The rate of energy lost from the storage tank to the environment, which is at temperature Tenv, is given by
The storage tank losses for the month can be obtained by integrating Eq. (11.50), considering that (UA)s and Tenv are constant for the month:
where Ts = monthly average storage tank temperature (°C).
Therefore, the total load on the solar energy system is the actual load required by a process and the storage tank losses. Because the storage tanks are usually well insulted, storage tank losses are small and the tank rarely drops below the minimum temperature. The fraction of the total load supplied by the solar energy system, including storage tank losses, is given by
Lu + Qst where
Ls = solar energy supplied to the load (GJ). Lu = useful load (GJ).
Therefore, after Qst is estimated, /TL can be obtained from the /-charts as usual. The solar fraction / can also be represented by Ls/Lu, i.e., the solar energy supplied to the load to the useful load, then Eq. (11.52) becomes f /rTL
Storage tank losses can be estimated by considering that the tank remains at Tmin during the month or by assuming that the average tank temperature is equal to the monthly average collector inlet temperature, Ti, which can be estimated by the $ charts. Finally, the average daily utilizability is given by (Klein and Beckman, 1979):
For the estimation of the tank losses with Eq. (11.51), Klein and Beckman (1979) recommend the use of the mean of Tmin and Ti. The process is iterative, i.e., T is assumed, from which Qst is estimated. From this, the fTL is estimated with the f-charts; subsequently, $ is estimated from Eq. (11.54) and Xc is obtained from $ charts, from which Ti is estimated from Eq. (11.45). This new value of Ti is compared with the initially assumed value and a new iteration is carried out if necessary. Finally, Eq. (11.53) is used to estimate the solar fraction, f.
Was this article helpful?