Joplings department store in Sunderland was one such client. Due to an electrical load limit and stringent requirements for low noise breakout, the site proved to be an economic and feasible location for absorption refrigeration.
The store required comfort cooling for its 8000 m2 store area within a budget of £1 million. Oscar Faber was appointed as the building services and structural consultants for the project. Initial surveys began in May 1998, and the project was completed in November 1999.
The site was not without its challenges. The client specified that none of the retail floor area could be compromised, not even for risers, and all works must be carried out without loss of trading time. There are roads on three facades and an adjacent building on the fourth, which resulted in restricted access and no on-site storage facility. The contractor carried out all works during night hours with materials located in remote site stores.
The need for air conditioning
Oscar Faber carried out a site survey to assess the existing building electrical, heating and future cooling loads, investigate potential options to provide fresh air and cooling the store, and to determine comparative capital and running costs for the cooling options.
While the building's heating loads are minimal due to all rooms on external walls being used as stores, the retail area has high internal heat gains from lighting and occupancy levels.
Air conditioning the store was justified on the basis that the fresh air supply to the sales floor is limited to natural ventilation and a few local extract fans. Aside from the floors being perceived as stuffy and overheated, the ventilation was considered not to comply with current Workplace Regulations, which stipulate a fresh air supply rate of 5 litres/s/person.
The loads on the building comprised internal gains from 300 staff and up to 1000 customers (one customer per 8 m2/sales floor area), lighting, small power and supply air of 75 W/m2.
Constraints imposed by the physical characteristics of the store limited the choice of comfort cooling to ceiling-mounted chilled water fan coils or variable refrigerant flow (vrf) units. Faber could find little to choose between either system in terms of capital and running costs, but the vrf units were not compatible with gas-fired absorption refrigeration.
The cooling options were based on three primary sources of energy for the operation of the refrigeration equipment:
Option appraisal
Faber's analysis of potential sources of refrigeration identified four options. Each one is defined by an acronym which relates to figures 1-3, over. The options were:
Examination of the maximum demand showed that all options could generate the required cooling within the maximum available capacity of 700 kVA (600 kW at a power factor of 0.85), except the electric chillers.
The electrical requirements that would arise from the use of electrically-driven refrigeration machinery was well in excess of the authorised maximum demand from Northern Electric. While the utility could increase the demand to 700 kVA using the existing transformer and incoming supply, this would not be enough for future store upgrades.
Figure 2 shows that the two chp options offer the best running costs. The heat rejection from gas absorption cooling is greater than a conventional system. However, the lower cost of the primary energy source and greater efficiency in part-load conditions results in annual running costs being lower than conventional electrically-driven chillers.
Figure 3 reveals that the capital costs for the chp options were much higher than for the two non-chp systems. The capital cost for chp-fired absorption came in at £1 350 000 with a 6·4 y payback, while gas-fired absorption would cost £1 060 000 with a payback of 2·2 y.
Due to the electrical load limit and calculations based on the life cycle cost, utility costs on-site, equipment capital costs and future forecast maintenance costs, gas absorption cooling was shown to be the most economic solution.
Site works
The chiller selected was a Sanyo lithium bromide unit, supplied by McQuay. The unit provides 649 kW of comfort cooling via a chilled water system operating at 6°C via the double-effect system. Heat rejection was provided by roof-mounted dry air coolers.
Considerable old and redundant services were present in ceiling voids and throughout the store. While existing service routes provided useful distribution space, live services were often situated adjacent to redundant plant, so all existing m&e had to be tested prior to removal. Many old works were also insulated with asbestos that required specialist removal.
Following a structural survey it was confirmed that new plant could be located on a rooftop frame. This required the equipment to be designed for a severe coastal/saline environment and noise limitations of 60 dBA on the site boundary.
The chiller was installed on-site, complete with integral micro-processor controls. These manage the chiller's lithium bromide/water solution concentrations so avoiding the operating problems of crystallisation.
The chiller operates automatically on timed control. It modulates to meet the store's cooling loads, which tend to be more or less constant throughout the day. This avoids frequent starting of the chiller.
Fresh air is provided by a new air handling unit (ahu) located externally on the roof. A balanced extract air system was provided by placing local extract fans on each floor. These are interlocked with the roof ahu.
The chilled water system serves fan coil units located within the ceiling void throughout the store. Each retail floor area has five notional zones for temperature control, governed via thermostats located on columns within the space. Areas such as lighting, audio and the cafe were zoned accordingly.
The supply air heating is provided by the original basement boilers via a secondary pumped circuit and heat exchanger. The project involved considerable enhancements to the lighting and electrical power supplies throughout the store, which also added to the internal heat gains.
Downloads
Figure 1: Comparison of cooling sources.
Other, Size 0 kbFigure 2: Annual running costs comparisons of the four options
Other, Size 0 kbFigure 3: Capital costs comparison.
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Source
Building Sustainable Design
Postscript
Kate Whitley BEng (Hons) Graduate CIBSE is a principal engineer with Oscar Faber Consulting Engineers. Further reading Good Practice Guide 256: An introduction to absorption cooling, DETR May 1999.
