PhD Thesis - Energy Systems Research Unit - University of Strathclyde

As 1 kWh = 3610.3 kJ, and using litres as the measure of volume of water stored
in the tank, rather than kg as a measure of weight, the heat stored in kWh may
be calculated using
Heat Stored (kWh) = V D Cp ∆T (5.47)
3610.3
where V = storage tank volume (litres)
D = density of water (kg/litre).
Using Equation 5.39, the threshold temperatures defined above may be
expressed in their equivalent amount of stored heat for a given storage tank size.
The water inlet temperature is taken as the lowest temperature that the tank will
experience, and the amount of heat stored at this temperature is zero. Room
temperature (below which heat can not be supplied to a room) is calculated as a
heat stored equivalent by using Equation 5.47, where ∆T is equal to room
temperature minus the water inlet temperature. To calculate the heat stored
equivalent of the minimum hot water supply temperature, ∆T is equal to the
minimum hot water supply temperature minus the water inlet temperature. To
calculate the maximum amount of heat that may be stored in the tank, ∆T is
equal to the maximum tank temperature minus the water inlet temperature. To
calculate the initial heat storage level, ∆T is equal to the initial tank temperature
minus the water inlet temperature. These threshold heat storage values are then
used to decide whether or not heat may be added or supplied using the
procedures outlined below.
Figure 5.25 outlines the algorithm for adding heat to the hot water storage tank
using excess heat, electricity or both. It is assumed that, when waste heat is
used to heat the stored water, that this is supplied at or above the maximum tank
temperature. This allows all the available heat (in kWh) to be transferred to the
tank, with no reduction in tank water temperature.
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