Model description

A model of a solar heating plant is built in TRNSYS. The collector array consists of 100 rows where the distance between the rows is assumed to be so large that the shadows between the rows have negligible influence on the collector performance. The energy consumption of a town is defined by a water mass flow rate, a return temperature and a flow temperature of 80°C.

If the temperature from the solar heat

exchanger is above 80°C the temperature is mixed down to 80°C with at three-way valve.

If the temperature from the solar heat

exchanger is below 80°C, an auxiliary boiler plant heats up the district heating water to 80°C.

An illustration of the TRNSYS model can be seen in Fig. 7 and Fig. 8 shows the mass flow rate and a flow and return temperature through out the year for the district heating net of the town. The annual heat consumption of the town is about 32500 MWh.

The collector performance is investigated for two locations:

• Copenhagen, Denmark, lat. 56°N, yearly average ambient temperature: 7.8°C. Weather data: DRY (Lund H. (1995). ).

• Uummannaq, Greenland, lat. 71 °N, yearly average ambient temperature: -4.2°C. Weather data: TRY (Kragh J. et al (2002). ).

The stratifying solar heat exchanger of COAX 390 consists of a tightly wound smooth coil which is surrounded on the inside and outside by a synthetic pipe (figure 1). The heat transfer fluid is carried in a spiral from top to bottom against the up-flow of the domestic hot water being heated. Thus the heated water can be carried into the middle of the tank or into the heating zone at the top, where the water is then released through openings. Sufficient radiation provided, the heated domestic hot water is immediately ready for con­sumption, which leads to fewer boiler start ups and the water remaining cool at the bottom of the tank for longer time periods, which increases the energy yields of solar applications.

A specific construction of these pipes, that optimises the channel widths for low-flow rates of the solar circuit as well as that for the domestic hot water leads to a more intensive im­mersion of the heat transferring surfaces, even at low-flow rates. The surface-specific heat-transfer-coefficient is therefore two to three times that of conventional immersed smooth coils. Figure 2 illustrates the heat-transfer-coefficient of an enamelled smooth coil heat exchanger (reference tank of ITW) in comparison to measurements of the COAX heat exchanger.

The result of this extremely efficient heat transfer is that the temperature at the entrance of the solar collector is only 1 to 5 degrees higher than the temperature at the bottom of the

Figure 2: Comparison of the COAX heat exchanger with a conventional enamelled heat exchanger (1" pipe, area 1.7 m2). Measurement: ITW, University Stuttgart

tank, which leads to minimised heat losses of the collector whilst achieving maximum yield. The heat exchanger has been developed especially with view to scaling problems of filigree heat exchanger structures, as they are used by other suppliers for low-flow: here the smooth stainless steel areas combined with the high current and the exposure to con­stantly changing temperatures show a significantly reduced tendency to scaling than con­ventional immersed heat exchangers. When the heat exchanger is hot the upward current starts up thus scale precipitates primarily in the water. It is then rinsed from the tank or sinks down to the bottom of the tank where it can be removed via the cold-water connec­tion.

The system efficiencies of the fagade-integrated solar collectors with green black, blue and white absorbers were measured (Svasand, 2003). The measurements were corrected for the thermal contributions of the system components and the collector efficiency was deduced. As the efficiency curves revealed large uncertainties for very small and large AT/I, Table 4 lists the collector efficiency n for AT/I = 0.03 (Km2)/W, which lays within the typical range of operation of the present collector.

For the green absorber the collector performance in a fagade with and without ventilated cavity is almost equal within the uncertainty. As confirmed by the absorptance measurements (Fig. 8), the efficiency of the green absorbers was very close to the black absorbers.

No measurements were available for the black absorbers in the fagade with ventilated cavity. The large uncertainties for the collector efficiency for the blue and white painted absorbers are caused by the fact that the measurements were carried out late in the year (September, October).

The solar power of 184W was successfully transmitted away from the focal area of the primary mirror to a laser head by 36 flexible optical fibers. Power transmitted by each fiber shows variation less than 0.5%, facilitating hence the further uniform pumping of a 4mm diameter Nd:YAG laser crystal. The optical fibers were mounted around the flow tube via an aluminium part that provided a half cylindrical 4 x 9 matrix fiber distribution as shown in Fig. 7a) and 7b).

Fig. 7(b) — Optical fiber bundle output end view

The output light characteristics of the presented two stage transmission system, allowed the use of thin laser rod with efficient light coupling which is advantageous as the cooling is more efficient, reducing both thermal gradient and thermal lensing effects.

To maximize the flux energy in the active medium, curve polishing at the output end of the optical fibers was done. This allows a better match of the spot-size in the focal region. Theoretical analysis was confirmed by performing the Zemax ray-tracing program.

Figure 8 — Zemax ray-tracing. a) Plane extremity, b) Curved extremity

To maximize the flux energy in the active medium, it was necessary to use a double pass scheme. This was accomplished by depositing a cylindrical reflector on half of the internal wall of the flow-tube.

The Nd:YAG rod, with 1.1at.% Nd3+, 4mm in diameter and 25mm in length was inserted in the center of flow-tube center, with 8mm external diameter and 12mm in length.

(a) (b)

Figure 9: (a) — Pumping scheme; (b) — Optical fibers coupling.

The resonant cavity was formed by a 94% reflectivity output coupler of -1.0m radius of curvature and a 100% mirror of the same curvature. The cavity length was 650mm and the Nd: YAG rod was positioned at the centre of the resonator.

In order to prevent laser rod from becoming solarized and to reduce unwanted temperature rise, a doped flow tube was used. Demineralised water was used as a coolant and the flow rate was 2.2l/min.

The laser output power was measured as a function of the input power (Figure 11). Here the input power is taken to mean the power collected at the entrance of the light guide. The variation of the input power was achieved by masking the primary mirror with ring pieces of non-reflecting material, which affected the collecting radius of the primary concentrator and the flux distribution on the focus.

The maximum output power obtained was 2.46W, for an overall efficiency of 0.69% and slope efficiency greater than 1.6%. This is a reasonable value for the conversion efficiency of a solar-pumped Nd:YAG laser. The output power was stable to within 3% for periods of several minutes. The threshold was determined to be approximately 220W of the power collected by the light guide.

8. Conclusion

The low efficiency and high lasing threshold was attributed to the low transmission efficiency of the fiber bundle due to the angle-dependent transmission and also to coupling loss between fibers and light guide. Only 50% of solar radiation at the light guide entrance was transmitted to the laser crystal, in comparison with the 85% obtained by Weksler and

J. Shwartz3 with a CPC.

Further improvements in homogeneity of the absorbed pumping power can be achieved by using large numerical aperture (NA=0.66) optical fibers mounted around the laser rod as shown in Fig. 12. To guarantee that the optical energy is focused on the rod due to the highest NA, the output extremity of fibers must be close to the rod.

01

9. References

(1) R. Winston “Light collection within the framework of geometrical optics”, J. Opt Soc. Am, 60, 245-247 (1970)

(2) H. Arashi, Y. Oka, N. Sasahara:”A Solar-Pumped cw 18W Nd. YAG Laser,” Japan. J. Appl. Phys., vol.23, No.8, 1051-1053, 1984

(3) M. Weksler, J. Shwartz, “Solar-pumped Solid-State Lasers”, IEEE Journal of quantum electronics, vol. 24, No. 6, June 1988

(4) D. Cooke, “Sun-pumped Lasers: revisiting an old problem with nonimaging optics,” Applied Optics, vol.31, No 36, 7541-7546, 20 december 1992

(5) D. Feuermann, J. M. Gordon, M. Huleihil, “Solar Fiber-optic mini-dish concentrators:first experimental results and field experience,” Solar Energy Vol.72,No.6., pp. 459-472,2002.

(6) R. John Koshel and I. A. Walmsley, “Modeling of the gain distribution for diode pumping of a solid-state laser rod with nonimaging optics,” Applied Optics, vol.32, No. 9, 1517-1527, 20 march 1993.

(7) N. Pavel, Y. Hirano, S. Yamammoto, Y. Koyata, T. Tajime, “Improved pump-beam distribution in a diode side_pumped solid-state laser with highly diffuse, cross-axis beam delivery system,” Applied Optics, vol.39, No 6, 986-992, 20 february 2000.

(8) L. R. Mashall, A. Kaz, and R. L. Burnham, “ Highly efficient TEM00 operation of transversely diode-pumped Nd. YAG lasers,” Opt. Lett. 17, 186-188 (1992).

(9) D. L. Evans, “On the performance of cylindrical parabolic solar concentrators with flat absorbers,” Solar Energy, Vol. 18, pp. 379-385 (1977).

(10) D. Liang, S. Duarte, J. Trindade, D. Ferreira and L. F. Monteiro, “High power solar energy transmission by solid-core fused silica light guides,”

(11) J. Harris and W. S. Duff, “Focal plane flux distributions produced by solar concentrating reflectors,” Solar Energy, Vol. 27, pp. 403-411 (1981).

An important possibility for the development of medium temperature collectors is to reduce heat losses by concentration. For example INETI in Portugal is investigating a stationary CPC type collector without vacuum for medium temperature applications /4/. The concentration factor is in the range of 2.

Also the developments of MaReCo’s (=maximum reflector collectors) of Vattenfall and Finsun Energy AB in Sweden is based on using reflectors for improved performance at higher temperatures /5/. Figure 3 shows an application of an early MaReCo collector in a district heating system in Sweden.

Small parabolic trough collectors

Especially for the temperature range of 150°C to 250°C it is extremely interesting to consider the parabolic trough collector technology. A lot of experience is available from the high temperature applications where parabolic trough collectors are used at 400°C to 600°C for electric power production. But adjustments have to be made for the medium temperature range. As examples for current investigations with this aim developments from Spain, Austria, and Germany are mentioned:

CIEMAT in Spain plans to develop a parabolic trough collector (‘FASOL’) with an aperture width of 2.62 m and a focal distance of 0.7m. The length of every parabolic trough module will be 6 m and about 8 troughs will form a collector with tracking device. The concentration factor will be in the range of 20 to 25 /6/.

AEE INTEC in Austria works on the development and optimisation of a small parabolic trough collector with glass cover for operating temperatures from 100 to 200°C. The prototype dimensions of a module are 0.5 m x 4 m and the focal length is 10 cm. The first prototype has a receiver with a diameter of 8 mm, a non — evacuated glass cover tube
and was coated with a non-selective varnish /7/, see Figure 4. This Austrian national project is funded by the Austrian Ministry of Transportation, Innovation and Technology as part of the research program ‘Fabrik der Zukunft’. Co-operation partners are the manufacturer of the parabolic trough (the company Knopf Design) and a number of other Austrian companies.

In Germany, a parabolic trough collector is developed by DLR and SOLITEM /8/. A first collector field was recently installed in an hotel in Turkey, see Figure 5.

In Israel, the company SOLEL is marketing a parabolic trough collector. It is for example installed in an application in which solar thermal energy is used for the production of heat, cooling and electricity.

The collector field consists out of a set of 4 modules of 6 rows of 48 m long parabolic troughs. The collecting area totals to 860 m2. The collector field is installed 10 meters above the company’s parking lot and working area, see Figure 6.

A water sprinkling system is mounted on each row of collectors to wash off dirt and prevent deterioration in performance. The water is filtered and recycled. The heat carrier of the solar loop is thermal oil. The collector output is used to heat the water in the steam generator at a temperature up to 180° C. This then powers a condensed steam turbine and produces 50 kW of electricity. The waste heat is stored in a 20 m3 water tank supplying enough hot water for space heating in winter as well as 30 tons of air conditioning (by absorption) in summer for the offices and the corporate manufacturing facilities. The system is installed in Beit Shemesh, Israel.

01

Conclusion

For applications where temperatures up to 250°C are needed the experiences are rather limited and therefore also suitable collectors, components and systems are missing. The aim of IEA Task 33/4 is to make use of the huge potential for solar heat in the industry and to open new market sectors for the solar thermal industry. The aim is to integrate solar thermal systems into industrial processes in the best and most suitable way. To achieve this, new ‘medium temperature collectors’ have to be developed for the temperature range of 80°C to 250°C. Improved flat plate collectors, collectors with reflectors and low concentration factors as well as appropriate parabolic trough collectors are under development. Solar heat for industrial processes is needed for applications such as air conditioning and cooling, solar ice production, heat for food industry, textile industry, washing, processes in diary farms, pasteurisation, sterilisation, water purification, disinfection and sea water desalination. In these sectors, the new medium temperature collectors will open new markets.

Acknowledgement

Many thanks for their input to this paper to the colleagues from Subtask C, especially to: Maria Joao Carvalho, Esther Rojas Bravo, Bjorn Karlsson, Klaus Hennecke, Dagmar Jahnig and to Werner Weiss.

References

/1/ Matthias Rommel, Andreas Gombert, Joachim Koschikowski, Arim Schafer, Yan Schmitt, Which Improvements can be achieved using single and double AR — glass covers in flat-plate collectors? European Solar Thermal Energy Conference estec 2003, 26 — 27 June 2003, Freiburg, Germany

/2/ Matthias ROMMEL, Joachim KOSCHIKOWSKI, Marcel WIEGHAUS, Thermally driven desalination plants based on membrane distillation, International Conference ‘RES for island — Tourism & Water, 26-28 May 2003, Crete, Greece

/3/ M. Hermann, J. Koschikowski, M. Rommel, Corrosion-free solar collectors for thermally driven seawater desalination, SOLAR ENERGY, Vol 72, No.5, pp. 415-426, 2002

/4/ Maria Joao Carvalho, Stationary CPC type collector without vacuum for medium temperature applications (air-conditioning, industrial applications) (2004) mjoao. carvalho@ineti. pt

/5/ Bjorn Karlsson and Gunnar Wilson, MaReCo design for horizontal, vertical or tilted installations, Eurosun 2000, Copenhagen (bjorn. karlsson@vattenfall. com)

/6/ Esther Rojas, FASOL collector, CIEMAT-PSA, ES, 2004 (esther. rojas@ciemat. es)

/7/ Dagmar Jahnig, Development and Optimisation of a smal-scale parabolic trough collector for production of process heat, AEE INTEC, 2004 (d. jaehnig@aee. at)

/8/ Klaus Hennecke, Solitem PTC 1800, 2004 (Klaus. Hennecke@dlr. de) and http:www/solitem. de

/9/ http://www. solel. com

E. Streicher, W. Heidemann, H. Muller-Steinhagen

University of Stuttgart, Institute for Thermodynamics and Thermal Engineering (ITW) Pfaffenwaldring 6, D-70550 Stuttgart Tel.: 0711 /685-3536, Fax: 0711 /685-3503 email: streicher@itw. uni-stuttgart. de

An important number for the assessment of thermal solar systems regarding environmental aspects is the energy payback time. This is the period, the system has to be in operation in order to save the amount of primary energy that has been spent for production, operation and maintenance of the system.

The present paper outlines the methodology for determination of the energy payback time of thermal solar systems. It is explained how factors like pump operating hours or fractional energy savings influence the energy payback time. This will be demonstrated by calculating the energy payback time for a typical solar domestic hot water system (SDHW system).

Solar heating systems for combined domestic hot water preparation and space heating, so-called solar combisystems, are more complex in their structure than SDHW systems. As solar space heating can be realized with different system concepts, a uniform methodology is necessary for comparison of different solar combisystems. This uniform methodology is presented in the second part of this paper.

Simon Furbo and Soren Knudsen
Department of Civil Engineering
Technical University of Denmark
Building 118, DK-2800 Kgs. Lyngby
Denmark

Fax: +45 45 93 17 55

1. INTRODUCTION

Investigations have shown that small SDHW systems are best designed as low flow systems with a vertical mantle tank [1], [2], [3], [4], [5], see Fig. 1.

A simulation model, MANTLSIM, for small low flow SDHW systems with a vertical mantle tank was originally developed and later modified at the Technical University of Denmark [6], [7], [8], [9], [10]. MANTLSIM can be used to calculate the yearly thermal performance of a solar heating system based on weather data from the Danish Test Reference Year TRY [11]. Recently the model was further improved and validated [12], [13]. The improvements were based on detailed studies of the fluid patterns and the heat transfer, both in the vertical mantle and in the inner domestic hot water tank. The studies were carried out by means of CFD (Computational Fluid Dynamics) models. These models were validated by means of experiments, both with a mantle tank in a heat storage test facility and by means of PIV (Particle Image Velocimetry) measurements with a transparent glass mantle tank.

With the CFD models parameter analyses were carried out for differently designed mantle tanks under typical operation conditions. Based on the analyses a number of Nusselt-Reynolds-Rayleigh heat transfer correlations were developed for the heat transfer between the solar collector fluid in the mantle and the inner and outer
mantle walls and between the tank wall and the domestic water in the hot water tank.

Thermal stratification is built up in the hot water tank due to natural convection in the tank. By means of CFD calculations for typical operation conditions, a method was developed to determine the heat transfer in the hot water tank caused by the natural convection.

Based on CFD calculations for typical operation conditions, a method to determine the mixing inside the mantle caused by the incoming solar collector fluid was developed.

The developed heat transfer correlations and methods to determine the heat transfer in the hot water tank caused by natural convection and to determine the mixing in the mantle were utilized in MANTLSIM. MANTLSIM was validated by means of measurements in a test facility for solar heating systems for two low flow solar heating systems with mantle tanks — one with the mantle inlet at the top of the mantle and one with the mantle inlet placed with a distance from the top of the mantle of about one fourth of the mantle height.

The measurements of the thermal performance of the two low flow systems as well as results of calculations of the yearly net utilized solar energy of low flow SDHW systems with differently designed mantle tanks will be presented in this paper.

Michael Hermann

Fraunhofer Institute for Solar Energy Systems
Heidenhofstr. 2, 79110 Freiburg, Germany
Tel.: +49 (0) 761 /45 88-54 09, Fax: +49 (0) 761 /45 88-94 09
michael. hermann@ise. fraunhofer. de. http://www. ise. fraunhofer. de

The energy efficiency of heat exchangers such as solar absorbers is determined both by their thermal efficiency — evaluated by the collector efficiency factor F — and the primary energy which is needed to drive the pump transporting the fluid. The former is strongly influenced by the uniformity of the volume flow whereas the latter also depends on the pressure drop in the fluid channels. Thus, in order to ob­tain a high energy efficiency, it is necessary to ensure a uniform flow distribution with low pressure drop.

However, conventional hydraulic structures often show a high pressure drop (serial flow) or a non-uniform flow distribution (parallel flow). In contrast to these channel designs, many natural structures are built of multiple branched channels ("fractals"). The aim of a current research work, which is funded by the German Federal Environmental Foundation (DBU), is to transfer those principles of fluid channel design to technical applications (bionic approach) and compare the struc­tures with conventional ones.

This paper describes how fractal hydraulic structures are generated and assessed using hydraulic and thermal simulations. Flow experiments as well as thermo­graphy with an absorber model are shown. Furthermore, investigations of flow phe­nomena using Computational Fluid Dynamics (CFD) are presented.

The efficiency of solar collector was evaluated by comparing the total radiant heat energy flux to the solar wall with the change in energy of the transpired air after passing through the collector. The estimates of efficiency are only presented for when irradiance level was 300W/m2, suction airflow rate (Vs) was 20~133.34m3/h/m2 and different wind speeds (U) were 0m/sec, 1.6m/sec, and 3.1m/sec.

The purpose of this work was to evaluate the practical performance of the solar collector as a function of wind speed. The efficiency as defined in Eq.2 was used since it measures the conversion rate of solar energy to useful energy in the transpired air stream. It was observed
that the most important factor in determining efficiency was the irradiance level and plenum temperature. Following this we compared the system efficiency for different wind speeds under different suction airflow rates.

Table 2 shows that peak efficiencies do not occur at high wind speed. The data seem to suggest that the collector operate at peak efficiency when the wind speed is 0 m/sec. It is clear from the results (see Fig.4) that the efficiency of solar collector increases proportionally with the increase of airflow rate and decreases with the increase of wind-speed.

Results show that at low airflow rate, the efficiency of the collector is low and at high suction velocity the efficiency of the solar collector is high. The efficiency of solar collector reaches a maximum value of 67% (at Vs =133.34m3/h/m2 & U=0m/sec) and a minimum value 17% (at Vs =20m3/h/m2 & U=3.1m/sec) being reduced by 50%. This reduction in efficiency is only due to the decrease in airflow rate. And reverse trend in efficiency has been observed for different wind speeds. Results show that different wind speeds effect the efficiency of solar absorber i. e. the efficiency decreases with the increase of wind speed.

The main reason of these variations in the efficiency is that the solar collector, which was installed perpendicularly in front of the artificial solar light source, absorbs the radiations and the energy absorbed by the collector bed increases the plenum temperature (see Table.3). The fan then propels the heat collected in the plenum to the duct. As we increase the airflow rate more heat is transferred to the duct, where the T-type thermocouple measured the temperature.

Table. 3. Plenum and ambient temperatures at different airflow rates under different wind — speed conditions.

The reason in decreased efficiency with the increase of wind speed is that, when wind deflector is in operational form, it deflects the air parallel to the face of the collector. When this wind with ambient temperature of 21oC ~ 22oC enter into the tiny holes of the collector bed it causes to decrease the plenum temperature. Also continuous sucking of hot air out of plenum area causes to decrease the efficiency.

The reliable and safe provision of fresh water is becoming an increasingly important issue world-wide either due to its scarcity (quantity problem) or because of its low quality (pollution or salt problems). The historical approach of developing new water sources to meet the rapidly increasing demand has reached its limits since inexpensive resources have already been developed and new ones are prohibitively expensive to exploit. In many areas, Desalination could be an alternative option for fresh water supply. Desalination is an energy intensive process and Renewable Energy Sources could provide an environmentally sustainable prospect to this problem.

The main objective of the present action is the development, installation testing and performance evaluation of an innovative stand-alone, solar desalination system. The system consists of vacuum tube solar collectors and produces mechanical work through the application of a Low Temperature Organic Rankine Cycle (SORC). The generated mechanical work drives a Reverse Osmosis (RO) desalination unit.

The system performance depends on the availability of solar source and the target potable water cost to be achieved. The latter depends strongly on the competitive price of alternative water supplies. It is known that in arid regions, where the quantity problems are predominant, water prices are high and even a low efficient desalination process (which is based on a low-capital cost plant) can be competitive in the real market meaning of the term. In non-arid regions the water problems are frequently related to quality problems. Especially for the first case SORC for RO desalination can provide a cheap water supply to a more complicated water producing technology.

The efficiency of RES-powered desalination systems depends strongly on the continuous energy supply. In case of applying solely a solar desalination technology, the efficiency is low because of the night cease of operation of the thermal unit. This can be acceptable for arid regions where the competitive fresh water supply may be rather expensive. However, solar Rankine cycle is characterised by the highest possible efficiency since approaches the efficiency of Carnot cycle. Its application for RO desalination can be ideal solution. Additionally, the proposed technology is flexible enough to be integrated to other thermal sources of continuous heat generation such as industrial thermal wastes, geothermal energy or even energy from biomass or Municipal Solid Wastes (MSW). The optimum design technique to be applied will assure a 24-hours per day water supply taking into consideration the adequate water storage. Water desalinisation has become increasingly important in many isolated areas and coastal regions where the natural inlet of rain water is sustainable below of the population demands either for the natural comfort or for the demands of new activities such as tourism.

Technologies for water desalinisation have moved from heat and vapour solutions (very costly in terms of energy use) to more efficient membrane separation technologies (RO in acronym form) that albeit continue to use energy (in electricity form) are much more energy efficient. Nevertheless all these technologies require a stable and substantial electricity supply that in some cases is not easily ready or affordable in economic terms.

For that reason, in parallel with membrane technologies based on grid electricity, new solutions have been developed that combine the use of renewable technologies (mainly wind energy or in some cases trough the combustion of biomass or residues) with that rO technologies to produce desalinised water in a smaller more decentralised scale.

Nevertheless, all these solutions have a technological drawback, which is the need to transform the firstly produced mechanical energy (in the form of wind force or combustion steam) into electricity (with an energy transformation loss of around 60%).

In this context, is where it steps this new technological approach that use the mechanical energy derived from solar collectors to run the mechanical parts associated with the
reverse osmosis technologies, so producing a very important leap forward in the energy efficiency of the whole system.

The benefits of the utilisation of SORC as energy carriers are the following:

• Solar energy is a sustainable and renewable resource, particularly in those countries, which have very little or no reserves of good quality fossil fuels as in all Southern EU countries.

• The application of SORC is much less harmful for the environment compared to conventional fuels.

• The integrated system is cost effective since SORC is a cheap technology (compared to other solar thermal technologies) with very low maintenance cost.

There is also a strong interest on EU policies to guarantee the viability of natural resources. For the years ahead, the water resources are going to be gradually declined. So, there is a requirement to regenerate this resource in an environmentally friendly way. The most safe and promising route to achieve this is the application of renewables. The exploitation of a natural resource (solar energy) to produce another natural resource (water) is an ideal solution.