Question 8.1: A geothermal aquifer supplies hot water at a temperature of ......
A geothermal aquifer supplies hot water at a temperature of 70°C at a flow rate of 10 L/s. The hot water is used in a district heating system at a temperature of 40°C. It is assumed that all the thermal energy can be transferred through a heat exchanger. The geothermal aquifer is used for 150 days each year. Calculate the amount of heat used for heating during a year. Calculate the amount of oil necessary to get the same amount of heat assuming an efficiency of 80% for the fuel-fired boiler.
The annual flow rate is 130\times10^{6} L/year and the heat transferred is that number multiplied by 30 (which is ΔT ) × 1000 (1 cal increases 1 cm³ of water of 1 °C). This gives 3.9\times10^{12}\mathrm{\ cal}=1.6\times10^{13}\mathrm{J}=4.5\ 10^{6}\mathrm{kWh}. One ton of oil corresponds to 42\,\mathrm{GJ},\;11,600\,\mathrm{kWh},\;\mathrm{or}\;\;10^{10}\mathrm{\ cal}. The amount of oil giving the same amount of heat is equal to 390/0.8 ≈ 490 tons of oil.