A 500 m2 light-colored swimming pool is located in a normal suburban site, where the measured wind speed at 10 m height is 3 m/s. The water temperature is 25°C, the ambient air temperature is 17°C, and relative humidity is 60%. There are no swimmers in the pool, the temperature of the make-up water is 22°C, and the solar irradiation on a horizontal surface for the day is 20.2 MJ/m2-d. How much energy must the solar system supply (gss) to the pool to keep its temperature to 25°C?

Solution

The energy balance of the pool is given by qe + qr + qc + q*

The velocity at 0.3 m above the pool surface is 0.3 X 3 = 0.9 m/s. The partial pressures for air and water are given by Eqs. (5.20) and (5.21). The saturation water vapor pressure at air temperature, ta, is also given by Eq. (5.21); therefore,

Ps = 1GG(G.GG4516 + G.GGG7l78t, + 6.944 X 1G~713) = 1GG(G.GG4516 + G.GGG7178 X 17

2.649 X 1G~612

Saturation water vapor pressure can also be obtained from Eq. (5.21) by using tw instead of ta. Therefore,

From Eq. (5.18), evaporation heat losses are qe = (5.64 + 5.96v0.3)(Pw - Pa) = (5.64 + 5.96 X 0.9)(3.166 - 1.162)

From Eq. (5.22), radiation heat losses are 24 X 3600 4 4

= 0.0864 X 0.95 X 5.67 X 10-8(2984 - 263.74) = 14.198MJ/m2-d

From Eq. (5.26), convection heat losses are qc = 0.0864(3.1 + 4.1v)(tw - ta) = 0.0864(3.1 + 4.1 X 0.9)(25 - 17)

From steam tables, hfg, the latent heat of vaporization of water at 25°C is equal to 2441.8 MJ/kg. Therefore, the daily evaporation rate is given by Eq. (5.28):

qc 4.693 X 103 , „„„, , 2a m = =-= 1.922 kg/m2-d evp hfg 2441.8

From Eq. (5.27), the heat losses due to the make-up water are qmuw = mevp€p (tmuw - tw) = 1.922 X 4.18(22 - 25)

= 24.10MJ/m2 -d (the negative sign is not used, because all the values are losses)

From Eq. (5.29), solar radiation heat gain is qs = aHt = 0.85 X 20.2 = 17.17MJ/m2-d Therefore, the energy required by the solar system to keep the pool at 25°C is qss = Qe + Qr + qc + qmuw - qs = 22.052 +14.198 + 4.693 + 24.1 -17.17

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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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