For fully mixed or unstratified energy storage, the capacity (Qs) of a liquid storage unit at uniform temperature, operating over a finite temperature difference (ATs), is given by:
where M = mass of storage capacity (kg).
The temperature range over which such a unit operates is limited by the requirements of the process. The upper limit is also determined by the vapor pressure of the liquid.
An energy balance of the storage tank gives
Qu = rate of collected solar energy delivered to the storage tank (W). Ql = rate of energy removed from storage tank to load (W). Qtl = rate of energy loss from storage tank (W).
The rate of storage tank energy loss is given by
(UA)s = storage tank loss coefficient and area product (W/°C).
Tenv = temperature of the environment where the storage tank is located (°C).
To determine the long-term performance of the storage tank, Eq. (5.31) may be rewritten in finite difference form as
(M cp )s where Ts-n = new storage tank temperature after time interval At (°C).
This equation assumes that the heat losses are constant in the period At. The most common time period for this estimation is an hour because the solar radiation data are also available on an hourly basis.
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