A user-defined type was developed to describe the behaviour of an ice-making ice-encapsulated cold storage. The storage can be represented like a stack of layers (nodes), each one containing a certain number of capsules. The number of capsules in each layer is directly calculated as follows:

N = Vtank і1′ Z)

capsules

y capsule 1 v layers і

where Vtank is the tank volume, Vcapsule is the capsule volume and є is the void fraction around the capsule, typically in the range 0.4 to 0.5 depending on the capsule shape. Heat balance equations over each layer yields the following:

^refVrefCpref j. ■ refCpref Tj-1 Tj NcapsulesQref^capsule

dt when mref is downward (3)

^refVrefCpref dt m refCPref Tj+1 Tj NcapsulesQref ^capsule

dt when mref is upward (4)

where Sref, Vref and Cpref are the refrigerant density, volume and specific heat respectively, mref is the refrigerant mass flow rate, Tj is the refrigerant temperature within the j layer and Qref^capsule is the heat transferred from refrigerant to capsule. The expression for Qref^capsule depends on the particular state of the PCM inside the capsule. Four conditions are possible: 1) PCM liquid, 2) PCM in transition from liquid to solid, 3) PCM in transition from solid to liquid and 4) PCM solid. When the PCM is completely liquid or solid, the lumped capacity model is used, assuming the PCM temperature is uniform in the capsule and equal to the temperature of the layer. During PCM phase transition inside the capsule, the following experimental equations, as described in [6], have been used:

where L is the latent heat of water, Vcapsule is the capsule volume, Ro is the capsule radius, 5wat and kwat are the density and thermal conductivity of water, 5ice and kice are the density and thermal conductivity of ice, Tpc is the phase change temperature, t is the time since phase change has started, and t* is the dimensionless time. For a detailed description see [8].

5 Simulation results

In a parametric study, the influences of the collector field area, size of the latent heat storage (commercially available nodule ice storage), the chilled mass flow that is to be cooled and the possible effect of dust on the reflector area, were investigated. Some results are presented below.

5.1 Evaluated Parameters

To rate the system two performance figures where evaluated: The extracted heat per incident beam radiation as total efficiency (ntotal) and the ratio between the cooling work carried out by the solar cooling system and the total cooling work required by the given process for the specified flow rate of the return flow as described in 5.3 (Solar fraction, SF).

extracted, solar

— |y°l

beam, incident

Qextracted, solar

Qtotal [%]