faces south. • The solar radiation in the collector plane and the ambient temperature was generated using Meteonorm /3/ for the location Rapperswil, Switzerland. • Domestic hot water load: The average daily domestic hot water (DHW) load volume is 200 litres at 45°C. In terms of energy, the annual DHW consumption amounts to about 3000 kWh/a. The draw-off pattern is identical to the one used at SPF for standard annual performance predictions with eight draw-offs per day. The draw-off pattern is identical on every day of the year. There is a seasonal dependency of the mains water temperature: The average is 9.5 K, the amplitude 5 K. The lowest mains temperature is reached on February 10. • The auxiliary heating is enabled only between midnight and 6 a.m. Restricting the operation of the electrical auxiliary heater to nightly hours reflects the usual set-up for DHW-preparation in Switzerland. • The storage ambient temperature is 18°C.

System configuration
• The solar systems (size; mode of operation; settings, e.g. overheating protection) are configured exactly as they were submitted for testing by the manufacturers. The only type of auxiliary heating used is the integrated electrical heating element. The auxiliary heater set points and the on/off temperature differences were determined during the system test.
With these auxiliary heater settings, the system is just able to meet the DHW demand without the hot water temperature dropping below 45°C if the mains temperature is 10°C and if there is no energy input from the solar collectors.

Test systems A 1 to A 6
The test systems A 1 to A 6 represent the most simple ones. There storage tanks are usually mass produced, made of enamelled steel and contain just one heat exchanger spiral in the lower part to transfer the solar energy to the domestic water. In the upper part a second heat exchanger is included to use an oil, gas or wood fired furnace as auxiliary source. In addition about half of the volume (150 to 250 l) can be heated by electricity. As already mentioned above this is because of the special Swiss circumstance to use cheap off peak night time electricity as auxiliary energy.

Figure 3: Scheme test systems A 1 to A 6
Test systems B 1 to B 3 The most significant difference to the test systems A is the additional solar heat exchanger in the upper part of the storage tank. But there are more differences: the operating mode of the collector loop is changed from standard to low-flow. This means the storage design, including the heat exchangers should support significant stratification. Depending on the feeding temperature the solar energy is directed to the upper heat exchanger before passing through the second one in the lower part. Under regular operating conditions the return temperature should be kept low, in a good low-flow design, just a few degrees over the cold water temperature. Depending on solar insolation the temperature difference over the collector can reach 50°C leading directly to convenient hot water temperature in the range of 60 to 70°C in the uppermost part of the storage tank. Figure 4: Scheme test systems B 1 to B 3

Test systems C 1 and C 2
As can be seen in figure 5 the test systems C 1 and C 2 show a quite different design! Instead of spiral heat exchangers double mantles are used. The advantage of double mantel heat exchangers are there large area, there low turbulence effects on the storage water and last but not least there excellent stratification behaviour. To maintain this advantages an adapted design is required. Both systems use different entrance heights depending on feeding and tank temperature.

SIMULATIONS AND DISCUSSION OF RESULTS
In this chapter results of different simulations are presented and discussed. The starting point are the results of the as tested systems. Since all systems are marketed in Switzerland they are optimized according to specific Swiss design criteria as presented in the next section. For the following steps of simulation the systems were changed to compare different aspects of system design.

Country specific design criteria
The criteria to design solar domestic hot water systems depend on a number of boundary conditions: • Climate (location, solar insolation, cold water temperature ...) • typical comfort needs (demand per typical family, temperature, typical daily or weekly variations) • available auxiliary sources and their prices • typical building conditions including location of the heating system (cellar, under the roof ...) • building regulations These criteria have a strong influence on the design of the system. As an example the conditions in Switzerland are described: The Swiss climate varies significantly between the different meteorological regions. Typical mid European climate can be found where more than 80 % of the population is living but also the alpine or the southern part is not neclectable. In Switzerland, especially in single family houses, electricity is still one of the main energies used for domestic hot water heating. Very important, using electricity as auxiliary energy, is the different price for day and night time electricity respectively. In general, night time off peak electricity is at least a factor of two cheaper than during day time. This circumstance has a strong influence on the dimensioning of the solar domestic hot water storage tank! An other important design parameter is the required comfort level. From one country to another a variation of a factor two is possible. In Switzerland people use about 35 to 50 l of water heated from 10°C up to 45°C. A typical Swiss family of four persons need about 6 to 9 kWh of domestic hot water per day.

Set-up for the first set of simulations
• The auxiliary heater is enabled during night from midnight to 6 a.m. • The auxiliary heater setting is able to meet the DHW demand without the hot water temperature dropping below 45°C if the mains temperature is 10°C and if there is no energy from the solar collectors. The as tested systems show large differences in the necessary auxiliary energy to cover the load. Large differences in necessary auxiliary energy are shown for a number of systems.
The traditional systems with only one heat exchanger in the bottom of the tank, system A 1 and A 3 show a remarkable difference. Both have about equal collector and heat exchanger area. Responsible for the lower performance of system A 1 are the small auxiliary volume leading to a higher set temperature for the auxiliary source and the fare poorer collector performance (at x = 0.1 m 2 K/W). Also outstanding is the large height of the heat exchanger and the low flowrate in the collector loop for system A 3 , both features seem to have a positive effect on the system performance. The B-systems perform all quite similar except for the high power consumption of system B 1 leading to an extraordinary high parasitic energy consumption. The low-flow systems using mantle heat exchanger, C 1 and C 2 , perform extremely different. Both systems designed for Swiss conditions shows a drastic difference in the auxiliary volume. System C 1 has just 160 l, which is leading to a very high set temperature and therefore reduces the efficiency of the whole system. In addition the collector area of systems C 1 is about 30% smaller than for system C 2 .
Another way to present system performances is shown in figure 7: The figure shows clearly the enormous differences between the systems. Again the systems C 1 and C 2 perform very different, the reason is already discussed above. B 1, B 3 , C 2 , A 3 and B 4 are performing well.

Set-up for the second set of simulations
Compared to the first set of simulations, these elements of the boundary conditions were altered: • The auxiliary heater is enabled during the day (between 6 a.m. and 10:30 p.m.), not during the night. • For all the systems, the switch on temperature of the electrical auxiliary heater is 50 C, the switch off temperature is 52 C. • The load file with daily and seasonal variations from Jordan /4/ was used to distribute the domestic hot water draw-off over the year. In this way the volumes and probabilities of different categories of DHW consumption are taken into account. With the combination of variable mains temperature and daily load volume the DHW load is significantly smaller in summer than in winter (see table 2 and figure 8). The other parameters, including those of the auxiliary heater (power, on/off temperature difference and the orientation (vertical/horizontal) are not modified but left as they were during the testing of the system. The position of the auxiliary heater and with it the volume heated by the auxiliary heater is not modified with respect to the tested system either and was not modified in any of the simulations described. The auxiliary volume is thus different from one system to the other. Parasitic energy difference to last set of simulation kWh/a Auxillary energy difference to last set of simulation kWh/a Figure 9: Second set of simulation results under the boundary conditions described above The large influence on efficiency between the first and second set of simulations is initiated mainly by the change from night to day time auxiliary heating. The large auxiliary volumes of some of the systems is nowin contrary to first set of simulations -a disadvantage. The lower auxiliary heating for system A 5 is caused by the lower hysteresis of the auxiliary source. In the as tested system, the hysteresis was in the order of 8 K which is now reduced to 2 K. Conspicuous is the strong increase off auxiliary energy for the systems B 1 to B 3 . The main reason is, the large auxiliary volume but connected with the arrangement of the upper heat exchanger. It is assumed, that the temperature sensor controlling the auxiliary source is below the upper heat exchanger and therefore is turned on even if enough energy is available in the top part of the storage tank. This disturbs the positive effect of the second heat exchanger drastically.

Set-up for the third set of simulations
Compared to the second set of simulations the configuration of the systems was made uniform for all the systems in the following points: • The position of the electrical heating element is horizontal and the vertical distance of the thermostat used to control the auxiliary heater and the heating element is identical for all systems. The power of the auxiliary heater is 3 kW. • The only overheating protection used is: cease collector loop operation if the temperature in the storage tank exceeds 90°C. • Power consumption of the collector loop pump is 45 W during times of operation. The controller power consumption is 3 W at all times. • There is no heat transferred from the collector loop pump to the collector loop fluid • These modifications were implemented to make the results of the next sets of simulations more comparable. The influence on auxiliary energy of the changed system configuration is rather small. The only remarkable decrease in auxiliary energy for systems A 6 and B 3 is caused by the increased overheating temperature from 65°C to 90°C. The strong decrease on parasitic energy for system B 1 may be explained by the reduced pump power consumption from 85 to 45 W.

Set-up for the fourth set of simulations
The set-up for the fourth set of simulations is based on the set-up for the third set of simulations. The only modifications made are: replacing the system-specific collector loop piping with a piping which is identical for all the systems. All collector loop piping has an inner diameter of 10 mm and is insulated according to the state of the art. The pipe lengths are identical in all simulations: 5 m indoor, 8 m outdoor for feed and return lines each.
Parasitic energy difference to last set of simulation kWh/a Auxillary energy difference to last set of simulation kWh/a Figure 11: Fourth set of simulation results under the boundary conditions described above Except for system A 4 , the influence of replacing the piping is surprisingly small. Obviously one of the week points of system A 4 was the insufficient piping insulation.  Figure 12: Solar fractional savings in relation to the specific solar gain (energy consumption reference system 3600 kWh/a) for the fourth set of simulations Figure 12 shows a nearly linear dependence of the solar fractional savings to the specific solar gain of the systems B and C. An exception is System B 2 where especially the high storage losses cut down the performance. The difference in performance between the A an the other systems is clearly related to the main difference: enhanced stratification during the solar charging process.

Set-up for the fifth set of simulations
For the fourth set of simulations all systems were combined with the same collector field: 4 m 2 of flat plate collectors.
Parasitic energy difference to last set of simulation kWh/a Auxillary energy difference to last set of simulation kWh/a Figure 13: Fifth set of simulation results under the boundary conditions described above As expected, systems with collector areas larger than 4 m 2 show an increase in auxiliary energy. This results show mainly the characteristics of the storage tank. The collector loop including piping is the same for every system. Of course still different is the flowrate in the collector loop. Except for system B 2 , system performances are much more leveled out than in the first (as tested) simulation set-up.

Set-up for the sixth set of simulations
The last step in trying to make systems more comparable with respect to heat exchangers and flow type (low-flow versus traditional flow) the auxiliary volume is reduced to 150 l for each of the systems.
Parasitic energy difference to last set of simulation kWh/a Auxillary energy difference to last set of simulation kWh/a Figure 14: Sixth set of simulation under the boundary conditions described above.
Except for system B 2 the more advanced systems B and C need less auxiliary energy (-5 to 10%) compared to traditional systems A. In figure 15 it can be clearly recognized which systems perform better! The difference between the best (C 2 ) and the worst (B 2 ) system is about 20 % in performance. The advantage using low-flow coupled with high stratification in the storage tank is clearly demonstrated! Even using one high single heat exchanger (for example 40 % of the storage height) using low-flow increases the system performance (A 5 and A 3 ).

CONCLUSIONS
The approach using results of component based system testing to investigate pros and cons of different system concepts has been successfully demonstrated. Varying boundary conditions is an efficient tool to improve the comparison of system design features. Systems have to be optimized to achieve maximum fractional solar savings under the conditions of the country (region) they are used. Under typical Swiss conditions the difference in performance of the 12 systems taken into consideration, is as high as 20 % (absolute) in fractional solar savings.
Using an advanced concept is not a guarantee for high performance; one design error -as seen in system C 1 under Swiss conditions -might lead to a drastic cut in performance.
Low-flow in the collector loop, including heat exchangers for improved stratification, is leading to systems with higher performance than traditional systems. The highest performance is reached with a mantle tank using even the bottom and top part of the storage tank as heat exchanger (tank in tank system). There is still a potential for further improvements, but of course related to the boundary conditions of the respective country.