The location of Bangkok is at latitude 13.44 degree above equator thus the cooling load as well as the ambient temperature are quite stable for the year round. The average global radiation (kWh/m2) and the average ambient temperature are showed in figure 3.

The load generally depends on the solar radiation as shown in figure 4. The average load capacity is 4 kW. The high cooling load can be observed during the dry season, from March to June.

Load (kW) incident solar radiation (kw/m2) — Qtoad total — *isoia0on_kwm2 — Osensl kW — Oat I kW Figure 4. Load characteristic

Generating temperature (°C) Figure 5. Efficiency of the ejector refrigeration subsystem

The performance of the ejector refrigeration subsystem strongly depends on the operating temperatures and the ejector design (Pridasawas, 2003). The COP of the refrigeration subsystem increases proportionally to the generation and evaporation temperature. A high generating temperature, however, requires a high temperature output from the solar collector, thus resulting in high heat losses and more expensive solar collectors. The performance of the ejector increases when the evaporator temperature increases, thus refrigeration subsystem should be provided at the highest evaporation temperature possible in the given application. In this case, the evaporation temperature is fixed at 10°C and the generating temperature is studied. The changing in cooling load does not significantly affect the COP, if the operating temperatures are kept constant. Generally, the generating temperature is high when the cooling load is high due to the high temperature heat gain from the solar collector. On the other hand, the solar collector efficiency is in inverse proportion to the temperature output of the solar collector. Figure 5 shows that the highest STR is obtained at the generating temperature between 90 and 110 °C. The average COP of the system of the simulation condition described above is about 0.3.

The simulation result for the whole year at 40 m2 solar collector area, 2 m3 storage tank volume and 400 kg/hr water flow rate is shown in figure 6. The highest performance is obtained from March to August. During the dry season (March to June), the ejector

subsystem operates at high cooling capacity and high performance due to the higher outlet temperature of the solar collector. The performance of the refrigeration subsystem does not only depend on the generating temperature, other operating conditions e. g. condensing temperature and evaporation temperature, the geometry of the ejector also influence the performance (Lundqvist, 1987). In this simulation other parameters are set constant at normal operating conditions, thus, their influence cannot be observed. Further information can be found in the literature of Lundqvist (1987), and Pridasawas (2003).

0 800 Jan Feb Mar Apr May Jun July Aug Sep Oct Now Dec ЗДічМИоп Tim# ««TWO, iv Figure 6. A year round COPejc

Figure 7. A year round solar collector efficiency

Figure 8. A year round system thermal ratio

The annual energy usage by pumps and auxiliary heater decreases remarkably when the solar collector area increases from 20 m2 to 50 m2. The optimum solar collector area should be traded off with the solar collector installation cost. Figure 9 shows the annual energy (heat and electricity) usage by the pumps and auxiliary heater at different solar collector area for the storage tank 2 m3 and the water flow rate 450 kg/hr. Ninety five percent of the energy is consumed by the auxiliary heater, 3% by the circulation pump in the solar collector subsystem and 2% by the pump in the refrigeration subsystem. It shows clearly that the auxiliary heater is needed for the reliability of the system.

The increase in size of the storage tank and the water flow rate (in the solar collector subsystem) does not significantly affect the energy usage.

The increase of the water flow rate every 100 kg/hr decreases the supplementary energy usage about 2% while the increase of the storage tank volumes every 1 m3 decreases the energy usage only about 0.5%. The system operates during daytime while the high radiation is proportional to the cooling load, thus the size of the storage tank does not significantly affect the system. If the system also operates during nighttime, the size of the storage tank becomes an important parameter. The model and the degree of stratification of the storage tank should also be taken in consideration.

CONCLUSION

The system thermal ratio (STR) is influenced by both the COPejc and the solar collector efficiency. The COPejc increases in proportion to the generating temperature while the efficiency of the solar collector decreases in inverse proportion to the outlet temperature. The optimum generating temperature for achieving the highest STR is about 100°C. The highest STR that can be obtained is about 0.25 at the COP of 0.55. The optimum solar collector area is about 50 m2 for the average cooling load at 4 kW. The size of the well — mixed vertical storage tank does not significantly affect the annual electricity usage of the system, which operates during daytime. The water flow rate in the solar collector subsystem slightly influences the electricity usage, however, it should be high enough to provide the heat for the generator and to maintain the reliability of the system.

NOMENCLATURE

TOC o "1-5" h z Asc = solar collector area (m2)

c = velocity of refrigerant in the refrigeration subsystem. (m/s)

COPejC = coefficient of performance of the ejector refrigeration system (-)

FR(za)e= Bliss coefficient (-)

(FrUl) = Bliss heat loss coefficient (kW/m2 K)

h = enthalpy (kJ/kg)

hg, exp = enthalpy of the driving fluid from the generator after expanded

through the nozzle. (kJ/kg)

I = Incident solar radiation (kW/m2)

m = mass flow rate (kg/s)

Qe = cooling capacity at an evaporator (kW)

Qg = energy input to a generator (kW)

Qs = incident solar radiation over the solar collector (kW)

Qu = useful energy output from a solar collector (kW) 03

SER = system electrical efficiency STR = system thermal ratio

T = absolute temperature (Kelvin)

Wpump = necessary work required by pump (kW)

r/sc = solar collector efficiency

• = mass ratio

Subscripts

1- 9 = the points in the refrigeration subsystem as shown on figure 2.

c = condenser

e = evaporator

el = electricity

g = generator

ht = auxiliary heater

p = pump