Heat recovery ventilation (HRV), also known as mechanical ventilation heat recovery (MVHR) is a ventilation system that recovers energy by operating between two air sources at different temperatures. It is 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 preheated (or pre-cooled) before it enters the room, or the air cooler of the air conditioning unit performs heat and moisture treatment. A typical heat recovery system in buildings comprises a core unit, channels for fresh 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 the exhaust air and have significantly improved the energy efficiency of buildings.
Energy recovery ventilation (ERV) is the energy recovery process in residential and commercial HVAC systems that exchanges the energy contained in normally exhausted air of a building or conditioned space, using it to treat (precondition) the incoming outdoor ventilation air. The specific equipment involved may be called an Energy Recovery Ventilator, also commonly referred to simply as an ERV.
An ERV is a type of air-to-air heat exchanger that transfers latent heat as well as sensible heat. Because both temperature and moisture are transferred, ERVs are described as total enthalpic devices. In contrast, a heat recovery ventilator (HRV) can only transfer sensible heat. HRVs can be considered sensible only devices because they only exchange sensible heat. In other words, all ERVs are HRVs, but not all HRVs are ERVs. It is incorrect to use the terms HRV, AAHX (air-to-air heat exchanger), and ERV interchangeably.The Healthy House Institute. Staff. "ERV". Understanding Ventilation: How to Design, Select, and Install Residential Ventilation Systems. 4 June 2009. 9 December 2009.
During the warmer seasons, an ERV system pre-cools and dehumidifies; during cooler seasons the system humidifies and pre-heats.Dieckmann, John. "Improving Humidity Control with Energy Recovery Ventilation." ASHRAE Journal. 50, no. 8, (2008) An ERV system helps HVAC design meet ventilation and energy standards (e.g., ASHRAE), improves indoor air quality and reduces total HVAC equipment capacity, thereby reducing energy consumption. ERV systems enable an HVAC system to maintain a 40-50% indoor relative humidity, essentially in all conditions. ERV's must use power for a blower to overcome the pressure drop in the system, hence incurring a slight energy demand.
Working principle A heat recovery system is designed to supply conditioned air to the occupied space to maintain a certain temperature. A heat recovery system helps keep a house ventilated while recovering heat being emitted from the inside environment. The purpose of heat recovery systems is to transfer 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. There is no direct interaction between fluid and fluid or fluid and solid in most heat recovery systems. In some heat recovery systems, fluid leakage is observed due to pressure differences between fluids, resulting in a mixture of the two fluids.The purpose of an energy recovery system is to reduce the energy required for heating, cooling, or ventilating the space by repurposing the exhaust air's energy.
Types Thermal wheel
Fixed plate heat exchanger
Fixed plate heat exchangers have no moving parts, and consist of alternating layers of plates that are separated and sealed. Typical flow is cross current and since the majority of plates are solid and non permeable, sensible only transfer is the result.
The tempering of incoming fresh air is done by a heat or energy recovery core. In this case, the core is made of aluminum or plastic plates. Humidity levels are adjusted through the transferring of water vapor. This is done with a rotating wheel either containing a desiccant material or permeable plates.Huelman, Pat, Wanda Olson. Common Questions about Heating and Energy Recovery Ventilators University of Minnesota Extension. 1999. 2010.
Enthalpy plates were introduced in 2006 by Paul, a special company for ventilation systems for passive houses. A crosscurrent countercurrent air-to-air heat exchanger built with a humidity permeable material. Polymer fixed-plate countercurrent energy recovery ventilators were introduced in 1998 by Building Performance Equipment (BPE), a residential, commercial, and industrial air-to-air energy recovery manufacturer. These heat exchangers can be both introduced as a retrofit for increased energy savings and fresh air as well as an alternative to new construction. In new construction situations, energy recovery will effectively reduce the required heating/cooling capacity of the system. The percentage of the total energy saved will depend on the efficiency of the device (up to 90% sensible) and the latitude of the building.
Due to the need to use multiple sections, fixed plate energy exchangers are often associated with high pressure drop and larger footprints. Due to their inability to offer a high amount of latent energy transfer these systems also have a high chance of frosting in colder climates.
The technology patented by Finnish company RecyclingEnergy Int. Corp.Recycling Energy is based on a regenerative plate heat exchanger taking advantage of humidity of air by cyclical condensation and evaporation, e.g. latent heat, enabling not only high annual thermal efficiency but also microbe-free plates due to self-cleaning/washing method. Therefore, the unit is called an enthalpy recovery ventilator rather than heat or energy recovery ventilator. Company's patented LatentHeatPump is based on its enthalpy recovery ventilator having COP of 33 in the summer and 15 in the winter.
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 air streams pass through these spaces, adjacent to each other. Heat transfer occurs as the temperature transfers through the plate from one air stream to the other. The efficiency of these devices has reached 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 Heat pipes are a heat recovery device that uses a multi-phase process to transfer heat from one air stream to another. Heat is transferred using an evaporator and condenser within a wicked, sealed pipe containing a fluid which undergoes a 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 air stream. The gas condenses back to a fluid in the condenser section where the thermal energy is dissipated into the cooler air stream 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 Run-around systems are 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. The general case of run-around heat recovery, two fixed plate heat exchangers are located in two separate air streams and are linked by a closed loop containing a fluid that is continually pumped between the two heat exchangers. The fluid is heated and cooled constantly as it flows around the loop, providing 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 Phase change materials, or 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 their ability to store heat and transfer heating and cooling demands from conventional peak times to off-peak times.
The concept of the 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 air streams. 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.
Fixed-bed regenerators
According to ASHRAE, a fixed-bed regenerator (FBR) is an exchanger where one or more stationary matrices are charged with energy from discharge air and transfer it to alternating flows of supply and exhaust air.
Building on this principle, FBRs serve as heat recovery ventilator (HRV) that help reduce the energy consumption of HVAC systems by pre-conditioning incoming fresh air with high apparent sensible effectiveness.
FBRs come in a variety of sizes and configurations to suit different airflow demands. They operate using one or two cores to accomplish energy transfer, reversing the direction of airflow during each recovery cycle. While single-core systems are more common in Europe, double-core systems, such as the patented DualCore® and DualCorePlus™ technologies by Tempeff Inc., are more prevalent in North America.
To improve handling, cleaning, and transport, cores are often divided into smaller cells. Most are made of aluminum, though polypropylene alternatives are available for lightweight or corrosion sensitive environments. The plates within each cell can have varying spacer textures, corrugation patterns, and thickness, typically spaced 0.1 to 0.5 inches apart.
Airflow is managed by damper systems (either multiblade dampers or single deflector plates) which rapidly switch flow direction between the two stationary exchangers. During each cycle, one core delivers fresh, conditioned air while the other core exhausts stale air. The airflow is reversed so that the cores can recover after a period which usually ranges from 50 to 70 seconds. This alternating pattern enables continuous ventilation with high energy recovery performance.
Some FBRs are also designed for latent recovery in heating mode. Applying a desiccant coating to the exchanger surfaces enables moisture transfer in a wide range of environmental conditions. In desiccant-treated FBRs, latent effectiveness typically ranges from 60% to 80%, whereas untreated systems may achieve up to 70%.
In free cooling mode, recovery periods may extend to 3 hours. Certain FBRs use varying recovery times, ranging from 20 to 120 seconds, to regulate the temperature and moisture content of exhaust air. Shorter cycles can improve overall heat exchange efficiency by increasing air transfer rates.
Applications
Fixed plate heat exchangers