Belfast Product Design: CAD, Prototyping and Engineering
Generating new product design concepts, developing prototypes and delivering production designs
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R & D

This project targeted the provision of effective vaccine refrigeration using solar power energy in off-grid locations (e.g. sub-saharan Africa). It uses heat, rather than electricity, to drive the refrigeration cycle.

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Research and Development

CASE STUDY :

SOLAR POWERED REFRIGERATOR

Solar refrigerators already exist, however they typically employ photovoltaic (electric) solar panels to drive a traditional vapour compression cycle. Less well known is the thermally powered jet-pump cycle. With no moving parts, the jet-pump replaces the compressor found in conventional electric refrigerators. Although such systems have been developed for large air conditioning (AC) applications, the viability of the cycle at lower temperatures and lower cooling capacities had not been fully explored. The system was originally conceived for use as a vaccine delivery system in the developing world, however the potential of the technology to reduce reliance on electrically powered cooling equipment (15% of world energy demand) should not be overlooked.

 
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THEORETICAL DESIGN

Computational fluid dynamic (CFD) was used to theoretically optimise the performance of the jet pump. High pressure vapour exits the nozzle of the jet pump and accelerates to supersonic speeds. This high velocity flow entrains refrigerant flow from the evaporator thereby generating the required cooling effect.

 
 
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JET PUMP PROTOTYPE

The jet-pump assembly was precision machined in brass. Silicon o-rings were used to seal the component surfaces against the +30bar inlet operating pressure. The assembly was designed to allow the position of the nozzle to be adjusted in 0.25mm increments - this allowed the theoretical performance of the system to be experimentally validated.

 
 
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ELECTRONIC CONTROL

Although the primary power source for the system was 100W of solar thermal energy, a small amount of electrical energy (<50W) was also needed to operate the control electronics. This monitored system conditions (e.g. temperatures, pressures etc) and actuated an innovative arrangement of fluid reservoirs to provide highly efficient cycling of refrigerant around the system.

 
 
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FULL SCALE PROTOTYPE

In order to fully evaluate the performance of the jet-pump it was necessary to build a fully functioning prototype, complete with a 100W refrigerator and 1.1kW condenser array. In many instances this required the design and fabrication of bespoke components. The performance of the system was tested over extended durations at a range of ambient temperatures.

 
 
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SYSTEM TESTING AND OPTIMISATION

Although maintaining a coefficient of performance above 0.1 was difficult below evaporator temperatures of 10°C, the experimental results showed that the jet-pump cycle was capable of attaining the 6°C target temperature (needed for the safe storage of vaccines). The graph shown highlights the cyclical operation of the innovative gravity feed reservoir system.