1. Introduction - Firenze University Press

load by the time horizon of the TS. 2,000 kWh could have been covered by direct heat-transfer from
solar thermal-energy and 1,140 kWh could have been covered from indirect heat-transfer using
storage. This means that the demand 3,140 kWh could have been covered by solar thermal-energy,
which is 42.2% of the overall heat demand. The rest of the demand, 4,295 kWh, should still been
covered from the utility with constant availability. However, the dependency on fossil fuels should
be decreased as much as possible, since this energy source has an impact on the environment.
5.5. Determination of storage size
In order to estimate the storage size, the amount of the heat stored or used should be determined in
each cTS separately e.g. the heat stored at the cTS1 is H = 220 kW6 h=1320 kWh (Table 2,
storage column and Fig 13). These calculated amounts of heat are presented in the boxes of Fig 13.
The cumulative amount of stored heat is represented by the numbers outside the boxes (Fig 13). Fig
13A presents the initial cumulative heat stored. As can be seen, in the last cTS12 the amount of
stored heat is more than zero. This indicates that smaller storage would also be sufficient. The
smallest storage, at which the heat recovery remains the same, would be when the cumulative
amount stored at the last cTS is equal to zero (Fig 13B).
The storage from this case study should be large enough to store 1,032.4 kWh of heat. The result
was determined by the maximal amount of heat within the cascade. However, in order to obtain a
proper trade-off besides the rate of heat recovery, also the investment of the storage should be
analysed.
Fig 13: Cascading the amount of heat in storage through different Time Slices at A) maximal
storage and B) reduced storage
6. Conclusions and future work
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