Operators of refrigeration systems can reduce energy by re-using waste heat, but the technology remains under-exploited. DK Heat Recovery is aiming to change that, writes Andrew Gaved
The subject of heat recovery is exercising the minds of a number of people in the cooling industry at the moment.
Anyone who has been to RAC’s Retail Question Time in the past couple of years will know that major retailers are all looking at how they can exploit the exhaust heat from their refrigeration systems in an efficient, controllable and cost-effective way.
The prospect of closer integration between cooling and heating, using an element of recovered heat, clearly offers economic benefits beyond energy saving by consolidating maintenance etc.
With the government actively looking at how waste heat can be incentivised under the likes of the Renewable Heat Incentive, there is clearly a mood to increase the take-up of heat recovery technology.
But given the wide range of potential applications, it is also clear that, to date, beyond the big retailers and a few pioneers, it is a resource that the cooling industry has yet to really wake up to.
Yorkshire-based DK Heat Recovery, which installs German-built recovery technology, which is able to produce food-grade hot water, is on a mission to change that.
The company’s Ken Riley says: “Heat recovery should be on the list of considerations for all refrigeration and air conditioning installations.
Wherever there is a need for hot water or space heating, harnessing the heat from refrigeration and air conditioning systems saves money and cuts carbon emissions by heating water for free.”
DK has been installing its systems in a wide range of applications, from convenience stores to black-pudding factories, Mr Riley says: “Heat recovery is a relatively new concept to the UK market, but in Europe no refrigeration system is installed without consideration being given to energy savings.
This means that DK has supplied thousands of systems throughout the continent, with the first systems now being over 40 years old and still operating.”
The DK system is connected into the discharge line between the compressor and the condenser, effectively taking on the role of the condenser, but instead generating food-grade standard hot water as opposed to discharging it as waste to the atmosphere (see schematic on p26).
“Unlike other forms of heat exchangers, the DK system is a twin wall construction (in simple terms one pipe within another pipe, with a safety circuit between the two pipes),” says Mr Riley. “This maximises the transfer of heat, allowing up to 100 per cent recovery of available heat.
Using multiple heat exchangers will allow the same level of system head pressure control as provided by a fan speed controller or multiple fans,” he says (see diagram overleaf).
Mr Riley notes that the twin wall construction ensures that it is impossible for refrigerant to enter the water side and vice versa. “With the addition of a leak detection device it meets the requirements of EN1717 for food-grade water.”
Another attraction of the DK configuration, Mr Riley stresses, is that with no moving parts it is “virtually maintenance-free”.
Importantly, he adds, while conventional plate heat exchangers are often used in this application, not only are they less efficient compared with the DK system but they could pose a substantial threat to the refrigeration plant in the event of a failure.
And, since plate heat exchangers do not conform to EN1717, they are illegal for use in food-grade water systems.
The systems are designed to work with all refrigerants, except ammonia. All systems come fitted with corrosion protection, while frost protection, Legionella control and temperature flow monitoring are all factory-fitted options.
The DK range of heat exchange systems start with a 50 litre capacity tanks – ideal for smaller applications such as the local butcher, the company notes – up to the largest 10,000-litre tanks for use in the food processing industry.
The heat exchangers are rated at 3 kW up to 18 kW and are fitted within the tank, so will continue to heat the water while the refrigeration plant is running, regardless of whether water is being drawn from the tank or not.
This, Mr Riley notes, enables the hot water to be available right from the start of the day at circa 70 deg C. DK can also provide external heat exchangers for large capacity applications.
The largest DK tank can house as many as 24 separate 18 kW internal heat exchangers, operating from individual condensing units or in parallel for connection to a large pack system – a configuration that can generate 8,500 litres of hot water per hour, Mr Riley notes.
The systems are available for use in providing hot water, building heating applications or a combination of both.
Among the current installations, two demonstrate the variety of applications: one is a Budgens convenience store where waste heat is harnessed from the store’s existing Hubbard pack system.
The installation’s 750-litre tank is fitted with an internal array of five heat exchangers with a total capacity of 60 kW. This supplies all the store’s heating, saving more than 120,000 kWh and reducing the annual carbon footprint by 24 tonnes.
Another project, currently under construction in Yorkshire, will see waste heat energy re-used from two new Franchill pack systems. Here, the 750-litre tank is fitted with an internal array of three heat exchangers with a total capacity of 24 kW to produce approximately 480 litres of hot water per hour.
A second 300-litre tank and a 12 kW heat exchanger provide hot water to the heating system for the offices.
The completed system is expected to provide energy savings of 115,000 kWh, 62 tonnes of C02 emissions per year and payback is forecast at less thanthree years.
Mr Riley also makes an appeal to the industry to promote the use of heat recovery higher up the design process. “Heat recovery should not been seen as an add-on but should be a core part of a contractor’s business – no installation design should be put forward without offering heat recovery as an opportunity for the customer.”
He also believes that heat recovery can be beneficial to the contractor in giving them a new ways to connect commercially with customers. “By adopting heat recovery as a core business line, other benefits will come.
Besides the standalone marketing opportunity, many enquiries for heat recovery become new customers, with installation and service revenues exceeding the original sale of heat recovery.
“The place to start is to get out your customer list and identify those who use hot water or need space heating.”
Bury Black Pudding Company
DK’s installation at the Bury factory has recently been independently verified by carbon consultant Deltamech to show the extent of the savings made.
Using the waste heat energy from seven new Franchill condensing units, a 5,000-litre tank fitted with an internal array of 12 heat exchangers with a total capacity of 96 kW produces approximately 2,000 litres of hot water per hour.
The factory has high requirement for hot water first thing in the morning, but heat rejection peaks from midday onwards, so the tank accumulate the hot water generated overnight.
Now, water reaches 70 deg C first thing in the morning – a temperature lift of 60 deg C – and once that peak has passed, sustains a temperature of 50 deg C.
The headline output is significant:
Annual hot water provision from heat recovery 3,042 cu m
Daily water consumption 11,770 litres (6,770 litres at 70 deg C, 5,000 litres at 50 deg C)
Daily energy consumption 666 kWh
Annual oil saving 20,280 litres
Annual CO2e saving 58 tonnes
Annual cost saving (50p/litre oil) £10,185