Closed Cycle Solar Refrigeration with the Calcium Chloride System

A closed cycle solid absorption intermittent refrigerator, using CaC12 absorbent and NH3 refrigerant, was constructed and tested to obtain the instantaneous and cumulative available overall COP. The combined collector/absorber/generator unit had double glazing of 1.14 m2 exposed areas. The system was fitted with a stagnant passive evaporative condenser with porous sandcrete wall, which produced condenser water temperatures varying from 3 – 10 deg C below ambient, during NH3 generation and condensation.The instantaneous available overall COP rose to a peak which depended on the solar fluxes and starting pressure, as well as on the condenser and ambient temperatures. The peak varied from 0.07 to 0.08. It fell as solar flux decreased towards late afternoon, but rose again slightly due to the combined effect of decreasing collector plate temperature and solar flux. The cumulative overall COP rose steadily to peak values in the range of 0.07 to 0.08, by the end of the generation period. The COP was a strong function of starting generator pressure or evaporating temperature, and fell as the pressure decreased. The cumulative overall COP is much lower than the peak instantaneous COP as a result of system inertia, caused by high collector plate and tube mass, and large system free volume.The refrigerator is capable of maintaining evaporator temperature of -10 OC during the cooling phase, and is well suited for vaccine and food storage applications

Improvement to the Design of a Solid Absorption Solar Refrigerator

The paper presents the highlights of the design of an existing solid absorption solar refrigerator, using CaCl2 stabilised with CaSO4 as the absorbent. The performances are also discussed. The sources of poor performance were identified as high thermal and pressure inertia of the system, high heat loss coefficient, and low absorbent thermal conductivity. Based on these, measures taken in the design of a new model refrigerator, to improve the refrigerator performance are then presented. 

Determination of the Thermophysical Properties of Nsukkanut: A Solid Absorbent for Solar Refrigeration

The thermophysical properties of 'Nsukkanut', - a CaCl2/CaSO4 absorbent mixture used in solid absorption solar refrigeration [1], were studied in this report. The transient experimental technique of Beck and Al-Araji [12] was used in determining the effective thermal conductivity, specific heat and bulk thermal diffusivity of granular (2.8 - 6.35 mm sizes) parkings of the absorbent. Additionally, the parking density crushing stress percentage swell and degree of refrigerant - NH3 absorption were determined. Effective k, c and a values were in the ranges of 0.098 - 0.111 W/mk, 1.123 -1.696 kJ/kg K and 1.404 - 1.053.10-7 m2/s, respectively, for a parking density of 621.4 kg/m3. Average crushing stress, the percentage swell and absorption factor values 018.17 N/mm2, 7.96% and 62.1 % respectively, obtained in this report compared well with values of 8.10 N/mm2, 11.6% and 59.5% correspondingly, as reported in [11].The effect of aluminium as a thermal conductivity improvement additive, on the thermophysical properties of the absorbent, were also studied. For aluminium fractions of up to 3% by mass, no sustained improvement in thermal properties were observed, while the strength and swell properties deteriorated.

Effects of Parallel Channel Interactions, Steam Flow, Liquid Subcool and Channel Heat Addition on Nuclear Reactor Reflood Transients

Tests were performed to examine the effects of parallel channel interactions, steam flow, liquid subcool and channel heat addition on the delivery of liquid from the upper plenum into the channels and lower plenum of Boiling Water Nuclear Power Reactors during reflood transients. Early liquid delivery into the channels, following a loss of Coolant Accident, will help prevent overheating and melt down of the reactor fuel bundles. The tests were performed at the General Electric Nuclear Energy Division Laboratory, California. The channels consisted of two 5.22m long *25.4mm long*23.6mm i.d. stainless steel tubes, with unequal orificing at the bottom, and equal orificing at the top. Provisions were made for electrical resistance heating of 3.5m of each tube, and for visual observation of flows through the tubes. Test fluids were steam and saturated or subcooled water. Subcools ranged from 3.3 deg C to 37.2 deg C, and system pressures varied from near atmospheric to a little over 1.7 bar. Test section heat fluxes were between 2.58 and 13.95 KW/m2. It was observed that channel heat additions tended to make each tube behave independently of the other. As a result of subcool and vapour condensation, vapour supply into the lower plenum increased liquid delivery into the channels, and decreased the system rewet and reflood times when the subcool was in excess of about 20 deg C. Parallel channel interactions were observed to produce co-current downflow in the less restricted tube, with counter-current flow existing in the more restricted tube. This is desirable. When conditions permitted, the interactions gave rise to the classical "steam bound" flow configuration - (i.e. water hold up in the upper plenum due to top orifice Counter Current Flow Limitation, partial filling of the more restricted channels, a partially full lower plenum, and pure vapour flow in the less restricted channel). This configuration is undesirable for thermal recovery of a reactor following a loss of coolant accident

Two Phase Flow Split Model for Parallel Channels

A model has been developed for the determination of two phase flow distributions between multiple parallel channels which communicate between a common upper and a common lower plenum. It utilizes the requirement of equal plenum to plenum pressure drops through the channels, continuity equations at the lower plenum channel intake boundaries, together with phase-split relationships at the channel inlets, to set up a series of nonlinear simultaneous equations. The equations are solved using the Broyden’smethod [4], a modification of the Newton’s method. The model and code are capable of handling single and two phase flows, steady states and transients, up to ten parallel flow paths, simple and complicated geometries, including the boilers of fossil steam generators and nuclear power plants. A test calculation has been made with a simplified three-channel system subjected to a two-phase flow transient, and the results have been very encouraging

Three-Step Model of Dispersed Flow Heat Transfer (Post CHF Vertical Flow)

The paper presents a three step model of the dispersed flow heat transfer process, using an analysis of a single drop motion and heat transfer, and a statistical representation of the overall behaviour of the drops. The resulting equation gives the total heat transferred to the flow in terms of the mass flux, flow quality, fluid properties, wall roughness, and wall superheat. It includes the effect of contact angle or change in the wet ability of the surface. The range of validity of the model and the equation extends from dry wall film boiling to transition boiling, and is limited on the low temperature end by the critical Heat Flux region. Since the equation is analytically derived, its differentiation with respect to wall superheat will yield the Minimum Heat Flux point. The equation and model provide a very powerful base for analysis and prediction of post Critical Heat Flux heat transfer. The stable film boiling data for dispersed vertical flow of liquid nitrogen from reference [1] have been compared with the prediction and the results have been favourable