Making a full recovery
Exhaust heat sources from a range of systems offer the potential for heat recovery by increasing the COP of heat pumps and through other heat-recovery mechanisms. There is a wide range of applications, as Chris Ree explains.
There are many space-heating, process-heating and ventilation applications in the UK where perceived low-grade or unusable heat is discharged from the building — essentially, wasted heat. However, if we are to achieve our objectives of minimising use of fossil fuels and associated carbon emissions, we need to explore every opportunity to reduce such wastage.
In this context, we should consider all heat sources discharged to atmosphere as having potential for recovery and re-utilisation for space heating, feedwater pre-heating and domestic hot water.
Mechanical ventilation systems, for example, supply heated fresh air with heat recovery on the extract. Typically, these systems will provide between 60 to 80% heat recovery. This exhaust heat can be effectively recovered through the evaporator coil on an air-to-water heat pump, transferring the recovered heat to the hot-water circuit for space heating or DHW supply.
For example, many applications have ventilation requirements of 3 to 15 air changes per hour with 60 to 70% heat recovery through an air-to-air heat exchanger. Thus, the exhaust to atmosphere is often around 7ºC — considerably above ambient for much of the heating season — and this air can be passed through the heat-pump heat-recovery coil to provide a substantial increase in COP.
For instance, an air-to-water heat pump will have a COP of about 3.2 at -4ºC, increasing to 4.0 to 4.3 at 7ºC and, if there is no heat recovery, 4.5 to 5.0 at 20ºC — based on 35ºC water flow rate for the heating system.
The heat-pump output will effectively increase by over 30% with 7ºC flow, compared to –4ºC, onto the heat-recovery coil. The typical ventilation flow rate through the heat pump coil will be 87 l/s/kW of heat-pump rating. This air volume closely matches the typical ventilation requirements of many buildings, including schools, colleges, offices, supermarkets, department stores, restaurants, sports facilities.
Outside the ‘HVAC’ box
In fact, mechanical ventilation systems are just one example of how this principle can be, and is beginning to be, applied. In many ways it is the most obvious because ventilation and heating both fall within the discipline of mechanical building services, so the same people are generally dealing with system design.
However, if we cast our minds further afield there are many other instances where heat is being wasted from non-HVAC systems, and there is no reason why this heat source cannot be harnessed for space heating and DHW in the same building or complex. Indeed, there is a vast range of commercial and industrial process applications that exhaust heat to atmosphere through air or water outlets.
One common example is refrigerated food storage, where recovered heat from the condenser heat discharge is used for building-services heating. This technique is already in use in many supermarkets and can also be applied to other process areas, such as milk cooling. Similarly, production plant such as plastic injection moulding machines require cooling circuits, and the heat can be recovered for space heating.
Other industries where heat can be recovered from liquid effluents include food processing with the production of soft drinks, packaging, cooking processes and sterilisation; the textile industry with washing and dye processes; the paper production industry; paint processes and plating. All these areas have the potential to contribute heat to other systems and are worthy of exploration by any building-services engineer working in these sectors.
Just as there is huge variation in the range of potential applications, there is potentially a vast range of temperatures to deal with. The temperature of the exhaust fluid to atmosphere may range from 0 to 200°C. It is possible to recover a high proportion of this heat through fluid-to-water heat exchangers, air-to-air heat exchangers, air-to-water and water-to-water heat exchangers such as the heat pump, thermal heat and moisture transfer through the use of thermal wheels and exhaust flue gas to water heat recovery.
Thermal wheels provide a simple cost effective solution for heat recovery with efficiencies in excess of 80% and the ability to recover moisture from the exhaust air and transfer this moisture to the supply air.
These recovery systems can be combined to optimise the total heat-recovery process with facilities for buffer storage and compatible heating, ventilation and heat-recovery units for space heating, calorifier storage for DHW and heat-recovery/transfer exchangers for process pre-heating.
With the supply of all these heat-recovery products and the controls interface it is possible to provide a cost-effective solution for clients in a wide range of industries, with each application being assessed for its fuel-saving, cost-reduction and payback potential.
Chris Ree is divisional manager for indoor climate systems at Hoval Ltd.