heat recoVery laTEnT and sEnsiBlE hEaT A thermal wheel cools and humidifies incoming air a SeNSiBLe aPProach to heat recoVery Heat-recovery technology has an important role to play in minimising energy use in commercial buildings where natural ventilation is not an option. Chris Jones, of Flkt Woods, considers the practicalities of recovering latent and sensible heat T he commercial sector contributes 38% to the nations carbon emissions, so has a significant role to play in helping the UK meet its carbon-reduction commitments. Heating and air conditioning, in particular, can account for a considerable amount of an organisations energy use, so it is essential that the system specified is as efficient as possible. In the new-build sector, the drive for airtight structures is positive news when it comes to reducing heat loss and improving efficiency. It does mean, however, that unless an effective ventilation system is also implemented, the quantities of fresh, outside air being introduced into the building will be compromised, and the potential for unacceptable levels of humidity will rise. The NHS recognises that common symptoms of illness can be attributed to poor indoor air quality (IAQ) sick building syndrome and it encourages businesses to invest in better ventilation to help reduce these.1 Specifiers should work closely with their chosen manufacturer to select a heating and mechanical ventilation solution that includes heat recovery, which is particularly important in buildings where effective natural ventilation is impossible. non-hygroscopic rotor transfers only sensible heat. In terms of heat recovery, if the outdoor air is sufficiently cold and the extract air is warm and humid, moisture will condense on the extract air side and evaporate on the supply air side. In this way, the rotor will transfer a certain amount of moisture to inside air, ensuring an acceptable level of humidity to meet best practice for indoor air quality. Without any supply of humidified air to the rooms, the rotor can manage outdoor temperatures as low as -15C without a frost problem. If outdoor air is very cold, however, frost will form inside the rotor and defrosting will be required. A hygroscopic rotor can transfer both sensible and latent heat under all conditions. Hygroscopic rotors have a surface coating with a high capacity for absorbing and emitting water molecules. When the rotor passage is on the side that has the highest vapour pressure, the water molecules will be adsorbed on the surface and later emitted to the air on the dry side. A hygroscopic rotor can cope with the potential of frost better than a non-hygroscopic one. When normal ventilation is used, the hygroscopic recovering heat Energy recovery sometimes referred to as heat recovery is already a proven solution for commercial premises that dont have effective natural ventilation. It works by extracting moist, stale air from inside the building, and replacing it with fresh outside air. The system uses Hygroscopic rotors have a surface coating with a high capacity for absorbing and emitting water molecules The Recooler HP (all images) has a reversible heat pump and recovery wheel heat from the outgoing air to warm the incoming air via a heat exchanger in an air handling unit (AHU). By using components, such as rotary heat exchangers, these systems can achieve high levels of heat recovery for a building ventilation system. However, it is important to understand the types of heat exchangers available and the differences between latent and sensible heat. Sensible heat is heat that causes a change in temperature of air or another substance; for example, when cool air is heated, its temperature rises as a result of this added heat. The reverse effect, when heat is removed from the air and its temperature falls, is also sensible heat. Latent heat, however, does not affect the temperature of a substance. For example, water remains at 100C while boiling, and the heat added to keep the water boiling is latent heat. Heat that causes a change of state, with no change in temperature, is latent heat. There are two types of rotary heat exchangers, or rotary wheels: nonhygroscopic and hygroscopic. Generally, non-hygroscopic rotors are used for heat recovery during the winter. In temperate climates, such as the UKs, cooling energy recovery during the summer is usually very limited, as a rotor can still operate, even if the outdoor temperature drops to -25C, without any frosting problem. Because the hygroscopic rotor transfers moisture, it contributes to a better indoor climate during the winter when the indoor air will typically have a low relative humidity. In the summer, when the outdoor air is warm and humid, the rotor dries it, and contributes drier and cooler air to the indoor climate. So a hygroscopic rotary wheel can cool and dehumidify the incoming air in summer, while heating and humidifying the incoming air in winter, instead of having to add a steam humidifier, for example, which would need additional plant to create the steam increasing energy use. Increasing the moisture content of the air using an evaporative humidifier would need a constant water supply, while the pressure drop associated with it would increase specific fan power (SFP). A high SFP could lead to noncompliance under Part L, meaning the AHU size would have to be increased. In the winter months, when the air is cold and dry with little moisture content, the rotary wheel is able to operate to its anticipated high performance, ensuring effective heat recovery while helping to sustain the optimal level of moisture in the air inside the building. This, in turn, lessens the impact that dry air can have on occupants reducing the potential for breathing difficulties and discomfort. During the summer, the recovery principles are the same, but with the reverse effect, decreasing the cooling load required by the overall system and removing humidity from the supply airstream. This summer operation also has a benefit when sizing cooling coils. If recovering only sensible energy in summer, you would reduce the cooling coil size by approximately 30%. If recovering latent energy too, this can be increased to 60%. This means a smaller cooling coil is needed, resulting in a smaller SFP because the pressure drop across the coil is not as high. The associated chilled water plant can also be reduced, freeing up plant space for other services. Financially, the hygroscopic thermal wheel costs about 10% more than a standard rotary wheel, and has no maintenance or longevity implications. a combined approach Combined systems that use, for example, a reversible heat pump and an energy recovery wheel in one unit, can deliver reduced operating costs as well as effective ventilation. Manufacturers are continuing to develop hybrid all in one solutions designed to meet IAQ demands, while also improving energy efficiency, meeting regulations and cutting life-cycle costs. cJ References 1 NHS Choices website: Sick building syndrome definition 2 Research conducted by Populus for Guardian Air Hygiene CHRIS JONES is national product sales manager at Flkt Woods