Heat recovery for warming

Utilizing heat for a nearby heat demand, like what is done in Combined Heat & Power (CHP) or Cogeneration, is typically the most efficient use of thermal energy.


heat_recovery_for_warming

Benefits

If you have waste heat and an existing heat demand, the simplest, most efficient, and often most cost effective way to use that heat is to allocate it to the heat demand. In many cases, it can displace a boiler that requires fuel to operate or electric heaters that need power from the grid. As a result, the primary benefits are:

  • Significant savings through a reduction in operating costs
  • Reduction in capital spend (avoidance of a boiler purchase)
  • Potential to monetize the heat by selling it to nearby facilities
  • Reduction or elimination of fuel used for heating
  • Reduced CO2 and other emissions
  • Less price volatility

Examples of using recovered heat

There are a variety of ways that waste heat can be used for warming. Like all waste heat utilization projects, there needs to be a heat source and a use for the heat somewhere in the area. The different heat for warming projects primarily fall into two buckets:

Heat recycling

 An example of combustion air pre-heating.

An example of combustion air pre-heating.

Heat can be used in a different stage of the very process that is producing the heat. Recycling the heat in an existing process is ideal because the transport complexities are reduced, and the heat production occurs at the same time as the heat demand.

Common examples of heat recycling are:

  • Combustion air pre-heating
  • Boiler feedwater pre-heating
  • Load/charge pre-heating
  • Water heating for use in the same process.

Large breweries will recycle heat to reduce the amount of steam they need to generate.  At the end of the brewing process, the liquid needs to be cooled from boiling to room temperature.  Brewers will use a heat exchanger to pull heat from the hot liquid and transfer it to another tank to begin preheating the water for another batch.

Heat allocation

 An example of combined heat and power using a reciprocating engine

An example of combined heat and power using a reciprocating engine

With heat allocation, waste heat is transferred to a heat demand that is not in the same process. Heat allocation creates a potential source of revenue for the heat producer, and can create savings for a local user of the heat.

Common examples of heat allocation are:

  • Warming a hospital, pool, or greenhouse
  • Water heating
  • Space heating
  • Steam generation

Heat allocation is common in Combined Heat & Power (CHP) systems whereby a generator creates heat as a byproduct of operating. The exhaust heat can be captured and delivered to a nearby heat demand.


Heat exchangers, and how it's done

There are many different ways to capture and utilize heat. The most common forms are various types of heat exchangers that transfer heat from one medium to another. A heat exchanger is a device used to transfer heat between one or more media. The media are often separated by a wall to prevent mixing and less commonly they may be in direct contact. 

A typical application for heat exchanger is the transfer of heat from a flue gas to a liquid, such as water. The water can then be pumped to a location that requires hot water, or where water can be used for heating. Check out our Heat recovery page for more details on how the heat is utilized from the heat source.

Once the heat is extracted, it must be put to use. The desired end use of the heat will dictate the way it is transmitted and eventually used. Below are a few options, shown with a heat exchanger transferring heat from one medium to another.

 

Heat inlet

  • Air or flue gas
  • Water
  • Steam
heat_exchanger_energy_recovery

Heat outlet

  • Hot air
  • Hot water
  • Steam
  • Other medium
 

Considerations

Heat exchanger technologies enable the flow of energy 'downhill' from a high temperature use to a lower temperature use. As such, a project will not achieve a higher temperature heat from a lower temperature heat. The exception to this is with a heat pump, which requires energy to be put back into the system.

As heat is extracted from a heat source, the temperature is reduced. So although an exhaust stream may be 400F, as heat is transferred, the exhaust will cool down. To see limitations on this for quantity of heat at various temperatures, check out our calculator.

Water and steam can be transported relatively long distances to deliver heat to buildings or users nearby. Great examples of this are on college campuses where steam loops deliver heat to various buildings all around campus from one central heat source.