A SOLAR-DRIVEN EJECTOR REFRIGERATION SYSTEM

A solar-driven ejector refrigeration system consists of two main sub-systems: a solar collector sub-system and a refrigeration sub-system. The major components in the system include solar collectors, a storage tank, an auxiliary heater, an ejector, a condenser, a regenerator, an evaporator, an expansion device and pumps.

The solar collector subsystem

This subsystem consists of solar collectors, a storage tank and an auxiliary heater. Solar radiation is converted to heat by the solar collectors; the heat is transferred to the heat supply medium (water) in the solar collector, then it is stored in a storage tank before being supplied to the refrigeration subsystem in the generator. The auxiliary heater is placed between the storage tank and the generator. The lowest generating temperature of the refrigeration subsystem is set at 80°C. The temperature difference between the heat source fluid (water from the storage tank) and the refrigerant in the generator is assumed
at 10 K. If the temperature of the heating medium is lower than 90°C the auxiliary heater will start and the working medium is heated until it reaches the set point.

Qu

AJ

FrUl (T — Ta)

(1)

шСр (T0 — T)

AJ

= FrI (та) e

In the model of the flat plate solar collector in TRNSYS, the solar collector efficiency is calculated from the heat balance in the flat plate solar collector by the Hottel-Whillier-Bliss equation. It is basically defined in the form of the average Bliss coefficient (FR(xa)e) and the heat loss coefficient (FRUL).

An ejector refrigeration subsystem

The main energy supply to this subsystem is heat, but a small amount of electricity (supplied to the pump) is required to circulate working fluid inside the system. In the refrigeration subsystem, the high velocity vapor stream (from the generator) goes through a converging-diverging nozzle in the ejector resulting in the vapor being sucked from the low temperature evaporator. Suction occurs, as the pressure is low at the narrowest section of the ejector. The stream from the evaporator reaches subsonic velocity. In a mixing zone at the end of the converging section, the two streams are mixed. After mixing, the combined stream becomes a transient supersonic stream and the velocity of the combined fluid must be high enough to increase the pressure after deceleration in the diffuser to a suitable condensing pressure. The vapor from the ejector goes to the condenser, condenses and heat is rejected to the environment. After the condenser, part of the liquid refrigerant is pumped to the generator and the rest goes to the evaporator, reaching an evaporating pressure by the expansion device. The process inside the cycle can be shown in figure 2.

The necessary heat input to the generator (Qg) is

(2)

(3)

Q = m (h — h . )

g g g, out g, rn

The cooling capacity at the evaporator (Qe) is,

Q = m (h — h. )

Q0 „

Q

2

Figure 2. Ejector refrigeration subsystem

e e e, out e, in

At the ejector, the energy balance at the mixing point is written as,

(4)

(mg + me) ■ hm = me ■ he + mg ■ hg, exp

COP

ejc

(5)

Q

Q,

The efficiency of the ejector system can be expressed by both an entrainment ratio (•, a ratio between the evaporation mass flow rate and the generation mass flow rate) and a coefficient of performance (COPejc). Neglecting the electricity supply to the pump, the COP of the ejector refrigeration system is defined as the ratio between cooling capacity and necessary heat input.

The ejector is the key component of the refrigeration subsystem, it is used to maintain the pressure difference between the condenser and the evaporator; the better the ejector, the higher the system performance. From the mass conservation, the impulse law and an energy balance around the ejector, the entrainment ratio (*) can also be written as:

# = me 1 mg = (cg 1 cc)“1 = [(h3 — h4 )/(h6 — h5 F -1 (6)

The ejector efficiency selected for this paper is typical for ejector performance as reported in the literature of Lundqvist (1987).

System Performances

Energy inputs to the system are heat to the generator, electricity to the pumps (both in the refrigeration subsystem and in the solar collector subsystem) and electricity to the auxiliary heater. The performance of the system can be defined as the system thermal ratio (STR).

(7)

STR =

Qsu

the heat supply to the STR, the ideal system is simply written as the

The electricity input to the pump is very small comparing to generator, thus it is generally neglected when calculating the performance can be shown as the system thermal ratio, which product of the collector efficiency and the COPejc.

(8)

STRideai = n„ • COPjJc

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