OFF-GRID PHTOVOLTAIC SUPPLIED COOLING APPLICATIONS FOR REMOTE AREAS

Transcription

1 OFF-GRID PHTOVOLTAIC SUPPLIED COOLING APPLICATIONS FOR REMOTE AREAS Rainer Rudischer, Ingo Rittmüller, Wolfgang Hernschier Institut für Luft- und Kältetechnik (ILK) ggmbh Dresden, Solar Technology Department, Bertolt-Brecht-Allee, Dresden, D-01309, Germany, phone: , fax: , Abstract This paper describes two applications developed for remote areas with high ambient temperatures, where the cooling demand especially for various food-stuff and medicine goods is absolutely evident. The design and development results of a -ft food-stuff cooling container is presented in which an inner room temperature level of 0 C min and a net volume of 26 m³ is realised. An other modification presented is a 10-ft medicines cooling container which with three different cooling rooms inside for cold storage of different medicine goods according to their individual cooling levels ( C for vaccines; C for ointments and <+ C for e. g. pills). 1. INTRODUCTION Special adapted photovoltaic (PV) supplied cooling systems are an alternative to diesel powered cooling systems especially in remote areas if high solar radiation rates correlate very well with the time dependent power consumption demand. Especially with regard to maintenance needs and lifetime solar supplied systems will be more advantageous and competitive compared with diesel ones, so that one will make real benefit in total system costs taking into account the long operation time. Based on those facts two different sized solar cooling containers for different cooling applications were designed, manufactured and tested. One of them is special adapted to the needs of food-stuff cooling as a - ft container and the other one is a modified 10-ft container for cooling of medicines. The following main subjects concerning the design should be taken into account. First of all it has to be guaranteed that the system runs continuously as well under high temperature conditions combined with low radiation rates, and should at the same time meet the demand for low system costs. That means that the system has to be optimised with regard to a reduction of power consumption according to the possibility to install the smallest PV-array and energy storage, because these components cause the main part of energy supply system costs. Further the system has to work extremely reliable, maintenance free and over a life time of years. These years has been defined by the lifetime of present available solar modules. Only if these conditions will be met such solar cooling containers will compete successfully with diesel-electric systems and will have a chance on the market. Therefore a system has to be designed which fulfils the above mentioned features and is simple to handle, like a compact standard container design. Furthermore it has to be easy to operate and to use, that means to operate automatically, and has to be easy to install, e.g. bringing PV-array into working position by servo drives. The design, construction and tests of such systems is described as follows for a modified -ft food-stuff cooling container and a 10-ft medicines cooling container. 2. COMPONENT SELECTION For the realisation of such efficient systems it is very important to work out the adequate system conception especially with regard to the cooling cycle, and to choose the most efficient components of the energy supply, and to develop an optimised control structure. Another need is the minimisation of the capacity of the battery storage, which means a reduction in single components costs, which have to be exchanged during assumed life time of years. The main energy storage is a cold-storage tank, which is integrated in the cooling cycle. Such cold storage systems work more efficient than batteries and secure as well a correct operation during days with low solar radiation rates and high ambient temperatures with very low electric energy consumption rates. Besides the definition of the cold storage the cooling cycle was designed. The target was to realise a high efficient cooling cycle which can operate according to the actual available power of the PV-generator to minimise the energy buffer stock of the system, e.g. the nominal battery capacity, and should use R134a as refrigerant [1], [2], with no ozone depletion potential (ODP=0), a similar to R22 coefficient of performance (COP), and which has due to high ambient temperatures acceptable compression temperatures. Therefore different compressors, which can be operated speed controlled, were tested [3]. To increase the efficiency of the cooling system components like valves and vans were also optimised. For the design of the PV-energy supply system three aspects were decisive. The first design aspect was to

2 integrate the PV-array into the container structure. Therefore adapted to the given container dimensions PVmodules with a high efficiency have to be used. The second aspect was the connection of the modules to get the lowest current losses concerning energy transfer between PV-generator and PV-Inverter. Third aspect was the conception of the battery storage with respect to the battery voltage level according to the PV- Inverter/battery-charger and the main dc-consumers. An efficient controller unit was selected according to an optimised control of the system. The chosen controller has definitely a low power consumption and can solve the controller problems. Especially to reduce the power consumption was a big problem because most of such systems are not designed according to this aspect. 3. SIMULATION AND DESIGN To have a preview on the steady-state behaviour of different system designs a simulation program [4] was written, which allows the user to simulate the total system design not only with mean values but also in small time steps, because it is very important to consider not only some working points but the whole spectrum to adapt the system to the variable solar insulation and ambient conditions to the best design of stand alone solar systems. By using this program the user can not only vary the location of operation, the individual use and the simulation time, but also characteristics of the system components, for example of PV-modules, PV-inverters, battery-chargers and compressors, and other components of the cooling cycle. By this way the designer can find out the optimal components as well as the optimal control algorithm for the system. The simulation runs in consideration of the following influences: hourly varying ambient temperature cold room inner and outer dimensions cold room insulation type depending heat transfer outer walls orientation, and shading by PV-area number of inner walls and temperatures of adjacent room cold room and/or temperatures of stored goods sort, weight, and input/output time of refrigerated goods [5] modified battery and ice storage tank discharge depth defroster heating type influence of solar radiation on wall overheating In the following an example shows how the simulation can be used to get qualified information about the system, which can help to design and optimise it. Fig. 1 and 2 show the calculated heat loads of the predesigned 10-ft medicine cooling container done by the developed simulation program. Using these calculation results it was possible to decide about the time depending discharging mode of the cold storage, and by this way sets the rules for the design of the cold storage system and the lay out of the different storage rooms. required cooling power [W] Fig. 1: percentage of total required cooling power [%] Fig. 2: room 1 (+2 C) room 2 (+8 C) room 3 (+ C) all hour of the day Calculated required cooling capacity for the system during a period of 1 day with a maximum ambient temperature of +50 C room 1 & 2 room hour of the day Calculated percentage of single room cooling capacity to the total required cooling power of the system during a period of 1 day with a maximum ambient temperature of +50 C 4. FOOD-STUFF COOLING Based on the simulation results in a first step a -ft PVsupplied food-stuff cooling container was designed, constructed and tested [6],[7]. The main features of this system are:

3 stand-alone exclusively PV-supplied food-stuff cooling container with adapted cooling and energy supply systems transportable by lorry, train, and, if possible, helicopter integrated and hinged besides wall and roof PV-areas for transportation total outer dimensions similar to -ft standard container servo driven PV-area installation mode for uncomplicated and short putting into service 26 m³ cooling room volume for food-stuff coldstorage inner room temperatures down to a level of 0 C ( + 32 F) possible max. ambient temperature + 50 C ( 131 F) special developed control modes ready for operation container design, practically no maintenance need ambient temperature [ C] Fig. 4: Ambient temperature on the testing site from to After a preliminary test of the system which was done at the ILK Dresden the container was tested under real which means semi-desert conditions at the PSA (Plataforma Solar de Almeria)in Spain over a period of 14 months (Fig. 3). irradiation [W/m²] Fig. 3: -ft PV-supplied food-stuff cooling container at the PSA (Plataforma Solar de Almeria; Spain) In the following some test results are shown for the time between and Fig. 5: Irradiation on the aperture surface at the testing site from to Fig. 4 and 5 show the ambient conditions during the testing period. It is shown that the ambient temperature does not exceed +35 C. Therefore one cooling cycle and one energy supply was switched off for testing the system under harder conditions. During the period between and the container was full loaded. Fig. 6 shows the ambient temperature and the inner room temperature behaviour during this time interval. The inner room temperature does not exceed a range of ±1 K of the desired minimum room temperature level of 0 C.

4 temperature [ C] ambient temperature room temperature Fig. 6: Inner room temperature and ambient temperature during the period from to operating with only one cooling cycle and under full load On the 23 rd and 24 th June the container was discharged. During this discharging the cold storage was also discharged, caused by the long time open standing door (Fig. 7). During the next 3 days the container was closed and the cold storage was loaded again. On the 28 th June. the container was loaded with 0 l of warm water, the temperature of which was monitored. After 72 hours this water which was filled in into a 0 l tank was cooled down. The cooling down time of 72 hours results of the position of the water tank in the inner room and represents the centre temperature in the tank. The inner room temperature during this process does not exceed a level of +1 C again. This result confirms that the system is well designed and is reached by one cooling and one energy supply unit only. loading Fig. 8 shows the battery voltage behaviour during this period. It is shown that the system works with a good balance between power consumption (cooling unit) and power supply (PV-generator). The battery works during the day as buffer stock. This buffer stock is needed to balance the system and for operating the dynamically discharging of the cold storage, and for the control units. battery voltage [V] Figure 8: battery voltage of the system from to These results show that the system design meets the demands. Otherwise the results got by the simulation program confirm that it can be used to design other applications. 5. MEDICINE COOLING Based on the development results of the -ft food-stuff cooling container a 10-ft medicines cooling container was designed, manufactured and preliminary tested. Main features of the 10-ft container are the following: temperature [ C] Fig. 7: unloading room tempearure water temperature Inner room temperature and load (water) temperature during unloading and loading outer dimensions thermal heat transfer coefficient destination ambient temperature power supply cooling machine refrigerant energy storage operation transportation placement wind steady : modified 10-ft container incl. PV-array (w x l x h) 2495 x 3068 x 95 mm : < 0,3 W/m²K : remote areas with high solar radiation rates : max. +50 C (+132 F) : exclusively by PV-array : hermetically sealed : CFC free R134a : 1. cold storage 2. battery storage : maintenance free : lorry, ship, helicopter : without fundament : up to 180 km/h

5 Based on a modified standard container with steel-made outer walls and an inner double skin thermal insulation system the container is specially designed for storage of different medicine goods like blood conserves, ointments and pills which has to be stored on individual storage temperature levels. Therefore the Medicines Storage Container has three different storage rooms (Tab. 1, Fig. 9). room Temperatur max. goods e volume C 0 l vaccines, serum 2 < +15 C 0 l ointments 3 < + C 2,5 m³ e. g. bandaging materials Table 1: Layout of rooms for different storage temperatures Fig. 10: Transport situation of the 10-ft PV-supplied medicines cooling container Fig. 11: Operation situation of the 10-ft exclusively PV-supplied medicines cooling container Fig. 9: View of inner cooling room of the 10-ft PVsupplied medicine cooling container (low temp. cabinets left; normal temp. right) The PV-generator is similar designed to the -ft foodstuff cooling container version and integrated into the structure of the container (Fig. 10, 11) as well. This allows safe transportation of the complete system by lorry or ship. The PV-arrays are supported by a servo drive for an easy putting the system into operation. Contrary to the realisation of the -ft food-stuff cooling container this one was designed for the use of standard devices concerning for instance the PV-inverters and the control system. The installation of a standard compressor unit was impossible because there are no speed controlled cooling units according to the required cooling capacity available using the refrigerant R134a. The main problem in cases of using standard devices is their normally high power consumption. Besides this the adjustment of the PV-inverters in most cases is difficult with regard to the nominal PV voltage and nominal battery voltage. There are two configurations possible. At the one hand a 0 V PV-generator and a 1 V battery and on the other hand a 100 V PV-generator and a 48 V battery can be connected. Both combinations have some disadvantages. The first configuration needs a separate dc/dc converter to supply dc-devices installed in the system, and the other one causes higher electrical power losses caused of higher occurring currents and besides this the total electric design will be much more expensive. Furthermore both configurations are available for 230 V ac islanding-

6 systems only. Therefore an Inverter with variable frequency output is needed as compressor supply system. 6. CONCLUSIONS The presented work shows that it is possible to design and construct very efficient cooling systems supplied by a photovoltaic energy supplying system only. First of all the new developed simulation software which was used to pre-calculate and optimise the systems and the system operation is the basis to do so. It was shown that at present most components which are available on the market are not optimal for stand-alone PV-supplied cooling systems. The Prototype of the -ft food-stuff cooling container which was tested over a period of 14 months at the Plataforma Solar de Almeria (Spain) shows that such systems can meet all demands concerning cooling and cold storage in remote areas and under high ambient temperatures. Based on the experiences which could be got during the conception work, the development and test phase of the -ft food-stuff cooling container a 10-ft medicines cooling container was designed, constructed and tested successfully under middle European ambient conditions. OTTI, Regensburg. [5] Mann G. and Hofer B. (1977) Kühlraum- Lastfaktoren, 1 st edn. Pp Verlag Technik, Berlin. [6] Rudischer R. and Rittmüller I. (1997) SYSTEM CONCEPTION OF STAND-ALONE PHOTOVOTAIC SUPPLIED COOLING CONTAINERS. In Proceedings 14 th European Photovoltaic Solar Energy Conference, 30 June -04 July, Barcelona, Spain, pp , H. S. Stephens & Associates, Bedford. [7] Rudischer R. and Rittmüller I. (1998) DESIGN AND OPERATION RESULTS OF STAND- ALONE PHOTOVOTAIC SUPPLIED COOLING CONTAINERS FOR REMOTE AREAS. In Proceedings 2nd World Conference on Photovoltaic Solar Energy Conversion, 6-10 July, Vienna, Austria, pp , Office for Official Publications of the EC, Luxembourg. 7. ACKNOLEDGMENTS This work is partly supported by the German Federal Ministry of Economics and Technology under the Project-N A, and as well by the Saxonian Freestate s Ministry of Economics and Labour. The authors are responsible for the content of the publication. REFERENCES [1] Wobst E. and Vollmer D. ILK Dresden Special brochure KÄLTEMITTEL - Stand / Tendenzen ILK-Thesen zur Kältemittelsubstitution (published) [2] Wobst E. and Hommann G. ILK-Dresden Special Report " Kältesysteme für Solar-Kühlcontainer", Report N ILK-B-3/ (internal report, unpublished) [3] Wobst E. and Andersch H. ILK-Dresden Special Report "Leistungsmessungen an einem frequenzgeregelten Rollkolbenverdichter", Report-N ILK-B-3/ (internal report, unpublished) [4] Rittmüller I. and Rudischer R. (1997) PVCOOL- Ein Simulationsprogramm zur Auslegung von PVversorgten Kühlanlagen. In Proceedings Zwölftes Symposium Photovoltaische Solarenergie, February, Staffelstein, Germany, pp ,