A solar energy company is combining solar thermal with solar power, i.e. they ar
ID: 1633158 • Letter: A
Question
A solar energy company is combining solar thermal with solar power, i.e. they are producing electricity and heat from the same solar panel. The reasoning for this is: if the solar panel is too hot, its efficiency will be very low, so harvesting the heat created on the solar panel maintains its efficiency for the production of electricity. The company states that a 1-m2 solar panel produces 136 W of electricity and 400 W of heat.
a. If you are using this system at home and want to use the heat energy to heat your 50-L water tank, what is the temperature of the water in the tank if the system runs for 1 hour and the water was at 25C initially?
Answer: 31.9 oC
b. Now, the company has a 40 MW solar plant generating electricity. They want to take advantage of the heat from the panels but want to heat up the water to 100C and produce steam. If the plant is run for 1 hour, what is the amount of water that would be needed to fully utilize the heat produced? Is this number reasonable? What would you recommend to make the utilization of the heat possible to produce steam? Consider the initial temperature of water at 25oC.
Answer: m=1.65x105kg
Explanation / Answer
given 1 m^2 solar panel produces 136 W electricity and 400 W heat
a. Heat supply in one hour = 400*60*60 J = 1,440,000 J
initial temperature of water, Ti = 25 C
Final temperature, T
heat capaciuty of water = 4184 J/C Kg
mass, m = 50 L 50 kg
H = mC(dT)
So we can write
1,440,000 = 50*4184(T - 25)
T = 31.883 C
b. if plant generates 40 MW electricity, it generates 400*40/136 MW of heat = 117.647 MW of heat
for one hour if plant works and water is evaporated from 25 C
we get
H = mC(100 - 25) + mL = m(75C + L)
now latent heat of vapourisation of water = L = 2260,000 J/kg
117.647*10^6 = m(75*4184 + 2260,000)
m = 1.65*10^5 kg
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