Notes for FSP0012 – Energy Recover Ventilation (ERV)
(Scroll to bottom for drawings of fixed-plate heat exchanger and enthalpy wheel)
Motivation:
In most places, most of the time, the air inside of our buildings is either too hot or too cold or too humid or too dry for our intended use. In order to bring the building space to our desired level of temperature and humidity, we have to contend with both the ambient conditions (that is the climate or weather outside) and also heat and humidity generated by activities inside the building (people working, appliance/machinery running). We use heating and air conditioning systems for this. Talked about refrigerant in #7.
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To make it less sort of syntactically awkward, I’ll talk about the situation where it’s hot outside and we’re cooling the inside of the building, but the concepts apply equally if we want to heat the inside of the building and it’s cold outside.
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So we use a lot of energy (and a lot of money) cooling the air inside the building (and also removing the excess humidity if we’re in a very humid place…I’m in Florida, so there’s plenty of excess humidity here). If we have good insulation and a tight building envelope (and an other such things) so that we can keep all of the air we cooled inside of the building and can can keep the outside heat outside, then our air conditioning system only has to contend with the heat generated inside the building. Cooling the already cool and dry recirculated air inside the building takes much less energy than cooling the hot humid air outside the building.
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Unfortunately it’s not that easy for us. We also have to contend with other factors of indoor air quality.
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CO2 buildup
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Formaldehyde and other VOC off-gassing from furniture and other materials in side the building
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chemical emission from processes.
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restrooms, food service, etc… that put a bunch of junk into the air that is either uncomfortable or unhealthy.
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The most common method we use to improve indoor air quality in the face of the things I just mentioned is ventilation. We exhaust some of the air inside the building to the outside and bring fresh outside air inside.
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We just spent a bunch of precious energy (= lots of money) cooling and dehumidifying that exhaust air. And we’re just dumping that energy and money outside.
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The fresh, ventilation air we’re bringing in from outside is hot and humid, so we need to spend a bunch more energy and money cooling it down and taking the humidity out. Then later on we’re going to dump it back outside and do it again.
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Obviously this is a waste of energy, but the ventilation function is important. It would be nice if we had a way to recover the energy from the conditioned air before dumping it outside.
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We do, and probably not surprisingly, it’s called ERV – Energy Recovery Ventilation.
Why ERV?
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Use less energy (save money). We use ERV when we can recover more energy than we spend in the ERV mechanism…obviously we won’t bother if we don’t have a net energy recovery (and in some circumstances we don’t and it’s not worth it).
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Use smaller cooling and heating equipment for a given exhaust requirement – because the net energy recovery we get from ERV is basically free energy from the point of view of our air conditioning equipment. Basically we use energy recovered from the exhausted air to give our AC plant a head start vs the outside air.
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comply with energy use regulations, codes, standards, or rating systems (LEED, Energy Star).
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these things can raise standards for indoor air quality, calling for increased ventilation
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…but also call for higher energy efficiency (which means recovering energy from exhausted air).
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ERV works very well in some places, not really worth it in other places.
HRV vs ERV (sensible vs latent heat energy recovery)
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Two forms of energy – sensible heat and latent heat – I talked about this a little bit in the context of refrigerant in #7 of this podcast, the one about refrigerant. Same concept applies here except were concerned about the latent heat related to the condensation of water vapor into liquid water
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Sensible heat is related to the change in temperature of the air.
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Latent heat is the energy required to cause the water in the air to change phase from vapor to liquid or the other way around. When we cool the hot, humid air from outside, we remove sensible heat from the air to change the temperature, but we also remove latent heat from the air. This is made apparent by, for example the large amount of condensate (liquid water) that drips off of the evaporator coils in our air conditioning equipment. All of that water was in the air, and we had to spend a lot of energy to get that water out, above the energy spent just bringing the air temperature down.
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When we exhaust air from our building, were not only throwing away the energy we spent to remove the sensible heat from the air, we’re also throwing away the energy we spent to remove the latent heat from the air.
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Some ERV systems only remove sensible heat, which means they use the cool outgoing exhaust air to cool down the hot incoming ventilation air without using the dryness of the exhaust air to pick up some of the moisture from the humid incoming air (and remember I’m defaulting to talking about a space we are cooling, but this concept works just as well if it’s cold outside and we’re trying to keep the inside warm). The systems that remove only sensible heat are more properly called Heat recovery ventilation (HRV) because they only attempt to recover the sensible heat and not the latent heat (although often people don’t distinguish between the 2 in casual conversation and call both ERV).
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Other ERV systems attempt to recover both sensible heat and latent heat from the exhaust air by using the coldness of the air to cool the incoming air and also the dryness of the air to remove humidity from the incoming air. These devices can be described as total enthalpy devices and are properly called ERV. Enthalpy is a term that references the total heat content of the system. In this context it refers to a system and accounts for both sensible heat and latent heat.
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Another term you might see to describe both ERV and HRV is air-to-air heat exchange…because, you know, that’s what we’re doing here.
Mechanically, how does it work? Several types of ERV devices.
ERV types (not all types are suitable for all applications, the type you use will be chosen by the design engineer)
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fixed-plate heat exchanger – ductwork for exhaust and ventilation makeup air are arranged so that they cross each other. Since we don’t want the air to mix (because then we would be putting the exhaust air back into the building) we need to put something in the crossroads of the ductwork to allow heat exchange between the 2 air streams but not allow mixing of the air. In a fixed plate heat exchanger, we do this by stacking a bunch of plates (probably aluminum) in the crossroads. This created a stack of channels that air can flow through. By closing off 2 sides of one of the spaces, we can allow one of the air streams to flow through that space but not the other. In the adjacent space we close off the other 2 sides so that the air from the other stream can flow through and alternate like that. The warmer air heats up the plates, while the colder air moving through the adjacent passage (which shares a plate) takes heat from the plate. This way we can transfer sensible heat between the 2 air streams without mixing them up. This is kid of hard to describe in audio-only format. I made a drawing which you can find by going to facilityscience.com and looking for the notes for this episode (this is #12). There might also be a video there with a 3D model. I haven’t actually finished the video yet at the time I am recording this, so the video may or may not be there when you are listening to this. In any case, there is a drawing. The size and thickness and material of the plates and the distance between plates is chosen to maximize heat transfer while allowing the required volume of air to flow through. We leave the details of this to the design engineers.
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fixed-membrane heat exchanger – the fixed-plate heat exchanger I just described only recovers sensible heat. If we want to also attempt to recover latent heat, we need a way to transfer moisture between the 2 air streams. We can use the concept of fixed-plate heat exchanger (that is a stack of plates creating alternating passages) except instead of aluminum plates we use some material that is moisture permeable but not air permeable. So this material will pick up moisture from the higher humidity stream, the moisture will then migrate through the material into the passage with the dryer air and be picked up and taken away by the dryer air. This type of fixed-plate heat exchanger is often called a fixed-membrane heat exchanger since we are using a moisture permeable membrane to separate the air passages rather than impermeable plates (but some people will still call it fixed-plate)
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Enthalpy wheel (total energy wheel)
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In this case our exhaust air and supply air are in adjacent ducts. We place a wheel (a circular device) in the ducts such that at any give time half of the wheel is in he exhaust duct and the other half is in the outside air duct. The wheel is oriented so that the air is hitting the face of the wheel (so there’s a half circle cross section in each duct).
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As the wheel rotates, the part of the wheel that was in one duct rotates into the other, and it does around and around.
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The wheel is made up of thin layers wrapping around and around the wheel so that there is a lot of surface area of the wheel material but there is enough space between the layers that air can be forced through via pressure differential.
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The wheel is constructed so that it will transfer both sensible heat and latent from one air stream to the other.
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The main construction will be of either a metal like aluminum or some type of polymer mesh material that can be alternately heated and cooled by the different air streams.
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The wheel material is the also coated with some type of desiccant material that readily absorbs water (either something like silica gel that you might be used to see in little packets inside product packaging, or a molecular sieve material (a fancy ceramic that is made out of complicated sciency things that we’ll call out-of-scope for this podcast). The desiccant material absorbs moisture from the wetter air stream and is then dried by the dryer air stream.
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Coil loop – Another method for sensible heat recovery (no latent heat) is a coil loop. Have a heat transfer coil in each air stream with piping connecting the 2 coils creating a loop. Energy is collected from air in one coil and deposited in the other air stream via the other coil. Pumps pump the heat transfer material inside the pipes.
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isolates ventilation air (incoming air) from exhaust air (outgoing air), prevents contamination of incoming air.
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No latent heat transfer (humidity) because air is completely isolated, sensible heat only.
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Requires pumps. Good choice when absolutely no cross contamination is acceptable.
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There are other ERV mechanisms
Fixed-Plate Heat Exchanger
Fixed-Plate Heat Exchanger Detail
Enthalpy Wheel