This project has been carried out by Fabian Ochs, a master student of the author, during 2001/02. Figure 10.25 shows the basic layout of the system: The PV generator consists of two parallel-connected, frameless 30 Wp modules. Located in the foundation structure are a maintenance-free lead-acid battery (12 V, 105 Ah) and a 200 W sine inverter (115 V, 60 Hz) with an integrated charge controller (6 A). A water tank cools all components. The output leads to a regular AC plug. All components are contained in a waterproof epoxy fiber glass tank. The prototype is 1.37 m long, 0.76 m high and 0.5 m deep and has a volume of 0.3 m3. It can be transported easily when empty (20 kg), and is fixed when filled up with water at the installation site (320 kg) without necessity to penetrate the roof surface (see Figs. 10.25, 10.26).
water in et
Inverter mcl. charge controller
AC outlet (115V, 60Hz, 200W)
Battery (12V. 105Ah)
Fig. 10.25. Internal structure of the Integrated Solar Home System (I-SHS).
A module elevation angle of 30° was chosen to achieve a good yield even in winter in most parts of Brazil. The tank has a volume of almost 300 liters, which results in a weight of 300 kg, when full. The tank acts as an efficient cooler for the PV modules. The aluminum back of the PV modules allows for good heat transfer to water stored in the tank. The water, with its high thermal capacity, limits solar cell temperatures to a range in proximity to that of ambient temperatures (see Fig. 10.27).
The increase of cell temperature relative to ambient temperature was measured for several days in Rio during March 2002 and is shown in Fig. 10.28 as a function of irradiance in comparison to the equivalent values for a conventional SHSs (see Messenger and Ventre 2000, Krauter 1998, Krauter and Schmid 1999).
Despite a relatively wide spread in values, mainly due to wind speed variations, the following linear approximations can be extracted:
0.03 • G (W/m2)-1 K 0.012 • G (W/m2)-1 K 0.0058 • G (W/m2)-1 K
G stands for global irradiance, TSHS for module operation temperature of a conventional SHS (“Reference Case”) as measured Krauter 1998, Krauter and Schmid 1999) or given in literature (Messenger and Ventre 2000, Krauter and Hanitsch 1996). Tupper stands for the temperature of the upper module and Tlower for the lower module in the I-SHS. All temperatures are given in Kelvin (K) or degree Celsius (°C).
100 200 300 400 500 600 700 800 900 1000
Global irradiance (W/m2)
Fig. 10.28. Differences between module and ambient temperature in comparison to the reference case (conventional SHS) plotted as a function of irradiance.
In previous experiments, a reduction in cell temperatures during operating time increases the electrical yield by up to 12% (Krauter 1995, Krauter 1996). Due to the stratification observed, the I-SHS showed just a 9% gain. Forcing circulation in the tank would certainly result in higher electrical yields, on the other hand, stratification serves very well for an optional thermal use of the system. Additionally, the hot water generated is sufficient for the consumption of a small household in Brazil. The upper module can also be replaced by a thermal absorber and would boost hot water generation.
For testing purposes a single prototype was manufactured. Mass production of the I-SHSs (e. g., by recycled PP or PE) would be fast and
inexpensive with production costs of less than 50 € for mass production. Material problems related to UV-stability and the drying of the plastic seem to be solved – manufacturers of similar tanks (used as floating docks for boats) give guarantees of ten years. The prototype’s form is shaped from a block of expanded polystyrene (EPS). Subsequently the container-tank, consisting of six parts, was laminated using a fiber glass and epoxy resin. To allow for modifications the modules are mounted in a detachable fashion. A cable channel through the tank was installed to simplify maintenance. Additionally, fixed integration of the PV modules within the tank would provide improved performance. Construction time for the prototype was less than a week. Material costs for the prototype have been 420 €.