Making sure that air velocity stays below the critical value, a constant sand flow and a steady state temperature field specific for cross flow heat exchangers similar to Fig. 2 can be obtained.

The heat transfer effectiveness is defined as the ratio of actual and maximum possible heat flow. In a system with different products of mass flow rate and heat capacity for the two media, the maximum possible heat flow is defined by the lower value:

s _ Q (4)

(m cp L •( — TS, in)

As for the power plant operation both high sand output temperature and low receiver air heat losses are desired, for the heat exchanger measurements it is aimed for balanced products of mass flow rate and heat capacity for air and sand.

For an effectiveness measurement with sand grain size 1-2 mm the following values were measured: Ta, hi=687 °C, TS, in=25 °C, mA =0.0143 kg/s, mS =0.0205 kg/s, TA, out=62.4 °C, Ts, out=427.5 °C

The lower product of mass flow and heat capacity is defined for air. Further decrease of sand flow is impeded by the valve design, otherwise higher sand output temperatures could be obtained.

The heat exchanger effectiveness comes to

mS • cp, S(Tr. f ^ T„„.,S) * (Tout, S — Tr. f )

(0.0143kg/s • cp )*(687°C — 25°c) (5)

0.0205 — • 920 J •(427.5°C — 25°C)

________ s kg • К v___________________ ’ _ 7591W

0.0143^ 4072 — l(687°C — 25°C) 10148W

s kg • К)y ’

The measured heat exchanger effectiveness is clearly below the values obtained from simulation of 0.8-0.9, according to section 2.2. This is partly explained by heat losses on the heat exchanger surfaces and air leakages. Additionally, it is a result of disturbing effects due to the limited heat exchanger width.

Figure 5 shows a front view on the air outflow area and the according air outflow temperatures. As expected the temperatures increase to the bottom, similar to the right line in the middle area of Fig. 2. However, moving on horizontal lines from the centre to the heat exchanger frame the temperatures increase, too. Whereas for determination of effectiveness the air and sand temperatures are measured in the centre section, the energy balance is largely determined by the differing temperatures on the heat exchanger frame region on the left and right side. The temperature increase is attributed to heat conduction effects in the steel frame and air leakages between steel frame and sand.