TCM parameter study

A parametric study has been carried out, in which the solar fraction of a thermochemical storage system for a low-energy house (6 GJ space heating) with a 40/25C low-temperature heating system and 9 GJ domestic hot water) was calculated as a function of collector area, collector type, charge and discharge temperatures, charge and discharge power, and reactor heat capacity and insulation (Zondag et al., 2008). Some typical results are displayed in Fig. 3 and Fig. 4. These calculations show an idealised case, in which good vacuum tubes were used, as well as a collector piping loop without heat loss, 100% efficient backup heaters and optimised TCM material (very fast reaction kinetics, ideal conductivity, small temperature hysteresis). All areas in the graphs refer to aperture.

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Fig. 3. Solar Fraction versus charge and discharge temperatures of TCM storage

Discharge power (W)

Fig. 4. Solar Fraction versus charge and discharge power of TCM storage From the simulations, the following conclusions can de drawn:

 

• The effect of the charging temperature of the TCM on the system performance depends strongly on the type of collectors used and the system dimensioning; generally, good vacuum tube collectors are preferred for TCM charging, since relatively high charging temperatures have to be reached. In addition, the simulations show that if the collector array is dimensioned large, an increase in the required TCM charge temperature only reduces the number of standstill hours, without much effect on the solar fraction. However, if the solar collector array is dimensioned small, an increase in required TCM charge temperature will imply that the storage is not filled anymore, thereby substantially reducing the solar fraction that can be obtained (see Fig. 3b).

In addition, the effect of the thermal capacity of the TCM reactor was investigated. It was found that the insulation of the TCM reactor was increasingly critical for reactors with a high thermal capacity, the worst case being that of a reactor integrated into an insufficiently insulated non- compartimented storage (as shown in Fig 1b).