Heat recovery ventilation

Heat recovery ventilation (HRV), also known as mechanical ventilation heat recovery (MVHR), 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 (pre-cooled), and the fresh air enthalpy is increased (reduced) 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 .Sensible and latent heat recoveryMechanically driven, requiring energy inputAir velocityHigh heat transfer coefficientLimited to two separate airstreamsOperating pressuteNo cross contaminationInternal fluid should match local climate conditionsContact timeNo cross contaminationDifficult to integrate into existing structuresFluid typeOffset peak energy demandsExpensive Heat recovery ventilation (HRV), also known as mechanical ventilation heat recovery (MVHR), 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 (pre-cooled), and the fresh air enthalpy is increased (reduced) 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 . A heat recovery system is designed to supply conditioned air to the occupied space to continue the desired level of comfort. Heat recovery system keeps the house fully ventilated by recovering the heat which is coming from inside environment. Heat recovery system basically works as transferring the thermal energy (enthalpy) from one fluid to another fluid, from one fluid to one 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 and that causes mixture of two fluids. 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 (desiccant) 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 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 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. 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.