The effect of mass flow-rate on collector optical efficiency

The mass flow-rate is an important parameter for thermal solar collectors. In most cases flow-rate will be the only system parameter that a domestic/industrial end user will be able to influence. Therefore the relationship linking mass flow and optical efficiency needs to be

Figure 4 — [a] and [b] The variation of optical efficiency with mass flow and the corresponding global heat losses, [c] the variation of 1/AT with mass flow and the deviation from the linear fit for intercept zero and [d] the plot of AT against mass flow and the applied correction to calculate the global heat losses for the collector

well understood in order to ascertain the effects for evacuated tubes collectors. The optical efficiency on the collector was monitored for mass flows ranging 0.02 to 1.50 kgs-1 at 590 and 1150 Wm-2 respectively.

The optical efficiency of the collector was found to be significantly affected by mass flow in the system. Figures 4a, b show two distinct regimes; for mass flow-rates <0.07 kgs-1 the efficiency was found to reduce by up to ~25 % with decreasing flow. However, at rates >0.07 kgs-1 the optical efficiency slowly increases with increasing flow. Ismail et al5 also observed these phenomena for flat-plate collectors with heat-pipes, in that case the efficiency stabilised to a constant value at higher flow-rates.

Morrison1 explains this as the ‘low flow penalty’; this was caused due to the fact that experimentally measured values of ATexp across the collector were less than the predicted values at low flow-rates, for UL independent of temperature. This was demonstrated in Figure 4c, the experimental data of 1/AT against flow-rate; at low mass flow, the experimental data deviates away from the linear fit (intercept zero) shown by the dashed black lines. This indicates that observed AT was less than would be expected for a system with no losses. As a result of the low mass flow the useful gain Qu (that directly depends on AT) decreases and therefore the corresponding efficiency of the collector also decreases; this is a fundamental property of low flow systems.

A simple empirical method was used to calculate the global heat losses from the collector at normal incidence under the described conditions. From eq.1 (the equation of useful heat gain) it was possible to determine an expression for AT assuming no heat losses given by eq.2

QU = mCpAT = AcFr[Gt{t+)-Ul T — Ta)]

where UL = 0, as seen in Figure 4d (green dashed line). A correction was applied to AT in order to match the experimentally measured values ATexp (red dashed line Figure 4d). This modification took the form shown in eq.3

where x = 0.98. Substituting this expression for ATx into eq.4 an approximation of the global heat losses from the collector could be calculated for the collector, where Ti > Ta.

The global heat losses from the collector were found to decrease with increasing mass flow-rate as shown in Figures 4a, b. The heat losses range from 6.0 to 0.2 Wm-2K-1 depending on the mass flow-rate and incident irradiance power density.