Heat recovery ventilation
Heat recovery ventilation, also known as mechanical ventilation heat recovery, is an energy recovery ventilation system which works between two sources at different temperatures. Heat recovery is a method which is increasingly used to reduce the heating and cooling demands of buildings. By recovering the residual heat in the exhaust gas, the fresh air introduced into the air conditioning system is pre-heated, and the fresh air enthalpy is increased before the fresh air enters the room or the air cooler of the air conditioning unit performs heat and moisture treatment. A typical heat recovery system in buildings consists of a core unit, channels for fresh air and exhaust air, and blower fans. Building exhaust air is used as either a heat source or heat sink depending on the climate conditions, time of year and requirements of the building. Heat recovery systems typically recover about 60–95% of the heat in exhaust air and have significantly improved the energy efficiency of buildings.
Working principle
A heat recovery system is designed to supply conditioned air to the occupied space to continue the desired level of comfort. The heat recovery system keeps the house fully ventilated by recovering the heat which is coming from the inside environment. Heat recovery systems basically work by transferring the thermal energy from one fluid to another fluid, from one fluid to a solid or from a solid surface to a fluid, at different temperatures and in thermal contact. Additionally, there is no direct interaction between fluid and fluid or fluid and solid in most of the heat recovery systems. In some application of heat recovery systems, fluid leakage is observed due to pressure differences which can cause mixture of the two fluids.Types
Rotary thermal wheels
Rotary thermal wheels are a mechanical means of heat recovery. A rotating porous metallic wheel transfers thermal energy from one air stream to another by passing through each fluid alternately. The system operates by working as a thermal storage mass whereby the heat from the air is temporarily stored within the wheel matrix until it is transferred to the cooler air stream.Two types of rotary thermal wheel exist, heat wheels and enthalpy wheels. Though there is geometrical similarity between heat and enthalpy wheels, there are differences which effect the operation of each design. In a system utilizing a desiccant wheel, the moisture in the airstream with the highest relative humidity is transferred to the opposite airstream after flowing through the wheel. This can work in both directions of incoming air to exhaust air and exhaust air to incoming air. The supply air can then be used directly or employed to further cool the air, this is an energy intensive process.
Fixed plate heat exchangers
Fixed plate heat exchangers are the most commonly used type of heat exchanger and have been developed for 40 years. Thin metal plates are stacked with a small spacing between plates. Two different airstreams pass through these spaces, adjacent to each other. The heat transfer occurs as the temperature transfers through the plate from one airstream to the other. The efficiency of these devices have shown values of 90% sensible heat efficiency in transferring sensible heat from one air stream to another. The high levels of efficiency are attributed to the high heat transfer coefficients of the materials used, operational pressure and temperature range.Heat pipes
s are a heat recovery device that use a multi-phase process to transfer heat from one airstream to another. Heat is transferred using an evaporator and condenser within a wicked, sealed pipe containing a fluid which undergoes constant phase change to transfer heat. The fluid within the pipes changes from a fluid to a gas in the evaporator section, absorbing the thermal energy from the warm airstream. The gas condenses back to a fluid in the condenser section where the thermal energy is dissipated into the cooler airstream raising the temperature. The fluid/gas is transported from one side of the heat pipe to the other through pressure, wick forces or gravity, depending on the arrangement of the heat pipe.s.|alt=Run-around
Run-around systems are a hybrid heat recovery system that incorporates characteristics from other heat recovery technology to form a single device, capable of recovering heat from one air stream and delivering to another a significant distance away. There is the general case of run-around heat recovery, two fixed plate heat exchangers are located in two separate airstreams and are linked by a closed loop containing a fluid which is continually pumped between the two heat exchangers. The fluid is heated and cooled constantly as it flows around the loop, providing the heat recovery. The constant flow of the fluid through the loop requires pumps to move between the two heat exchangers. Though this is an additional energy demand, using pumps to circulate fluid is less energy intensive than fans to circulate air.Phase change materials
, commonly referred to as PCMs, are a technology that is used to store sensible and latent heat within a building structure at a higher storage capacity than standard building materials. PCMs have been studied extensively due to the ability to store heat and transfer heating and cooling demands from conventional peak times to off-peak times.The concept of thermal mass of a building for heat storage, that the physical structure of the building absorbs heat to help cool the air, has long been understood and investigated. A study of PCMs in comparison to traditional building materials has shown that the thermal storage capacity of PCMs is twelve times higher than standard building materials over the same temperature range. The pressure drop across PCMs has not been investigated to be able to comment on the effect that the material may have on airstreams. However, as the PCM can be incorporated directly into the building structure, this would not affect the flow in the same way other heat exchanger technologies do, it can be suggested that there is no pressure loss created by the inclusion of PCMs in the building fabric.
Applications
Rotary thermal wheel
O’Connor et al. studied the effect that a rotary thermal wheel has on the supply air flow rates in to a building. A computational model was created to simulate the effects of a rotary thermal wheel on air flow rates when incorporated into a commercial wind tower system. The simulation was validated with a scale model experiment in a closed-loop subsonic wind tunnel. The data obtained from both tests were compared in order to analyse the flow rates. Although the flow rates were reduced compared to a wind tower which did not include a rotary thermal wheel, the guideline ventilation rates for occupants in a school or office building were met above a external wind speed of 3 m/s, which is lower than the average wind speed of the UK.No full scale experimental or field test data was completed in this study; therefore it cannot be conclusively proved that rotary thermal wheels are feasible for integration into a commercial wind tower system. However, despite the air flow rates decrease within the building after the introduction of the rotary thermal wheel, the reduction was not large enough to prevent the ventilation guideline rates being met. Sufficient research has not yet been conducted to determine the suitability of rotary thermal wheels in natural ventilation, ventilation supply rates can be met but the thermal capabilities of the rotary thermal wheel have not yet been investigated. Further work would beneficial to increase understanding of the system.
Fixed plate heat exchangers
Mardiana et al. integrated a fixed plate heat exchanger into a commercial wind tower, highlighting the advantages of this type of system as a means of zero energy ventilation which can be simply modified. Full scale laboratory testing was undertaken in order to determine the effects and efficiency of the combined system. A wind tower was integrated with a fixed plate heat exchanger and was mounted centrally on a sealed test room.The results from this study indicate that the combination of a wind tower passive ventilation system and a fixed plate heat recovery device could provide an effective combined technology to recover waste heat from exhaust air and cool incoming warm air with zero energy demand. Though no quantitative data for the ventilation rates within the test room was provided, it can be assumed that due to the high pressure loss across the heat exchanger that these were significantly reduced from standard operation of a wind tower. Further investigation of this combined of technology is essential in understanding the air flow characteristics of the system.
Heat pipes
Due to the low pressure loss of heat pipe systems, more research has been conducted into the integration of this technology into passive ventilation than other heat recovery systems. Commercial wind towers were again used as the passive ventilation system for integrating this heat recovery technology. This further enhances the suggestion that commercial wind towers provide a worthwhile alternative to mechanical ventilation, capable of supplying and exhausting air at the same time.Run-around systems
Flaga-Maryanczyk et al. conducted a study in Sweden which examined a passive ventilation system which integrated a run-around system using a ground source heat pump as the heat source to warm incoming air. Experimental measurements and weather data were taken from the passive house used in the study. A CFD model of the passive house was created with the measurements taken from the sensors and weather station used as input data. The model was run to calculate the effectiveness of the run-around system and the capabilities of the ground source heat pump.Ground source heat pumps provide a reliable source of consistent thermal energy when buried 10–20 m below the ground surface. The ground temperature is warmer than ambient air in winter and cooler than ambient air in summer, providing both heat source and heat sink. It was found that in February, the coldest month in the climate, that the ground source heat pump was capable of delivering almost 25% of the heating needs of the house and occupants.
Phase change materials
The majority of research interest in PCMs is the application of phase change material integration into traditional porous building materials such as concrete and wallboards. Kosny et al. analysed the thermal performance of buildings which have PCMenhanced construction materials within the structure. Analysis showed that the addition of PCMs is beneficial in terms of improving the thermal performance.A significant drawback of PCM use in a passive ventilation system for heat recovery is the lack of instantaneous heat transfer across different airstreams. Phase change materials are a heat storage technology, whereby the heat is stored within the PCM until the air temperature has fallen to a significant level where it can be released back into the air stream. No research has been conducted into the use of PCMs between two airstreams of different temperature where continuous, instantaneous heat transfer can occur. An investigation into this area would be beneficial for passive ventilation heat recovery research.