Decarbonization for non-engineers: heat recovery

Using “waste heat,” or heat generated from various campus processes that is usually lost to the environment, as a source of energy is a bit like finding a twenty dollar bill in your pocket that you forgot was there: it’s a resource you’ve always had. You just need to capture it to use it.

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By Madeline Rawson

Heat recovery takes the fairly simple principle of heat transfer, which we can see in an ice cream cone melting on a hot day or in warming your hands by ambushing an unsuspecting friend, and using it to capture an otherwise wasted resource. How heat recovery systems do this, however, is not so simple. In this article, I dive into how heat recovery actually works, the applications of the technology, and the benefits to using it, in terms even your unsuspecting friend can understand.

How does heat recovery work?

Heat recovery is the process of capturing and reusing thermal energy that would otherwise be lost to the environment. In buildings, this “waste heat” can be a significant source of energy when repurposed, reducing heating and cooling costs, decreasing greenhouse gas emissions, and improving energy efficiency.

Heat recovery systems work by using a heat exchanger to transfer thermal energy between two energy streams (e.g., water, air, etc.). In the winter, heat that is rejected from the building will be transferred to an incoming air source. For example, exhaust air from a building transfers heat to cold outside air, preheating it.

Applying heat recovery to building systems

Energy recovery ventilators (ERVs)

An energy recovery ventilator (ERV) is a mechanical ventilation system that brings fresh air into a building while simultaneously exhausting stale indoor air. One of its key functions is to recover energy from the indoor air.

In the winter, the warm, outgoing air passes through the heat exchanger, transferring its heat to the cold, incoming air (illustrated below). This “pre-heats” the fresh air before it enters the building’s heating system, significantly reducing the energy needed to bring it to a comfortable temperature. In the summer, the process reverses. The cool, outgoing air transfers some of its coolness to the warm, incoming air, pre-cooling it and reducing the load on the building’s air conditioning system. This technology can be applied to any area in a building with a high outside air load; primarily labs, bathrooms, gyms, and other recreating areas.

illustration of an energy recovery ventilator

Variable refrigerant flow heat pumps

Variable refrigerant flow (VRF) heat pumps can be ground- or air-sourced (see article on how heat pumps work here). They meet simultaneous heating and cooling needs within a building by capturing and using waste heat. In a VRF heat recovery system, the refrigerant doesn’t just transfer heat from inside to outside; it moves heat within the building from where it’s not wanted to where it is (illustrated below). This can lead to significant energy savings, as the system is essentially running both a cooling and heating cycle at the same time, with one helping to power the other.

illustration of variable refrigerant flow heat pumps

Wastewater heat recovery

Wastewater, especially from showers and laundry, carries a significant amount of thermal energy. Wastewater heat recovery is the process of capturing and reusing thermal energy from used warm water (or “greywater”). These systems use a heat exchanger to preheat incoming cold water before it enters the water heater, reducing the energy needed to reach the desired temperature.

Heat recovery chillers

Large and complex buildings can generate considerable waste heat from refrigeration and cooling systems. Heat-recovery chillers maximize energy efficiency by recovering waste heat generated during the cooling process. This otherwise wasted heat can be used for various purposes, including heating buildings and water. These chillers are ideal for buildings or campus loops with simultaneous heating and cooling needs. Examples include laboratories, supercomputing centers, hospitals, food service facilities, and residential complexes.

illustration of a heat recovery chiller on cooling mode

illustration of a heat recovery chiller on heating mode

Benefits

The application of heat recovery in building systems offers numerous benefits:

Energy efficiency and cost savings. By reclaiming waste heat, buildings use less energy for heating and cooling, leading to substantial reductions in utility bills.
Improved indoor air quality. Heat recovery ventilators provide continuous, controlled fresh air, which helps remove pollutants and stale air without the energy penalty of opening windows.
Reduced GHG emissions. Less energy consumption directly translates to lower greenhouse gas emissions, supporting decarbonization efforts.
Right-sized equipment. By reducing heating and cooling loads, heat recovery allows for the installation of smaller, less expensive HVAC equipment.

Installing a heat recovery system, or implementing any decarbonization solution, differs from building to building. GreenerU is here to be your decarbonization partner, so please reach out to us with any questions you have!