7-19 A commercial district is currently heated by coal-burning heaters in winter with an average efficiency of 75 percent. The unit price of coal is $0.25/kg. It is proposed to heat this district by geo- thermal water. On an average winter day, hot geothermal water is supplied to the district at 80C at a rate of 20 kg/s and returns at 60C. How much revenue can be generated per year if the winter period can be taken to be equivalent to 3500 h of these average conditions, and geothermal heat is sold at the same price as coal?