Energy recovery ventilation

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Energy recovery ventilation (ERV) is the energy recovery process of exchanging the energy contained in normally exhausted building or space air and using it to treat (precondition) the incoming outdoor ventilation air in residential and commercial HVAC systems. During the warmer seasons, the system pre-cools and dehumidifies while humidifying and pre-heating in the cooler seasons. [1] The benefit of using energy recovery is the ability to meet the ASHRAE ventilation & energy standards, while improving indoor air quality and reducing total HVAC equipment capacity.

Contents

This technology has not only demonstrated an effective means of reducing energy cost and heating and cooling loads, but has allowed for the scaling down of equipment. Additionally, this system will allow for the indoor environment to maintain a relative humidity of 40% to 50%. This range can be maintained under essentially all conditions. The only energy penalty is the power needed for the blower to overcome the pressure drop in the system. [1]

Importance

Nearly half of global energy is used in buildings, [2] and half of heating/cooling cost is caused by ventilation when it is done by the "open window" method according to the regulations [define method and include citation]. Secondly, energy generation and grid is made to meet the peak demand of power. To use proper ventilation; recovery is a cost-efficient, sustainable and quick way to reduce global energy consumption and give better indoor air quality (IAQ) and protect buildings, and environment.

Methods of transfer

An energy recovery ventilator (also abbreviated ERV) is a type of air-to-air heat exchanger that not only transfers sensible heat but also latent heat. Because both temperature and moisture are transferred, ERVs can be considered total enthalpic devices. On the other hand, 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, whereas all ERVs are HRVs, not all HRVs are ERVs, but many people use the terms HRV, AAHX (air-to-air heat exchanger), and ERV interchangeably. [3]

Throughout the cooling season, the system works to cool and dehumidify the incoming, outside air. This is accomplished by the system taking the rejected heat and sending it into the exhaust airstream. Subsequently, this air cools the condenser coil at a lower temperature than if the rejected heat had not entered the exhaust airstream. During the heating seasons, the system works in reverse. Instead of discharging the heat into the exhaust airstream, the system draws heat from the exhaust airstream in order to pre-heat the incoming air. At this stage, the air passes through a primary unit and then into a space. With this type of system, it is normal, during the cooling seasons, for the exhaust air to be cooler than the ventilation air and, during the heating seasons, warmer than the ventilation air. It is for this reason the system works very efficiently and effectively. The coefficient of performance (COP) will increase as the conditions become more extreme (i.e., more hot and humid for cooling and colder for heating). [4]

Efficiency

The efficiency of an ERV system is the ratio of energy transferred between the two air streams compared with the total energy transported through the heat exchanger. [5] [6]

With the variety of products on the market, efficiency will vary as well. Some of these systems have been known to have heat exchange efficiencies as high as 70-80% while others have as low as 50%. Even though this lower figure is preferable to the basic HVAC system, it is not up to par with the rest of its class. Studies are being done to increase the heat transfer efficiency to 90%. [5]

The use of modern low-cost gas-phase heat exchanger technology will allow for significant improvements in efficiency. The use of high conductivity porous material is believed to produce an exchange effectiveness in excess of 90%. By exceeding a 90% effective rate, an improvement of up to five factors in energy loss can be seen. [5]

The Home Ventilating Institute (HVI) has developed a standard test for any and all units manufactured within the United States. Regardless, not all have been tested. It is imperative to investigate efficiency claims, comparing data produced by HVI as well as that produced by the manufacturer. (Note: all units sold in Canada are placed through the R-2000 program, a standard test synonymous to the HVI test). [6]

Types of energy recovery devices

Energy recovery deviceType of transfer
Rotary enthalpy wheel Total & sensible
Fixed plate Total** & sensible
Heat pipe Sensible
Run around coil Sensible
Thermosiphon Sensible
Twin towers [7] Sensible

**Total energy exchange only available on hygroscopic units and condensate return units

Rotary air-to-air enthalpy wheel

The rotating wheel heat exchanger is composed of a rotating cylinder filled with an air permeable material resulting in a large surface area. The surface area is the medium for the sensible energy transfer. As the wheel rotates between the supply and exhaust air streams it picks up heat energy and releases it into the colder air stream. The driving force behind the exchange is the difference in temperatures between the opposing air streams which is also called the thermal gradient. Typical media used consists of polymer, aluminium, and synthetic fiber.

The enthalpy exchange is accomplished through the use of desiccants. Desiccants transfer moisture through the process of adsorption which is predominately driven by the difference in the partial pressure of vapor within the opposing air-streams. Typical desiccants consist of silica gel, and molecular sieves.

Enthalpy wheels are the most effective devices to transfer both latent and sensible energy but there are many different types of construction that dictate the wheel's durability. The most common materials used in the construction of the rotor are polymer, aluminium and fiberglass.

When using rotary energy recovery devices the two air streams must be adjacent to one another to allow for the local transfer of energy. Also, there should be special considerations paid in colder climates to avoid wheel frosting. Systems can avoid frosting by modulating wheel speed, preheating the air, or stop/jogging the system.

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. [8]

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 for frosting in colder climates.

The technology patented by Finnish company RecyclingEnergy Int. Corp. [9] 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.

Related Research Articles

Heating, ventilation, and air conditioning Technology of indoor and vehicular environmental comfort

Heating, ventilation, and air conditioning (HVAC) is the technology of indoor and vehicular environmental comfort. Its goal is to provide thermal comfort and acceptable indoor air quality. HVAC system design is a subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics and heat transfer. "Refrigeration" is sometimes added to the field's abbreviation, as HVAC&R or HVACR or "ventilation" is dropped, as in HACR.

Dehumidifier

A dehumidifier is an electrical appliance which reduces and maintains the level of humidity in the air, usually for health or comfort reasons, or to eliminate musty odor and to prevent the growth of mildew by extracting water from the air. It can be used for household, commercial, or industrial applications. Large dehumidifiers are used in commercial buildings such as indoor ice rinks and swimming pools, as well as manufacturing plants or storage warehouses.

An evaporative cooler is a device that cools air through the evaporation of water. Evaporative cooling differs from typical air conditioning systems, which use vapor-compression or absorption refrigeration cycles. Evaporative cooling uses the fact that water will absorb a relatively large amount of heat in order to evaporate. The temperature of dry air can be dropped significantly through the phase transition of liquid water to water vapor (evaporation). This can cool air using much less energy than refrigeration. In extremely dry climates, evaporative cooling of air has the added benefit of conditioning the air with more moisture for the comfort of building occupants.

Psychrometrics field of engineering concerned with the physical and thermodynamic properties of gas-vapor mixtures

Psychrometrics, psychrometry, and hygrometry are names for the field of engineering concerned with the physical and thermodynamic properties of gas-vapor mixtures. The term comes from the Greek psuchron (ψυχρόν) meaning "cold" and metron (μέτρον) meaning "means of measurement".

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.

Recuperator

A recuperator is a special purpose counter-flow energy recovery heat exchanger positioned within the supply and exhaust air streams of an air handling system, or in the exhaust gases of an industrial process, in order to recover the waste heat. Generally, they are used to extract heat from the exhaust and use it to preheat air entering the combustion system. In this way they use waste energy to heat the air, offsetting some of the fuel, and thereby improves the energy efficiency of the system as a whole.

Air handler air handler

An air handler, or air handling unit, is a device used to regulate and circulate air as part of a heating, ventilating, and air-conditioning (HVAC) system. An air handler is usually a large metal box containing a blower, heating or cooling elements, filter racks or chambers, sound attenuators, and dampers. Air handlers usually connect to a ductwork ventilation system that distributes the conditioned air through the building and returns it to the AHU. Sometimes AHUs discharge (supply) and admit (return) air directly to and from the space served without ductwork

Radon mitigation is any process used to reduce radon gas concentrations in the breathing zones of occupied buildings, or radon from water supplies. Radon is a significant contributor to environmental radioactivity.

A ground-coupled heat exchanger is an underground heat exchanger that can capture heat from and/or dissipate heat to the ground. They use the Earth's near constant subterranean temperature to warm or cool air or other fluids for residential, agricultural or industrial uses. If building air is blown through the heat exchanger for heat recovery ventilation, they are called earth tubes (also termed earth cooling tubes, earth warming tubes, earth-air heat exchangers, air-to-soil heat exchanger, earth channels, earth canals, earth-air tunnel systems, ground tube heat exchanger, hypocausts, subsoil heat exchangers, thermal labyrinths, underground air pipes, and others.

Underfloor heating and cooling is a form of central heating and cooling which achieves indoor climate control for thermal comfort using conduction, radiation and convection. The terms radiant heating and radiant cooling are commonly used to describe this approach because radiation is responsible for a significant portion of the resulting thermal comfort but this usage is technically correct only when radiation composes more than 50% of the heat exchange between the floor and the rest of the space.

Solar air conditioning refers to any air conditioning (cooling) system that uses solar power.

Energy recovery

Energy recovery includes any technique or method of minimizing the input of energy to an overall system by the exchange of energy from one sub-system of the overall system with another. The energy can be in any form in either subsystem, but most energy recovery systems exchange thermal energy in either sensible or latent form.

Natural ventilation process of supplying air to and removing air from an indoor space without using mechanical systems

Natural ventilation is the process of supplying air to and removing air from an indoor space without using mechanical systems. It refers to the flow of external air to an indoor space as a result of pressure differences arising from natural forces. There are two types of natural ventilation occurring in buildings: wind driven ventilation and buoyancy-driven ventilation. Wind driven ventilation arises from the different pressures created by wind around a building or structure, and openings being formed on the perimeter which then permit flow through the building. Buoyancy-driven ventilation occurs as a result of the directional buoyancy force that results from temperature differences between the interior and exterior. Since the internal heat gains which create temperature differences between the interior and exterior are created by natural processes, including the heat from people, and wind effects are variable, naturally ventilated buildings are sometimes called "breathing buildings".

Infiltration is the unintentional or accidental introduction of outside air into a building, typically through cracks in the building envelope and through use of doors for passage. Infiltration is sometimes called air leakage. The leakage of room air out of a building, intentionally or not, is called exfiltration. Infiltration is caused by wind, negative pressurization of the building, and by air buoyancy forces known commonly as the stack effect.

HVAC is a major subdiscipline of mechanical engineering. The goal of HVAC design is to balance indoor environmental comfort with other factors such as installation cost, ease of maintenance, and energy efficiency. The discipline of HVAC includes a large number of specialized terms and acronyms, many of which are summarized in this glossary.

Dedicated outdoor air system type of heating, ventilation and air-conditioning system

A dedicated outdoor air system (DOAS) is a type of heating, ventilation and air-conditioning (HVAC) system that consists of two parallel systems: a dedicated system for delivering outdoor air ventilation that handles both the latent and sensible loads of conditioning the ventilation air, and a parallel system to handle the loads generated by indoor/process sources and those that pass through the building enclosure.

Thermal wheel

A thermal wheel, also known as a rotary heat exchanger, or rotary air-to-air enthalpy wheel, or heat recovery wheel, is a type of energy recovery heat exchanger positioned within the supply and exhaust air streams of an air-handling system or in the exhaust gases of an industrial process, in order to recover the heat energy. Other variants include enthalpy wheels and desiccant wheels. A cooling-specific thermal wheel is sometimes referred to as a Kyoto wheel.

Run-around coil

A run-around coil is a type of energy recovery heat exchanger most often positioned within the supply and exhaust air streams of an air handling system, or in the exhaust gases of an industrial process, to recover the heat energy. Generally, it refers to any intermediate stream used to transfer heat between two streams that are not directly connected for reasons of safety or practicality. It may also be referred to as a run-around loop, a pump-around coil or a liquid coupled heat exchanger.

Radiant heating and cooling Systems using temperature-controlled surfaces to exchange heat with their surrounding environment through convection and radiation

Radiant heating and cooling systems are temperature-controlled surfaces that exchange heat with their surrounding environment through convection and radiation. By definition, in radiant heating and cooling systems, thermal radiation covers more than 50% of heat exchange within the space. Hydronic radiant heating and cooling systems are water-based. It refers to panels or embedded building components. Other types include air-based and electrical systems. Important portions of building surfaces are usually required for the radiant exchange.

Cooling load is the rate at which sensible and latent heat must be removed from the space to maintain a constant space dry-bulb air temperature and humidity. Sensible heat into the space causes its air temperature to rise while latent heat is associated with the rise of the moisture content in the space. The building design, internal equipment, occupants, and outdoor weather conditions may affect the cooling load in a building using different heat transfer mechanisms. The SI units are watts.

References

  1. 1 2 Dieckmann, John. "Improving Humidity Control with Energy Recovery Ventilation." ASHRAE Journal. 50, no. 8, (2008)
  2. http://www.interacademycouncil.net/CMS/Reports/11840/11914/11920.aspx
  3. The Healthy House Institute. Staff. "ERV". Understanding Ventilation: How to Design, Select, and Install Residential Ventilation Systems. June 4, 2009. December 9, 2009.
  4. Braun, James E, Kevin B Mercer. "Symposium Papers - OR-05-11 - Energy Recovery Ventilation: Energy, Humidity, and Economic Implications - Evaluation of a Ventilation Heat Pump for Small Commercial Buildings." ASHRAE Transactions. 111, no. 1, (2005)
  5. 1 2 3 Pulsifer, J. E., A. R. Raffray, and M. S. Tillack. "Improved Performance of Energy Recovery Ventilators Using Advanced Porous Heat Transfer Media." UCSD-ENG-089. December 2001.
  6. 1 2 Christensen, Bill. “Sustainable Building Sourcebook.” City of Austin’s Green Building Program. Guidelines 3.0. 1994.
  7. "Chapter 44: Air-Air Energy Recovery" (PDF). ASHRAE Systems and Equipment Handbook. American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). July 2000. p.  44.17. ISBN   978-1883413804.
  8. Huelman, Pat, Wanda Olson. Common Questions about Heating and Energy Recovery Ventilators Archived 2010-12-30 at the Wayback Machine University of Minnesota Extension. 1999. 2010.
  9. Recycling Energy