The storage concepts have been compared in a number of different aspects including storage density, cost for materials, temperature requirements as well as system simulation results (for three of the concepts) with the same boundary conditions [11]. Data from lab testing of prototypes [4] and estimations of storage size for 70 and 1000 kWh storage capacity showed the following:

• The storage density for cold (based on total system volume), when compared to water, is more favourable than for heat. For the ClimateWell 10 commercial heat pump/store, the storage density for cold is 4.7 that of water whereas for heat it is only 1.2 times greater. This is due to the fact that the temperature range available for water storage for cold is smaller (~10°C) than for heat (~60°C).

• For short term heat storage, the best technologies have an energy density 2 — 2.5 times that of water. This is relatively low due to the space required for reactors and condenser/evaporator in addition to the store. In addition all of the storage systems have irreversibilities in the processes themselves during charge and discharge resulting in lower store efficiencies.

• For longer term storage (1000 kWh) the energy density for the TCA technology and NaOH storage systems is nearly three times that of water, for Monosorp twice and for MgSO4.7H2O 2.5.

In addition, once the sensible heat from the solution has been lost (or at best recovered), the energy can be stored indefinitely, a significant advantage compared to water.

• In terms of material cost, all materials are expensive compared to water. However, NaOH, is significantly less expensive than the other materials reported: zeolite, LiCl, silica gel,

MgSO4.7H2O and zeolite 13X. The cost for the whole storage system has not been estimated here. For the ClimateWell 10, the projected cost is ~8000€ for a heat pump system consisting of two units in parallel, with a total heat storage capacity of 70 kWh.

The fractional energy savings (Fsav, th) of three systems, based on system simulations are shown in Fig.2. FSC’ [13] is a quantity that reflects the theoretical amount of energy a system with given boundary conditions can save, if it is 100% efficient from solar to heat delivery. It is dependent on the amount of radiation incident on the collector, and thus collector size, orientation and slope, and heat demand. It is also dependent on storage capacity. Systems with a large storage capacity

compared to the load can have an FSC’ value larger than 1, while systems with small heat capacity have a maximum FSC’value of near 1. The curves show that the performance of the AEE Intec closed adsorption system and that of the ECN chemical storage system are fairly similar, if the storage size of the ECN system is similar to that of the AEE Intec system. A larger storage size gives improved savings. The ITW Monosorp open adsorption system achieves significantly greater savings compared to the others. This is partly due to the fact that the collector is slightly better, but also because the system uses a ventilation heat recovery system that is not in the other systems. This means that the Monosorp system in practice has a smaller space heating load than the other systems, which automatically results in greater savings without affecting the FSC’value.

It must be pointed out that these simulation results are for systems in different stages of development, using models with different degrees of detail. The curves are also trendlines for a number of points, with dotted sections being extrapolations.