Angewandte Systemanalyse 34 : Performance and cost analysis of pressurized and non-pressurized domestic solar water heating systems with thermosyphonic flow

Two commercially available domestic hot water heating systems, in which natural convection maintains the flow of water from the collector to the tank were studied experimentally under identical meteorological conditions. One of the systems consisted of a meander type of flat plate collector (1.5 m$^{2}$) connected to a 110 l storage tank, which was maintained under the pressure of city water supply (system I . The other system consisted of two flat plate collectors (2 x 0.88 m$^{2}$) connected in parallel to each other with a 200 1 pressure-free water storage tank (system II). Corresponding to a nice sunny day i. e. 10th June, 1982 at Juelich, West Germany (latitude 50°55'33"), the average temperature in the tank of the pressurized water system reached a maximum of about 51 °C startingfrom an initial temperature of 15 °C in the morning, while the average tank temperature in the non-pressurized system reached a maximum of about 44 ° C corresponding to the same initial temperature. For an incident solar energy of 6.75 kWh per m$^{2}$ of collector area, the useful energy available from the pressurized and the non-pressurized system is 3.0 kWh and 3.8 kWh per unit collector area respectively, yielding over the day an average efficiency of 45 and 57 percent. By using Hagen-Poiseuille's law a theoretical expression for the mass flow in thermosyphonic systems is derived with the socalled effective tube length as the only system-dependent parameter. Experimental values of the mass flow obtained only from temperature measurements in collector and tank and a simple energy balance equation are in good accordance with the theoretical formula. With the collector parameters obtained from the HWB-collector model and meteorological data, the time-variation of the average tank temperature is evaluated and compared with the experimental data. The effect of reverse flow and heat losses during the night for both systems are shown by measurements,as well as the effect of alternatively using a double glass cover in the collector of the pressurized system. A cost analysis helps to calculate the cost of energy supplied by the water heating systems. Numerical calculations are performed for typical market conditions of two developing countries, namely Egypt and India. These show that such domestic solar water heating systems have considerable market potential in comparison to conventional hot water systems such as gas boilers and electrical boilers