Hypothetical scenario – a very simplified illustration
A new cold storage warehouse is being designed and engineered for a seafood exporter.
The engineer has determined the following details:
The cold storage warehouse will have a cooling duty of 300 kW(average) to chill and maintain produce at -10oC.
The cost of local power from the grid has a peak tariff for 12hrs/day of $0.20/kWh and an off-peak tariff for 12hrs/day of $0.10/kWh
Option 1 – “conventional”
Install a single refrigeration unit with an 85% expected efficiency of converting grid power to actual cooling duty and size the refrigeration unit at 125% of average duty ie. 440kW.
Power cost per year (365 days) = 300kW / 85% x 24 hours x 365 days x A$0.15(av. power cost) = A$464,000 per annum
Option 2 -PCM “assisted”
Install a single refrigeration unit with an 85% expected efficiency of converting grid power to actual cooling duty and size the refrigeration unit at 200% of average duty ie. 706kW.
Also install a suitable quantity of encapsulated PC-10 with a passive cooling duty (at around -5oC) of 300kW for 12 hours (per day).
How much PC-10 to absorb 300kW of power for 12 hours?
Heat capacity over “useful range” (-15 to -5oC) for PC-10 = 310 kJ/kg (max)
300 kW = 300 kJ/sec = 12,960,000 kJ per 12 hours.
12,960,000/310 = 42 tonnes of PC-10 required.
Power cost per year (365 days)
Power drawn only at off-peak time = 300kW for chilling duty + 300kW for PC-10 “reactivation” duty = 600 kW / 85% x 12 x 365 x A$0.10 = A$309,000 per annum
The gross operating savings in power cost is ~ A$155,000 per annum by adopting the PCM Option 2.
A determination/judgement then needs to be made whether the additional capital cost of the PCM Option 2 is acceptable in return for the operating savings.
