Working Principles of the TCA

The thermochemical accumulator (TCA) is an absorption process that uses a working pair, not only in the liquid, vapour and solution phases but also with solid sorbent (Olsson et al., 2000). This makes it a three-phase system, with significantly different properties from the traditional absorption processes, where there are only two phases: either solution + vapour or solid + vapour. Figure 2 shows the schematic of a single TCA unit, which is similar in
principle to that of Figure 1. In a practical unit the vessels are evacuated and the solution is pumped over a heat exchanger to increase the wetted area and improve heat transfer.

During desorption in the reactor, the solution is saturated and further desorption at the heat exchanger results in the formation of solid crystals that fall under gravity into the vessel. Here they are prevented from following the solution into the pump by a sieve, thus forming a form of slurry in the bottom of the vessel.

This gives the TCA the following characteristics:

Figure 2. Schematic of a single unit thermochemical accumulator.

High energy density storage in the solid crystals.

• Good heat and mass transfer, as this occurs with solution.

• Constant operating conditions, with constant ATequ for a given solution temperature.

For discharging, where the process is reversed, saturated solution is pumped over the heat exchanger in the reactor where it absorbs the vapour evaporated in the evaporator using the cooling load. The solution becomes unsaturated on the heat exchanger, but when it falls into the vessel it has to pass through the slurry of crystals, where some of the crystals are dissolved to make the solution fully saturated again. In this way the solution is always saturated and the net result is a dissolving of the crystals into saturated solution.

LiCl Porperties

The first TCA units have been built using water/LiCl as the active pair. The physical properties of this pair have been summarised in the literature (Conde, 2004) and empirical equations have been created for them based on data from a large number of studies over the last 100 years. The solubility line for LiCl can be seen in Figure 3 a), where it is readily apparent that there are several different hydrates for LiCl. However, for the operating range of the TCA, the solution is generally operating at temperatures of 20-50°C for discharge and 65-95°C for charge, all of which are within the monohydrate range for saturated solution. The figure shows that the mass fraction for saturated solution is a function of the solution temperature, and thus Equ. 1 is simplified to Equ. 2 for the TCA. This in practice means that ATequ is constant for a given set of boundary conditions resulting in constant operating conditions during charging/discharging.

ATequ ~ fsat (Tsol) Equ. 2

The equations for vapour pressure derived by Conde were used to create a Duhring chart for LiCl, Figure 3 b). This shows that the maximum value for ATequ is for the saturated solution, and that for the operating conditions of the TCA with an ambient temperature of 35°C, ATequ is 37°C for discharging (comfort cooling), and 53°C for charging.

SHAPE * MERGEFORMAT

Figure 3. Data for LiCI: a) solubility line (Conde, 2004); b) water vapour pressure above the solution for varying mass fractions of sorbent and solution temperature (Tso). Measurements made by ClimateWell using the solution used in the TCA are shown as filled squares.

Figure 4. Relationship of ATequ to the

saturated solution temperature.

Figure 4 shows the relationship of ATequ to the temperature of the saturated solution. ClimateWell have made their own measurements at different times with the mixture of LiCl that they use in the TCA. The lower line shows the correlation ClimateWell use in their control system, whereas the filled squares represent data for the latest measurements. The data from 2004 agree well with Conde’s equations at higher solution temperatures, but deviate somewhat at lower solution temperatures, as can also be seen in Figure 3 b).

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