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Content archived on 2022-12-27

MODIFICATIONS TO A LARGE REFRIGERATION SYSTEM TO INCORPORATE AN INTEGRATED DESIGN FOR REDUCED ENERGY COSTS

Objective

The aim was to demonstrate the installation of a lithium bromide absorption chiller and ice storage system in a secondary refrigerant circuit with fluctuating loads and where sub-zero temperatures apply. The aim was to show the integration and use of absorption chilling and ice storage in a large industrial refrigeration plant and the potential electrical cost saving benefits.
The project was also to show the additional benefits accruing where the self generation of electricity using steam turbines exists.
The design reduces the Brewery's electricity consumption and maximum electrical demand as well as transferring a large proportion of the total refrigeration power consumption on to the cheaper night rate tariff.
The expected annual benefits from the installation of 18,820 kW of ice storage and the integration of absorption chilling with a cooling capacity of 1,600 kW are :
- 1,478,000 kWh electricity savings.
- 1,478,000 kWh additional electricity produced.
The introduction of ice storage into a large refrigeration/cooling system for electrical load spreading has been successfully proven. The integration of the absorption chiller into the circuit has not shown all the benefits originally envisaged due to lower secondary refrigerant return temperatures.
The overall electricity saving, however, has been achieved through the improved system efficiency.
The project was a technical success where the energy saved and transferred objectives were in-line with the forecast (based on 12 months Monitoring Period) as follows:
FORECAST ACTUAL kWh Saved in Total 1,478,400 1,042,139 kWh Transferred (day to night) 2,366,000 2,372,635
Prior to the new installation the total day unit electricity consumption was 6,920,000 kWh/annum, this has been reduced to 4,547,365 kWh.
The total refrigeration/cooling plant power consumption has been reduced from 9,160,000 kWh to 8,117,861 kWh/annum.
In terms of load spreading; the installation has lowered the brewery electricity maximum demand from in excess of 6 MW to now below 5MW.
The annual electricity operational cost for the new installation is ?344,600; this compares with the previous costs at ?471,600/annum, giving an annual cost saving benefit of ?127,000.
Thermal storage applied to process cooling systems shows substantial reductions in the overall operational costs because of the lower 'off-peak' - night period tariff structure and improved efficiency by the optimum use of compressors.
Ice Storage systems are designed to generate and store cooling capacity overnight for use during the day hours, for process cooling where chilled water temperatures of +1 deg. C are suitable. The latent heat of fusion of ice permits large capacity storage in a relatively small space. Water when frozen can provide 335 kj/kg of cooling compared to chilled water, which has a capacity of 35 kj/kg. The Electricity tariff structure in Ireland is such that there is a considerable cost difference between the peak day units and off-peak night units. With this particular design it was not practical to accumulate the total daily requirements of the brewery for the following reasons:
(i) Sub-zero final temperatures were required by the process design and the fact that ice exists at 0 deg. C.
(ii) The compressor input power required to build the ice would leave insufficient capacity to carry the night load.
At the brewery there are two individual refrigeration systems for process cooling, both using ammonia as the primary refrigerant. By assigning one of the existing refrigeration plant screw compressors for ice making during the 9 hour night period but leaving sufficient capacity to cater for other night duties, 13 million kCal/hr of "cold" (or cooling capacity) is achievable.
To maintain the maximum benefit from the ice melt-down cycle a reasonably constant duty is required. As the return temperatures from the process vary, the new design incorporates an absorption chiller to assist the even ice melt-down. This method of cooling utilises a heat source to generate 'cold' and in this particular instance low pressure steam at 2.1 bar.
The new installation involved the integration of a lithium bromide absorption chiller with an ice storagesystem for cooling the secondary refrigerant (water/gylcol solution) flows to less than 3?C and further to reduce the cooling demand on the primary refrigerant compressors. The absorption chiller modulates to lower the secondary refrigerant return temperature to +6 deg. C, and in so doing, maintains a reasonably constant temperature (load) for the ice storage system. The ice water, cooled by the melting ice from the storage system, is circulated through a plate heat exchanger to provide the second phase of cooling to below 3 deg. C.
The microprocessor based control system for the existing cooling plant was modified and extended to facilitate the integration of the new installation.
The modifications incorporated to the control system were :
- The ability to select any of the four compressors for ice building while maintaining parallel operation of the remaining plant.
- Directional control of the secondary refrigerant return to optimise on the use of the chiller and ice storage.

Call for proposal

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Coordinator

ARTHUR GUINNESS & SONS LTD
EU contribution
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Address
ST. JAMES'S GATE BREWERY
8 Dublin
Ireland

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Total cost
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