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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[1]. 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 .[2]

Contents

Working principleEdit

 
Ventilation unit with heat pump & ground heat exchanger - cooling
 
Ventilation unit with heat pump & ground heat exchanger

A heat recovery system is designed to supply conditioned air to the occupied space to continue the desired level of comfort[3]. 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[4].

TypesEdit

Rotary thermal wheelsEdit

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.[2]

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. [5]

Fixed plate heat exchangersEdit

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 has been shown values of 90% sensible heat efficiency in transferring sensible heat from one air stream to another.[6]The high levels of efficiency are attributed to the high heat transfer coefficients of the materials used, operational pressure and temperature range. [7]

Heat pipesEdit

Heat pipes are a heat recovery device that use a multi-phase process to transfer heat from one airstream to another[7].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-aroundEdit

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.[8]

Phase change materialsEdit

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.[9] 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.[10]

ApplicationsEdit

 
Heat recovery ventilation with an earth-to-air heat exchanger, which is essential to achieve German Passivhaus standards.

Rotary thermal wheelEdit

O’Connor et al. [11]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 an external wind speed of 3 m/s, which is lower than the average wind speed of the UK (4–5 m/s).

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.[10]

Fixed plate heat exchangersEdit

 
Plate ground heat exchanger inside the foundation walls

Mardiana et al.[12] 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.[10]

Heat pipesEdit

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.[10]

Run-around systemsEdit

Flaga-Maryanczyk et al.[13] 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.[10]

Phase change materialsEdit

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.[14]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.[10]

Advantages and disadvantages[10]Edit

Type of HRV Advantages Disadvantages Performance Parameters Efficiency % Pressure Drop (Pa) Humidity Control
Rotary thermal wheel High efficiency

Sensible and latent heat recovery

Compact design

Frost control available

Cross contamination possible Requires adjacent airstreams

Mechanically driven, requiring energy input

Rotation speed

Air velocity

Wheel Porosity

80+ 4-45 Yes
Fixed plate No moving parts hence high reliability

High heat transfer coefficient

No cross contamination

Frost control possible

Sensible and latent heat recovery

High pressure loss across exchanger

Limited to two separate airstreams

Condensation build up

Frost building up in cold climates

Material type

Operating pressute

Temperature

Flow arrangement

70-90 7-30 Yes
Heat pipes No moving parts, high reliability

No cross contamination

Low pressure loss

Compact design

Heat recovery in two directions possible

Requires close airstreams

Internal fluid should match local climate conditions

Fluid type

Contact time

Arrangement/configuration

Structure

80 1-5 No
Run-around Airstreams can be separate

No cross contamination

Low pressure loss

Multiple sources of heat recovery

Multiple pumps required to move fluid

Difficult to integrate into existing structures

Low efficiency

Cost

Exchanger type

Fluid type

Heat source

50-80 ~1 No
Phase change materials Easy incorporation into building materials

Offset peak energy demands

No pressure loss

No cross contamination

No moving parts

Long life cycle

Thermal storage as opposed to instantaneous transfer

Expensive

Not proven technology

Difficulty in selecting appropriate material

Impregnation method ~ 0 No

Environmental impacts[15]Edit

Energy saving is one of the key issues for both fossil fuel consumption and protection of global environment.The rising cost of energy and the global warming underlined that developing of the improved energy systems is necessary to increase the energy efficiency while reducing greenhouse gas emissions. The most effective way to reduce energy demand is to use energy more efficiently. Therefore, waste heat recovery is becoming popular in recent years since it improves energy efficiency. About 26% of industrial energy is still wasted as hot gas or fluid in many countries.[16] However, during last two decades there has been remarkable attention to recover waste heat from various industries and to optimize the units which are used to absorb heat from waste gases. Thus, these attempts enhance reducing of global warming as well as of energy demand.

Energy consumptionEdit

In most industrialized countries, HVAC are responsible for one-third of the total energy consumption. Moreover, cooling and dehumidifying fresh ventilation air composes 20–40% of the total energy load for HVAC in hot and humid climatic regions. However, that percentage can be higher where 100% fresh air ventilation is required. This means more energy is needed to meet the fresh air requirements of the occupants. The heat recovery is getting a necessity due to an increasing energy cost for treatment of fresh air. The main purpose of heat recovery systems is to mitigate the energy consumption of buildings for heating, cooling and ventilation by recovering the waste heat. In this regard, stand alone or combined heat recovery systems can be incorporated into the residential or commercial buildings for energy saving. Reduction in energy consumption levels can also notably contribute in reducing greenhouse gas emissions for a sustainable world.

Greenhouse gasesEdit

CO2, N2O and CH4 are common greenhouse gases and CO2 is the most deleterious one in terms of environmental issues. Therefore the greenhouse gas emissions are frequently denoted as CO2 equivalent emissions. Total global greenhouse gas emissions increased 12.7% between 2000 and 2005. In 2005, around 8.3 Gt CO2 was released by building sector. Moreover buildings are responsible for more than 30% of greenhouse gas emissions each year in most of developed countries. According to another study, buildings in European Union countries cause about 50% of the CO2 emissions in the atmosphere. It is possible to mitigate the greenhouse gas emissions by 70% compared to the levels expected to be seen in 2030 if the proper measures are taken. The increase in greenhouse gas emissions due to high demand of energy use concluded as global warming. In this regard, mitigating gas emissions in the atmosphere stands out as one of the most crucial problems of the world today that should be resolved. Heat recovery systems have a remarkable potential to contribute in decreasing greenhouse gas emissions. The Scotch Whisky Association has carried out a project at Glenmorangie distillery to recover latent heat from new wash stills to heat other process waters. They have found that 175 t a year of CO2 will be saved with a payback period of under one year. In another report, it is underlined that 10 MW of recovered heat can be utilized for saving 350,000€ per year in emission costs. UK Climate Change Act of 2008 is targeting a 34% reduction in greenhouse gas emissions by 2020 compared with 1990 levels and an 80% reduction by 2050. They emphasize the notable potential and importance of heat recovery technologies to achieve this goal.

See alsoEdit

ReferencesEdit

  1. ^ Zhongzheng Lu,Zunyuan Xie, Qian Lu, Zhijin Zhao (2000). An Encyclopedia of Architecture & Civil Engineering of China. China Architecture & Building Press.
  2. ^ a b Mardiana-Idayu, A.; Riffat, S.B. (2012-02). "Review on heat recovery technologies for building applications". Renewable and Sustainable Energy Reviews. 16 (2): 1241–1255. doi:10.1016/j.rser.2011.09.026. ISSN 1364-0321. Check date values in: |date= (help)
  3. ^ S. C. Sugarman (2005). HVAC fundamentals. The Fairmont Press, Inc.
  4. ^ Ramesh K. Shah, Dusan P. Sekulic (2003). Fundamentals of Heat Exchanger Design. New Jersey: John Wiley & Sons, Inc.
  5. ^ Fehrm, Mats; Reiners, Wilhelm; Ungemach, Matthias (2002-06). "Exhaust air heat recovery in buildings". International Journal of Refrigeration. 25 (4): 439–449. doi:10.1016/s0140-7007(01)00035-4. ISSN 0140-7007. Check date values in: |date= (help)
  6. ^ Nielsen, Toke Rammer; Rose, Jørgen; Kragh, Jesper (2009-02). "Dynamic model of counter flow air to air heat exchanger for comfort ventilation with condensation and frost formation". Applied Thermal Engineering. 29 (2–3): 462–468. doi:10.1016/j.applthermaleng.2008.03.006. ISSN 1359-4311. Check date values in: |date= (help)
  7. ^ a b Mardiana-Idayu, A.; Riffat, S.B. (2012-02). "Review on heat recovery technologies for building applications". Renewable and Sustainable Energy Reviews. 16 (2): 1241–1255. doi:10.1016/j.rser.2011.09.026. ISSN 1364-0321. Check date values in: |date= (help)
  8. ^ Vali, Alireza; Simonson, Carey J.; Besant, Robert W.; Mahmood, Gazi (2009-12). "Numerical model and effectiveness correlations for a run-around heat recovery system with combined counter and cross flow exchangers". International Journal of Heat and Mass Transfer. 52 (25–26): 5827–5840. doi:10.1016/j.ijheatmasstransfer.2009.07.020. ISSN 0017-9310. Check date values in: |date= (help)
  9. ^ Feldman, D.; Banu, D.; Hawes, D.W. (1995-02). "Development and application of organic phase change mixtures in thermal storage gypsum wallboard". Solar Energy Materials and Solar Cells. 36 (2): 147–157. doi:10.1016/0927-0248(94)00168-r. ISSN 0927-0248. Check date values in: |date= (help)
  10. ^ a b c d e f g O’Connor, Dominic; Calautit, John Kaiser S.; Hughes, Ben Richard (2016-02). "A review of heat recovery technology for passive ventilation applications". Renewable and Sustainable Energy Reviews. 54: 1481–1493. doi:10.1016/j.rser.2015.10.039. ISSN 1364-0321. Check date values in: |date= (help)
  11. ^ O’Connor, Dominic; Calautit, John Kaiser; Hughes, Ben Richard (2014-10). "A study of passive ventilation integrated with heat recovery". Energy and Buildings. 82: 799–811. doi:10.1016/j.enbuild.2014.05.050. ISSN 0378-7788. Check date values in: |date= (help)
  12. ^ Mardiana A, Riffat SB, Worall M. Integrated heat recovery system with windcatcher for building applications: towards energy-efficienct technologies. In: Mendez-Vilas A, editor. Materials and processes for energy: communicating current research and technological developments. Badajoz: Formatex Research Center; 2013. line feed character in |title= at position 82 (help)
  13. ^ Flaga-Maryanczyk, Agnieszka; Schnotale, Jacek; Radon, Jan; Was, Krzysztof (2014-01). "Experimental measurements and CFD simulation of a ground source heat exchanger operating at a cold climate for a passive house ventilation system". Energy and Buildings. 68: 562–570. doi:10.1016/j.enbuild.2013.09.008. ISSN 0378-7788. Check date values in: |date= (help)
  14. ^ Kosny J, Yarbrough D, Miller W, Petrie T, Childs P, Syed AM, Leuthold D. Thermal performance of PCM-enhanced building envelope systems. In: Proceedings of the ASHRAE/DOE/BTECC conference on the thermal perfomance of the exterior envelopes of whole buildings X. Clear Water Beach, FL; 2–7 December 2007. p. 1–8. line feed character in |title= at position 73 (help)
  15. ^ Cuce, Pinar Mert; Riffat, Saffa (2015-07). "A comprehensive review of heat recovery systems for building applications". Renewable and Sustainable Energy Reviews. 47: 665–682. doi:10.1016/j.rser.2015.03.087. ISSN 1364-0321. Check date values in: |date= (help)
  16. ^ Teke, İsmail; Ağra, Özden; Atayılmaz, Ş. Özgür; Demir, Hakan (2010-05). "Determining the best type of heat exchangers for heat recovery". Applied Thermal Engineering. 30 (6–7): 577–583. doi:10.1016/j.applthermaleng.2009.10.021. ISSN 1359-4311. Check date values in: |date= (help)

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