Figure 3 shows a schematic view of CAVICAL1. It is a calorimeter which acts as a conic heat exchanger with water as a cooling fluid. It consists of two concentric cones. The inner one, which is made of copper and receives the solar concentrated radiation coming from the mirrors, has a vertex angle of 15o, a height of 16 cm, a base diameter of 8.56 cm, a base aperture diameter of 3.24 cm and a wall thick of 0.3 cm. The outer cone is made of stainless steel, it has 0.8 cm wall thick and there is a separation gap of 1.0 cm between the cones. Water can enters to the calorimeter at the vertex, flows between the cones and exit the device at the aperture or can flow in opposite direction.

The concentrated radiative energy coming from the mirrors, and passes through the calorimeter aperture is absorbed by the surface of inner cone and is transmitted to the interior of the wall by conduction. The water that circulates between the cones removes that energy by forced convection. Also, part of the irradiance entering through the calorimeter aperture is lost by reflection, and thermal emission loss back through the aperture, by heat conduction from the calorimeter through the insulation to surroundings and by natural convection in air through the aperture.

The design of the cavity calorimeter has as a first objective to diminish these losses.

In order to determine the heat losses through the cavity aperture due to convective process, a detailed simulation was developed using a CFD code.

Fig. 3. Schematic view of CAVICAL.

The high solar flux incident to the aperture of the calorimeter and absorbed by its inner cone is equal to the heat removed by the circulated water in the calorimeter (Qc), which can be calculated measuring the mass flow rate m and the inlet (Ti) and outlet (To) temperatures, that is,

Qc = mCp( — Ti) (1)

where Cp is the heat capacity of the cooling fluid. Thus, if Qin is the concentrated solar energy incident on the calorimetric cavity, then the following holds,

Qc = а ‘ Qin (2)

where a is the apparent absortance of the cavity calorimeter. Therefore, knowing a, Qin can be determined, and knowing Qin, a can be determined. It is clear that the energy Qin which is not absorbed by the cavity, is reflected by it, because the cavity is opaque. Therefore, knowing the apparent absortance a, the apparent reflectance p of the cavity is calculated by p = 1 — a.