Many buildings need a constant exchange of clean air from outside with contaminated air generated within the building. This is because the maintenance of particular indoor air conditions is vital for the health and comfort of the occupants. However, this creates a problem, because often the temperature and humidity requirements of the inside air are different than those found outside. The incurrent air needs to be altered to be suitable for traveling into the building, which can demand lots of energy – in fact, cooling comprises a huge portion of energy usage worldwide. This is where energy recovery wheels (ERWs) come in.
What is an Energy Recovery Wheel?
ERWs are an integral part of heating, ventilation and air conditioning (HVAC). They are a form of energy recovery ventilators (ERVs), and can passively transfer heat (sensible) energy, moisture (latent) energy, or both, between the incurrent and excurrent air into and out of a building.
The concept of an ERW is simple. At the point in a building’s ventilation system where fresh air and exhaust air run counter currently, but adjacent to each other, an ERW can be placed in the ventilation system such that half of it will be exposed to fresh, incurrent air, and the other half to contaminated, excurrent air. This allows the ERW to absorb the desired temperature and humidity properties of the excurrent air (which has been conditioned to be suitable for within the building), and transfer them to the incurrent air. In this way, the incurrent air is preconditioned to be close to a desired temperature and humidity. This process is passive, and can significantly reduce the energetic and monetary cost of using new energy and moisture to treat incurrent air. In some conditions, energy recovery wheels can reduce the energetic demand of conditioning incurrent air by up to 95% (Fischer 1998). Essentially, an ERW is a mechanism for recycling sensible and latent energy.
How they work
ERWs make use of the first law of thermodynamics: “Energy can neither be created nor destroyed; rather, it transforms from one form to another.” The ERW is made of a complex lattice of sensible energy absorbing metal alloy, which is usually coated with a moisture absorbing substance such as silica gel or a molecular sieve. The honeycomb matrix of metal and moisture absorbing material maximizes the surface area the air passes over, to optimize the area for energy transfer. As the excurrent air passes through the ERW, the half of the wheel exposed will absorb sensible and latent energy from the air, and then carry that energy as it slowly rotates, to be exposed and transferred to the incurrent air to be released. The physical properties of the ERW then allow it to be adjusted to either warm up or cool down, as well as humidify or desiccate, incurrent air into a building. This adjustment capability is critical for the functionality of the ERW in different seasons. In a hot, humid summer the ERW can be used to passively cool down and desiccate the incurrent air into a building, and in winter then ERW can be used to warm up and humidify the incurrent air into the building (see figure 1 below). In both cases, the ERW recycles the desirable properties of the air already inside the building, transferring that energy, rather than forcing the use of new energy.
Figure 1. The adjustable settings of an enthalpy wheel, moderating the humidity and temperature of the incurrent and excurrent air differently between summer (top panel) and winter (bottom panel). Taken from “The Facts about Energy Recovery Ventilators” (Hoger 2009).
There are three broad types of ERWs. Thus far the ERWs that have been discussed have been ones that are capable of absorbing and transferring sensible and latent energy (enthalpy recovery wheels). However, there are two other types: ERW that is designed to deal with only sensible energy (purely heat transfer; heat recovery ventilator HRV), and those that only transfer latent energy (moisture only). The two that deal with only one aspect of energy transfer are often cheaper or easier to implement. In some cases, the primary concern concerning the difference between the indoor and outdoor air conditions may only be the relative humidity or temperature. In these cases, one might opt for a particular form of ERW that deals with the appropriate type of energy recovery. However, it has been shown that ERWs that deal with both are often the most cost-effective, and energy-efficient form.
Why are ERWs needed?
The need for ERWs is primarily in their contribution towards energy savings, which ultimately greatly reduce energy costs. Energy recovery wheels pay back the cost of their installation within three years. After this, they will begin saving a large amount of money for the rest of the device’s lifespan, which can be considerable (more than 20 years).
They are also important because of the movement towards greener energy and design and lowering carbon footprints. Energy savings and green conscious technology are becoming both fashionable and mandatory in legislation, so ERWs and similar energy-saving HVAC technology are of great interest and import to the whole modern world. By recycling the energy from exhausted air, ERWs make the most of a bad situation.
With increasing health and safety regulations in all public work places, from schools to restaurants to factories, there has been increasing focus on HVAC systems, and their role in the health and comfort. A key change in regulations has been to increase the volume and rate of turnover of fresh air into and out of buildings. This poses a significant challenge of regulating the quality and properties (temperature and relative humidity) of much larger volumes of incurrent air. HVAC systems already comprise a huge portion of energy usage, accounting for up to 50 % of energy usage by some building complexes.
Traditionally, this energy usage to condition incurrent air was not well regulated, and would often be independently cooled to regulate humidity, and then reheated to control temperature. This was a massive waste of energy. ERWs pose a considerable advancement in air conditioning technology, and stand to provide new and old (if fitted retroactively) buildings with large energy savings in an ever more energy conscious global society.