Influences of working fluids on two-stage collectors

Economical and technological performances as well as collector efficiency have to be taken into consideration to evaluate the low temperature solar thermal power system with two – stage collectors. However, this work is concerned about influences of working fluids on heat collection and power conversion efficiency. In addition to key factors such as irradiation and environmental temperature that affect the efficiency of single-stage collectors, the proportion of FPC area to the total collector area plays an important role in both the overall heat collection efficiency and cost-effectiveness of the two-stage collectors. Figure 3 displays the heat collection efficiency of two-stage collectors varying with the proportion of FPC area to the total collector area y for each of the working fluids. The environment temperature is 20°C and irradiation is 750 W / m2 . The mass flow rates of the fluids are the same as those presented in table 4. The total collector area for each fluid is 690 m2 and keeps constant when FPC proportion varies. The overall collector efficiency climbs when the FPC area increases in the lower proportion range. However, it drops with further increment of FPC area in the higher proportion range. There exists an optimal FPC proportion yopt, at which the overall collector efficiency reaches the maximum for each fluid. The optimal FPC proportion alters when the working fluid is different.

Table 5 reveals the optimal FPC proportion and the maximum heat collection efficiency variation with working fluids and irradiation. Mass flow rate of each fluid through Pump 1 is equal to that through Pump 2. Thus, the dryness of the working fluids at the evaporator outlet should be 0.5 under normal condition without heat storage or heat release. On the use of Pump 2, superheating is avoidable even if irradiation is strong. Electricity is generated in a wide range of irradiation, and heat transfer between conduction oil and organic fluid is strengthened.

For each fluid, the optimal FPC proportion becomes larger when irradiation is weaker. The decrement of yopt for R123, R113, R245fa, pentane or butane is about 4.2%, 5.5%, 3.9%, 3.1% or 3.0% respectively when irradiation changes from 850 W / m2 to 650 W / m2 .

Among the five fluids, R245fa exhibits the highest heat collection efficiency accompanied with the largest FPC proportion. The ratio of heat required in the sub-cooled heating process to the total heat absorbed by fluid in the ORC process (hl – h2′)/ (h4 – h2′ )for R245fa is the highest as shown in table 3. It seems that the preheating concept of FPC is especially suitable for fluids that have a large heat proportion in the sub-cooled heating process.

On condition of irradiation of 750 W / m2, the maximum heat collection efficiency for R123, R113, R245fa, pentane or butane is about 49.23%, 49.18%, 50.12%, 48.56% or 50.04% respectively. And the relative increment of heat collection efficiency is 5.94%, 6.80%, 6.60%, 4.73% or 6.45% respectively as compared with that of single-stage collectors (table 4).

6. Conclusion

Heat transfer irreversibility between conduction oil and organic fluids will be large if single­stage collectors are adopted. The low temperature solar thermal electric generation with two-stage collectors and heat storage units gives a flexible system which can react to different operation conditions. Besides, this kind of system displays superior heat collection efficiency as well as cost-effectiveness.

The regenerator can significantly warm working fluids and complement the heat supplied from outside. On the condition of evaporation temperature 120°C, environment temperature 20°C and irradiation 750 W / m2, the ORC efficiency for R123, R113, R245fa, pentane or butane is 0.154, 0.161, 0.148, 0.160 or 0.147 respectively. Although R113 and pentane have the best ORC performance the highest collector efficiency is obtained on the use of R245fa and butane. And the heat collection efficiency is 49.23%, 49.18%, 50.12%, 48.56% or 50.04% respectively. The proportion of FPC area to the total collector area plays an important role in both the overall heat collection efficiency and cost-effectiveness of the two-stage collectors. And the optimal FPC proportion for R123, R113, R245fa, pentane or butane is 18.9%, 18.8%, 24%, 17.6% or 21.7% respectively. In consideration of frictional resistance of conduction oil as discussed in Section 4.4, the global electricity would be about 7.49%, 7.83%, 7.31%, 7.68%, 7.25% respectively.

[1] The efforts to meet the basic and mandatory mankind demand for quality of life will be discussed after the discussions on environment and energy, since these may shape up the former, although in general these issues are interconnected.