A Rankine cycle power plant is being developed to operate an irrigation system. In this power plant solar energy will be used to boil a low boiling point fluid within glazed flat plate solar collectors. The working fluid that has been selected is the commonly used refrigerant R 134a (1, 1, 1, 2 tetrafluoroethane), tabulated properties of which may be found in Tables A11, A12 and A13 at the rear of the prescribed text.
The plates being used for the solar collector are made of 2 layers of aluminium bonded together for much of their area and incorporating tubular passages of oval cross-section in the unbounded areas, within which the refrigerant boils. The working pressure of this construction is 1600 kPa. It is intended that the refrigerant will boil at this pressure and be slightly superheated, thereby bringing the temperature up to 60° C. The quality of the glazing and insulation on the solar collectors is such that at these fluid conditions, and with a solar intensity of 1 kW per square metre onto the glazing, the efficiency of solar energy collection into the fluid is 70%.
The irrigation pump is responsible for taking cold water from a stream, through the condenser of the power plant and on from that to drip irrigation trickle hoses in the fields nearby. The condenser and a water pump are positioned in a concrete lined pit below the stream level to ensure the pump is always primed and the condenser water tubes are free of air locks. The trickle hoses are laid out in the fields above the stream level. When the power plant is operating at the design condition of 1 kW per square metre solar intensity, the water flow rate through the condenser is to be such that it rises in temperature from 10 to 30°C. At this condition the saturation temperature of the refrigerant in the condenser will be 35° C. The refrigerant is to enter the boiler feed pump at the saturation temperature as a fully condensed liquid.
1. For the ideal case where the feed pump and turbine are isentropic and the electric motor on the feed pump and the electric generator are 100% efficient:
a. Determine the specific enthalpy of the refrigerant at the entry and exit of the feed pump, and the entry and exit of the turbine;