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Heat Recovery put to Good Use
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In HVAC system terms, heat recovery is a process of capturing, and putting to good use, heat that would, in the normal course of events, be “rejected to atmosphere”. One example is heat energy that is absorbed and rejected by an air conditioning system during its cooling cycle. This, to all intents and purposes, is free heat that can be successfully redirected for space heating, domestic water heating or any other process that requires heat.
Heat recovery is being successfully applied in all types of buildings, schools, manufacturing plants and office buildings. It typically provides a very attractive return on investment for building owner/operators.
Hotels with their relatively high year-round internal cooling loads combined with high hot water demand are excellent opportunities for “green” heat pump, heat recovery chiller technology.
Importantly, heat pump heat recovery systems can allow for substantially downsizing or even eliminating fossil fuel, combustion based heating equipment such as gas boilers. This was the case in a project undertaken and described here by the Daikin Distributor, Space Airconditioning plc.
Space Air has enjoyed a partnering agreement with a leading UK hotel group for over 20 years and, for a refurbishment project in the heart of London, supplied a 320kW Daikin air-cooled chiller with a heat recovery option. In addition to satisfying specific, almost continuous cooling demands, this chiller also provides warmed water to 4 x 500 litre indirect DHW cylinders.
The hotel’s hot water supply is pre-heated by the chiller’s heat recovery process; this in turn dramatically reduces the output required of new high efficiency gas fired water boilers.
The result is significant kW savings in energy and huge reductions in mains gas consumption.
The Hotel Group had committed to refurbish the Covent Garden Hotel including all guest bedrooms as well as function rooms, reception areas and restaurants.
Space Air technicians in conjunction carried out a survey with the hotel’s maintenance company. It was determined that whilst the existing Air Handling Unit serving the common areas and restaurants etc. was in good condition and could be re used, an existing air cooled chiller needed replacing, due to the fact that it was designed to operate with R22 refrigerant (the use of which for service purposes will be banned from 2015) and was in a state of disrepair with only one working compressor circuit.
The concept that a new Daikin Chiller to replace the old machine, could also provide cost effective heat recovery features, created interest because the hotel’s existing gas boilers were due to be replaced.
Space Air presented the features and specification of the Daikin Heat Recovery system to the client. Space Air was able to illustrate cost savings the Heat Recovery option would provide, and combined with the reduced costs for the smaller gas boilers, substantial reductions in the hotel’s hot water services operational costs could be realised.
The Hotel group decided it made good financial sense to proceed with the Heat Recovery option.
The new Daikin heat recovery chiller operates 24/7 and an existing back up (cooling only) Daikin chiller is only used in case of unlikely
failure. This means the new chiller provides virtually continuous pre-heated water for the DHWS.
On a seasonal basis it has been calculated that the chiller will produce an average of 236.6kW of hot water, free of charge. This takes into account the chiller operating for 24 hours, 7 days a week, 365 day a year (8760 hours).
Four x 500 Litre indirect cylinders provide pre-heated water on a separate circuit to the secondary heat exchanger in the basement, which serves the main feed to the showers, and DHWS for the hotel guestrooms.
Allowing for losses in the cylinders and pipework etc., the system achieves delta T of approximately 20°C. So the mains water supply to the DHWS heat exchanger is increased by this temperature rise resulting in a much lower demand than usual on the new gas boilers.
To put this into perspective the cost to heat 2000L of water based on a ∆T of 20°C with a gas boiler was calculated and compared with the ‘extra’ cost of providing a heat recovery option on the chiller price to perform the same function.
Approx. 46.44kWh (per hour) of energy is required. Over one year based on 24/7, 365 days (8760 hours) operation and with gas at 4p per kWh, the cost of gas would be approx. £16k.
The premium cost for the optional HR circuit on the Daikin Chiller plus the required tanks, pumps, pipework etc. Installed = £30/40k (Estimated). This equates to a payback period on the investment of less than 3 years.
So far the client is delighted with the cost savings achieved and is looking to apply similar heat recovery principles in more of his property portfolio.
