Category Archives: EuroSun2008-10

Evaluation of virtual case studies

Out of the large number of virtual case studies, a handy number of standard system configurations, which work best under different conditions, are identified. Based on these, the industry partners will provide consistent package solutions. These will enable planers and independent craftsmen to install reliable systems. The economical and ecological rating of the virtual case studies will also allow identifying the most promising markets, where systems are yet at the edge of economical breakeven point or beyond. Last but not least the results of the virtual case studies will be made available online with an easy to handle web-based tool, which can query it under different aspects.

3.3. Training on package solutions

Special training courses for solar thermal installers on standard system configurations and package solutions will be prepared and 15 pilot courses will be evaluated. Target group are (solar thermal) installers, because the goal of the packaged solutions is to avoid the need of engineering.

3.4. Dissemination, communication and training

Tailored dissemination, communication and training plans were elaborated to reach the different key actors. They include besides the presentation of results at relevant conferences and trade fairs addressing a wider audience (i) the dissemination of both the elaborated brochure and the online tool to query the virtual study cases towards professional groups (HVAC planners, architects, engineers, building industry), through their interest groups and associations (e. g. ESTIF, ECTP, chambers), where possible on the occasion of annual meetings or in synergy with related national and international projects, (ii) the provision of information and advice to (national) authorities on the potential of Solar Combi+ with the aim to include it in support programmes and (iii) the approach of local authorities in promising regions promoting pilot installations. Finally, information through public media in the most promising regions should give an important push to market entry. On the website all public deliverables will be available for download and most attention is given to the integration of the webpage in the existing information network on solar heating in general and combined solar heating and cooling in special.

4. Market analysis

Analysis of Results

In spite of an early settlement of the monitoring system, the STS has undergone a period of either reduced load and/or deficient operation, which has prevented the collection of a representative set of monitoring data enabling a thorough analysis to the system.

To the present, monitoring operations have suited particularly the detection of system faults, rather than evaluation of system behavior and performance. Nevertheless, and beside a short fault examples list, the data acquired allows a preliminary analysis of system performance and trends.

2.1 Data system results

The actual monitoring period started in June 2007, when the building was becoming occupied and starting to be close to project conditions. The deployment of the STS started three months before, with the solar field working at limited capacity considering the low occupancy of the building (only a single 4 collectors group uncovered on each orientation).

2.1.1. System Deficiencies

After the deployment of the system, a number of fault situations were detected; either related to installation problems or inadequate load conditions.

The system proved to be hydraulically unbalanced in its East-facing collectors.

There were leaks observed in the pumping area.

After the leakages were fixed and pressure reset they reappeared (after 1 month) as well as in the East-facing collectors.

Design of Hot Water Heating System for Low-rise Apartment House

S. Kaneko1* , M. Udagawa1 and T. Kusunoki1

1 Kogakuin University, 1-24-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo, 163-8677, JAPAN
* Corresponding Author, dm07014@ns. kogakuin. ac. jp

Abstract

Total performance of solar hot water heating(DHW) system for an apartment house of 10 housing units was examined using the detailed simulation with EESLISM [4]. The simulation was carried out to examine the appropriate collector area and the storage tank volume for the central type of DHW supply system. The result for an apartment house showed that the collector area of 30-50m2 is appropriate to expect the solar contribution of 37.1-91.8% while the solar contribution is strongly depending on DHW supply rate. The appropriate storage tank volume is 1.0-1.5m3 in the studied cases. The economical efficiency of DHW system showed that the equipment cost should be suppressed below 2 million JPY if expecting the pay back period of 10 years.

Keywords: Collector area, Storage tank volume, DHW heating load, Equipment cost

1. Introduction

In the past studies [1-3], the solar hot water heating(DHW) system for an apartment house of 10 housing units was simulated in order to examine the difference of tilt angles and azimuths of the collector. In this study, in order to find the relationship of collector area and storage tank volume of the DHW system, the simulation study was carried out. The suitable combination of the collector area and the storage tank volume with considering initial cost are examined by simulating the yearly performance of the solar DHW system for the apartment house.

Rise in energy prices

The simulations of the base case were calculated with a comparably small rise in energy prices of

1.3 Подпись: 0Подпись:Подпись:image254%/a for natural gas and 0.3 %/a over 20 years for electricity [6]. However the current development of prices in Germany amounts to 9.7 %/a (non inflation-adjusted) over the last seven years [7] which indicates a higher annual rate of growth.

Therefore the rise in energy prices has been varied between

1.3 Подпись: 12%/a and 7.5 %/a. Figure 3 (e) shows that as expected the lowest values of the objective function get smaller with a growing rise in energy prices which means that the heat generation costs per kWh become smaller. More surprisingly, the dimensions of each optimal system in terms of the underlying objective function and therefore also the primary energy savings stay constant, showing that the dimensioning does not depend on the rise in energy prices within the examined range. Figure 5 shows an extrapolation of the simulation results calculated with different rises in energy prices. The trend line derived from the calculated points indicates that the analysed solar heating system would be economically rewarding with a rise in energy prices of 9 %/a without any subsidies.

4.3 Subsumption of simulation results

Reducing the price of the solar collectors by 30 % improves the cost/benefit ratio by 21 %. The resulting optimal collector area increases by 3.5 m2, whereby the storage device capacity keeps unaltered. Using a high efficiency flat-plate collector instead of the initially defined model reduces the cost/benefit ratio by 8 %, whereas a low efficiency flat-plate collector increases the cost/benefit ratio by 13 % without having stronger impact on the dimensioning of the system. A 40 % reduction in the storage cost improves the cost/benefit ratio by 24 % again without changing the optimal dimensioning. The difference in the cost/benefit ratio between a system with 7.5 cm of storage insulation and a system with 17.5 cm of storage insulation amounts to 21 %, whereas the optimal system with the thickest isolation consists of a storage 120 litres larger than the thin isolated tank, connected to solar collectors that are 3.2 m2 smaller than the pendant with the thin isolated tank. Considering a rise in energy prices of 7.5 %/a instead of 1.3 %/a leads to a reduction of additional costs of 250 € /a with equal dimensioning parameters.

Comparison of the Thermal Performance of Different Working. Fluids in a Closed Two-phase Solar Water Heating Thermosyphon

A. Ordaz-Flores1, O. Garcfa-Valladares2*, V. H. Gomez2

1 Posgrado en Ingenieria (Energia), Universidad National Autonoma de Mexico, Privada Xochicalco s/n,

Temixco, Mor. 62580, Mexico

2 Centro de Investigation en Energia, Universidad National Autonoma de Mexico, Privada Xochicalco s/n,

Temixco, Mor. 62580, Mexico

* Corresponding author, ogv@cie. unam. mx
Abstract

A closed two-phase thermosyphon solar system was designed and built to produce hot water for sanitary purposes. The aim of this work is to compare the thermal performance of a two — phase closed thermosyphon using different phase change working fluids (acetone, R134a and R410A). The choice of using a closed two-phase thermosyphon, instead of a conven­tional solar water heating thermosyphons obeys to the some advantages as the lower freez­ing point of the two-phase system compared to water, and elimination of fueling, scaling and corrosion. Disadvantages of these systems are the higher cost because of the working fluid used and the additional coil heat exchanger; moreover, refrigerants reach high pressures. A witness conventional solar water heating system has being installed to compare its perform­ance versus that of the two-phase closed system. The two-phase system consists of a flat plate solar collector coupled to a thermotank by a continuous copper tubing in which the working fluid circulates. The working fluid evaporates in the collector and condensates in the thermotank transferring its latent heat to the water through a coil heat exchanger. The conventional thermosyphon system has the same characteristics (materials and dimensions), with the exception that it lacks the coil presented in the two-phase system. Data were col­lected from the two kind of solar water heating systems, operating simultaneously, and com­parisons of performance were made. Results show that the performance of the two-phase systems is strongly dependent on the load of the working fluid: an optimum point should be found. R134a and R410A show better performance than acetone. The two-phase closed sys­tem shows hardly any difference in performance (when working with both R134a and R410A) compared to the conventional solar water heating thermosyphon.

Keywords: acetone, test, R134a, R410A, phase change.

1. Introduction

The increasing interest of preserving the non-renewable resources has led to focus on sustainable growing, based mainly on using renewable energy. The use of renewable sources helps to save economical expenses, as well as to prevent the inherent environmental impact of conventional sources. Renewable energy sources are the Sun, biomass, hydrogen, wind, etc. The Sun leads to thermosolar and photovoltaic technologies, mainly.

The current paper has special interest in Solar Domestic Water Heating Systems (SDWHS). SDWHS permit to diminish the consumption of liquid gas and electricity, helping to reduce the quantity of pollutants expelled to the atmosphere. In 2004, Kalogirou [1] studied the environmental impact of energy utilisation and the potential benefits to swap conventional for solar assisted sys-

tems. He estimated that, for the case of solar water heating (one of the two most widely used re­newable energy) the savings would reach up to 80%. Hence, the importance of solar water heating.

For instance, in Mexico, the use of flat plate solar collectors to heat 500 L of daily water would yield savings of 433 kg/year of LP gas [2].

The most common currently available solar equipments to heat water are the thermosyphons in which the water is heated in a flat plate solar collector and stored in a thermotank. Active systems use a pump to circulate the water, while in passive systems the water circulates by the thermosy­phon effect. The water presented in the flat plate solar collector is heated by the Sun energy, so its density diminishes; the lower density of the water in the collector, compared to that of the thermo­tank makes the water to circulate: that is the thermosyphon effect. In direct systems, the water is heated in the collector; in indirect systems, some fluid is heated in the collector, and it transfers the energy to the water by means of a heat exchanger; in a closed system, the working fluid is sealed from the atmosphere, in an open system, the heat transfer fluid is in contact with the atmosphere. If the fluid changes its phase in the collector, the system is called a two-phase or a phase-change sys­tem.

The system studied in this paper is a passive, indirect, closed, two-phase system. This kind of sys­tem prevents problems like freezing, corrosion, scaling and fouling [3], which are presented in the conventional systems, increasing the life of the system.

In 1979, Soin et al. [3] described an experimental set up to evaluate the performance of a solar col­lector with a phase change working fluid. They used acetone and petroleum ether as working flu­ids, because of their high boiling and condensation heat transfer coefficient. They demonstrated that the collector efficiency increases linearly with liquid level.

In 1981, Schreyer [4] used a refrigerant, trichlorofluoromethane, to evaluate the energy recovery in a solar collector coupled to a heat exchanger, and the latter to a storage tank. The primary loop was passive and the secondary needed a recirculation pump. His system recovered up to 83% energy at low collector temperature difference.

Evaluation of R134a (among others) as replacing working fluids of ozone depletion promoting chlorofluorocarbons was made by Calm and Didion [5]. They concluded that there is no perfect fluid to prevent every environmental impact. R134a has a high latent heat of vaporization, does not contributes to ozone depletion but, yet low, does have impact on global warming.

Ong and Haider-E-Alahi [6] studied the performance of a heat pipe filled up with R134a, and found that the heat flux transferred increased with high refrigerant flow rates, high fill ratios and greater temperature difference between bath and condenser.

More recently, Hussein [7] studied a two-phase closed thermosyphon with the heat exchanger (condenser) in the solar collector; however, he did not mention the working fluid used. He carried out both experimental and numerical tests and set some dimensionless variables to determine ade­quate storage dimensions for the tank to improve the solar energy gain.

In 2005, Esen and Esen [8] studied a thermosyphon heat-pipe solar collector, to evaluate its ther­mal performance using three different working fluids, R134a, R407C and R410A. They found that the latter offered the highest solar energy collection.

In this work, refrigerants R134a and R410A were chosen due to their availability, low cost and small impact to environment. Acetone is also cheap and available, but it avoids the high pressures reached with the former ones; on the other hand, acetone is flammable.

2. Experiment

A water heating two-phase closed thermosyphon, using either R134a, R410A and acetone as work­ing fluids, and a conventional natural thermosyphon are compared simultaneously. Both systems have the same geometry, except for the coil presented in the two-phase system. The construction materials for the whole system are the same. Each collector has an absorption area of 1.62 m2 and the volume capacity of each thermotank is 160 L. The two-phase system consists of a flat plate solar collector coupled to a thermotank by a copper tubing circuit in which the working fluid circu­lates. A scheme of the systems is shown in Fig. 1.

Focusing on the fluid refrigerant behaviour, the solar collector is the evaporator of the system and the copper coil immersed in the thermotank is the condenser. The incoming solar radiation makes the temperature of the refrigerant in the collector to grow higher to reach the saturation liquid state. From this point, the working fluid starts to evaporate to reach the saturated vapour state and even the superheated vapour zone. As the refrigerant has a higher temperature than the water, the former donates its phase change latent heat to the latter and leaves the thermotank as sub-cooled liquid to come back to the solar collector to repeat the cycle.

image159

Fig. 1. Two-phase closed thermosyphon and conventional thermosyphon.

Refrigerant R134a is one of the replacing working fluids of chlorofluorocarbons since it does not contribute to ozone depletion. R134a evaporates at -26.1 °C at atmospheric pressure [9] with an enthalpy of vaporisation of 216.98 kJ/kg; its freezing point at this pressure is -101 °С.

Acetone (also known as propanone) is a colourless liquid, used mainly as solvent, for cleaning, or as a drying agent; is flammable, and should not be inhaled. At atmospheric pressure, it evaporates at 56.05 °С [10] with an enthalpy of vaporisation of 501.03 kJ/kg and a freezing point of -94.7 °С.

R410A is a mixture of refrigerants R32 and R125 (50% of the volume of each one), it is used in air conditioning as substitute of R22; it is not toxic and does not contribute to ozone depletion; its boiling point at atmospheric pressure is -52.7 °С; its enthalpy of vaporisation is 275.93 kJ/kg. The freezing point of R410A is not determined yet, but the freezing points of its components are -103°C for R125 and -136°C for R32, at atmospheric pressure [9].

The combination of boiling point temperature (the lower, the better) and heat of vaporization (the higher, the better) will show which of the fluids is more suitable for these operating conditions; other parameters as viscosity and pressure must also be considered.

The main disadvantage of R134a and R410A is that they reach high pressures; for instance, the pressure of these fluids at 50°C is 13.18 bar for R134a and 30.71 bar for R410A; their main advan­tage of the refrigerants is their low boiling points; that means that the heat transfer will start soon after the beginning of the test. Acetone does not have problems of pressure: at 50°C, it only reaches 0.81 bar; and its enthalpy of vaporisation is higher related to the refrigerants, but it lacks of a low boiling point at atmospheric pressure: 56.5°C.

The two-phase system was loaded up to 91% when operating with R134a, up to 83% when operat­ing with acetone, and up to 62% when operating with R410A. The systems were loaded differently because of the characteristic of the fluids and the difficulty to load refrigerants. On the other hand, acetone is very easy to load and permits to have better control.

Energy input from the solar loop

The solar heat exchanger is a cupper coil immerged at the bottom of the tank. One or more elements contain this exchanger as the user can decide. We consider that the heat provided by the

4

solar loop is transferred in a time step to one or more elements depending of their temperature. For example, if the last three elements have a lower temperature than the hot water produced by the solar exchanger than only those three elements will be heated. No influence is considered for the fourth element, during that time step.

The solar panel temperature is computed depending on the operation of the pump: if the pump is not running than

Tpanel(i + 1) = Tpanel(i) + •

panel

• Esun(i + [ • P-K1 • (Tpanel(i) — T0(i + 1))-K2 • (Tpanel(i) — T0(i + 1 ))2

image225 Подпись: (9)

And if the pump is running than

Where mpanel stands for the mass of water contained by the panels in kg, msolar stands for the water quantity (kg) flowing through the solar loop in a time step p, S panel is the active surface of the panels, Tho and Thi stand for the outlet and inlet temperatures of the exchanger, and T0 is the ambient temperature.

• (Tho(i + 1) — T0(i + 1))2 .• Spanel • P

Подпись: Esolar (i + 1) Подпись: Esun(i + 1) •P -K1 • (Tho(i + 1) - T0(i + V) -K2

Using the panel temperature, the control system can decide on the pump operation for the next time step, and finally we can compute the energy input of the solar loop if the pump is on.

(10)

Market situations and trends of small scale chillers

The first part of the project was devoted to an analysis of possible markets for Solar Combi+ systems. Since the competing technology are conventional (non-solar) air conditioning systems, the available technological solutions with small cooling capacity as well as their markets in Europe were analysed.

The European Air Conditioning (AC) market has grown rapidly during the last 5 years. The size of AC markets in the seven major European countries (France, Germany, Greece, Italy, Russia, Spain and the UK) expanded from some 2.4 million sold units in 2000 to 5 million sold units in 2004. A further breakdown of the European AC market reveals that Italy and Spain are holding the largest market of about 1.4 to 1.7 million units per year (after 2004), followed by France, Greece and UK at 300,000 to 500,000 units each [5, 6].

Подпись: Fig. 3. Evolution of the air conditioning market of individual air conditioners with a capacity below 17,5 kW in France. Sold units in the years 1998-2003 Подпись: Fig. 4. Share of air conditioners with different capacities on the overall Italian market in the years 2005 and 2006 [7]

For small cooling demands, typically room air-conditioning units or multi-split systems are used, as can be seen from a closer look at the France and the Italian markets can be stated that especially the monosplit units with small capacities are responsible for more than 50% of the overall sold units (see Figure 3 and 4). Application areas for these systems are mainly in smaller buildings such as the trade and residential sector and small office buildings where in the past mainly local solutions were used.

Подпись: Fig. 5. Number of systems sold until Feb. 2008, as reported by the SolarCombi+ industry partners Подпись: Fig. 6. Markets which are considered of high priority by the SolarCombi+ industry partners

However, small chiller systems have an increasing market share in many European countries. These smaller buildings are seen as the most promising target market for solar combi plus systems, which offer a central chiller system powered by solar heat (see Fig.5). The survey among the industrial participants of the SolarCombi+ project also showed that they see the most interesting markets for their Solar Combi+ systems in Spain, Italy and France (see Fig. 6).

System Evaluation

Only in the middle of March 2008 the system was totally uncovered, the pumps fixed and the pressure reset. From that time on, the monitoring process has been more conclusive in the diagnosis of the performance of the STS.

Because the East-facing collector field is hydraulically unbalanced, it reaches a constant value of temperature at least 60° C higher than the totalizing channel. As an example, Fig. 3 shows this behaviour.

The West-facing collector field is well balanced and shows no sign of abnormal behaviour thus the temperature read in the temperature probe is real and representative of the whole west facing collectors.

In April 2008 the STS presented its highest performance level (52%). Since then and because of the leakages previously referred the performance has been steadily decreasing at a pace of around 6% per month (see Fig.4).

Temperature Profile in a Typical Day

—West Col. Output —Inlet Temperature —East Col. Output —Outlet

image241

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

7/06/2008

Fig. 3. Typical temperature profile for the STS

image242

Fig. 4. Evolution of the performance.

But the system is operating satisfactorily with the right behaviour regarding the two azimuthal blocks of collectors. Fig 5 shows the time operation of both blocks through time of pump operation in a typical day of this last monitoring period. East sector begins first in the morning, then there is a period in the central part of the day with both sectors operating simultaneously and finally, in the last period of the day, only West sector is in operation.

Fig 5. Pump relays vs Flow

The solar DHW system

image188

The simulated building is an apartment house of two storied with 10 housing units. The central

type of DHW supply system was assumed, which composed of solar collector units, a storage tank and a boiler on the roof as shown in Fig.1. As the simulation result of the past study [1-3] showed that the efficiency of the DHW supply system is not strongly affected by the tilt angle and the azimuth of the collector when the tilt angle is from 20 to 40 degrees and the azimuth is from -15 to 15degrees (0 degree means to face to south). In this study, it is designed that the azimuth of the collector is faced to south and the tilt angle is 30 degrees.

Collector area [m2]

20

30

40

50

0.5

A20V0.5

A30V0.5

A40V0.5

A50V0.5

Storage

1.0

A20V1.0

A30V1.0

A40V1.0

A50V1.0

tank

1.5

A20V1.5

A30V1.5

A40V1.5

A50V1.5

volume

2.0

A20V2.0

A30V2.0

A40V2.0

A50V2.0

[m3]

2.5

A20V2.5

A30V2.5

A40V2.5

A50V2.5

3.0

A20V3.0

A30V3.0

A40V3.0

A50V3.0

Table 1 Simulation cases of the solar DHW system for the apartment house.

Подпись: Simulation cases As shown in Table 1, four cases of collector area was assumed from 20m2 to 50m2, and six cases of the storage tank was assumed from 0.5m3 to 3.0m3. Furthermore, three cases of DHW supply rate was assumed taking account of the DHW supply rate, which may vary according to family and lifestyle. DHW supply profile data that extracted 10 housing units from measurements of the DHW supply rate of 30 housing units were arranged to the supply rate of 10 minutes. As shown in Fig.2, the DHW supply profile data are prepared for a week and the same data are repeated in the next week. Three cases of DHW supply rate are Case A (average of 241 liters/day for a housing unit), Case B (average of 152 liters /day for a housing unit) and Case C (average of 66 liters /day for a housing unit). In addition, DHW supply rate varies depending on the season as shown in Table 2. The DHW supply temperature is set to 60 degrees C using the auxiliary boiler. The boiler Подпись: Room101 ,1 , J, 1. ill. Room102 ,1 „ 1 1 1 „ . Room103 . . .1 1 ,„ll . . .1 1 Room104 і и. 1 Room105 . .її ,i 1 , II 1. 1 ,1 Room201 і. 1 и. .lllll nil.. .1, ill, .1. 1 .ll Room202 1 . 1 1 . . ,. i,. 1 , , ,1.1 ll. 1 1,1. J . Room203 1 .1 . Il 1. 1. 1. ll ,J. Room204 ...і. 1.1 1, ll . ll .ll., ,1 l.l J . .1 , Room205 ii 1. 1., .ll Lu 1 . LLJ 0 12 24 12 24 12 24 12 24 12 24 12 24 12 24 „ 30

.E 20 1 10 “ 30

.£ 20 1 10 “ 30

.£ 20 1 10 “ 30

.£ 20 1 10 “ 30

.E 20 1 10 “ 30

.E 20 1 10 “ 30

.E 20 1 10 “ 30

.E 20 S 10 “ 30

.E 20 1 10 “ 30

.E 20 1 10 “ 0

Fig.2 DHW supply profile of 10 housing units in the simulation (Case A DHW supply).

Table 2 Simulation cases of DHW supply rate for a housing unit.

Simulation cases

Case A

Case B

Case C

Average of DHW supply rate for a

housing unit [liters/day]

Year

241

152

66

Winter

(1/1-3/31, 11/1-12/31)

269

163

72

Spring, autumn (4/1-6/30,10/1-10/31)

231

155

64

Summer

(7/1-9/30)

209

130

59

image191
image192

Summary and outlook

The directive for optimising the solar collector area and the storage device capacity based on economical and ecological criteria leads to a comprehensive evaluation of the system and its components. The variation of parameters concerning the thermal behaviour as well as the costs of solar collectors has revealed a high potential for reducing both the solar heat generation costs and the primary energy savings while improving the cost/benefit ratio. In opposition to the impact of

collector variations on the dimensioning, the variation of rises in energy prices does not alter the resulting dimensions; however the additional costs are strongly dependent on the rise in energy prices. Bearing in mind that this study was performed on a theoretical MaxLean system concept, the general validity of the conclusions drawn have to be confirmed in further studies involving standard market solar systems and current trends within the energy market.

Подпись: be negligible. For consistency reasons in this case only the influence of a reduction of the insulation thickness in comparison to the base case is presented.

Table 3. Influence of the varied parameters on the cost/benefit ratio and the dimensioning of the system. The symbols ++, + and о indicate a strong, medium or low sensitivity. (1) range of the additional costs within the variation; (2) range of the primary energy savings within the variation; (3) resulting solar collector area within variation (4) resulting storage device capacity within variation

Acknowledgements

The financial support of the Swiss Federal Office of Energy is gratefully acknowledged. Parts of

the work presented have been contributed to the IEA SHC Task 32. The authors would especially

like to thank the members of Subtask D for the fruitful discussions.

References

[1] Haberl, R., Vogelsanger, P.: Technical Report of IEA SHC Task 32 Subtask D, Simulation and Optimization of the MaxLean System, http://www. iea-shc. org/task32/publications/task32- MaxLean_Concept. pdf, 2007.

[2] Antony, F. et. al., Solarthermische Anlagen, DGS, Deutsche Gesellschaft fur Sonnenenergie, 2004.

[3] Haberl, R., Vogelsanger, P., Frank, E.: Dimensionierung solarer Kombisysteme, OTTI Symposium Thermische Solarenergie, Tagungsband, Bad Staffelstein, 23. — 25. April 2008, S. 422 — 427.

[4] Institut fuer Solartechnik SPF: Collector data base on SPF InfoCD-ROM, 2008, Collector factsheets also available on http://www. spf. ch.

[5] Huttmann, M. et al.,: Marktubersicht Solarspeicher 2007, solid GmbH, Furth, 2007.

[6] Schulz, W. et al.: Energiereport IV, Die Entwicklung der Markte bis zum Jahr 2030, Prognos AG, Koln, Basel, http://www. prognos. ch/pdf/Energiereport%20IV_Kurzfassung_d. pdf, 2007.

[7] Statistisches Bundesamt: Daten zur Energiepreisentwicklung — Lange Reihen von Januar 2000 bis Mai 2008, Wiesbaden, http://www. destatis. de, publication date: 26.06.2008.