Waste heat

[edit] Introduction

Waste heat is a by-product of other applications. It can sometimes be collected (recovered) from those applications and re-used for heating and other purposes as a means to save energy and reduce both running costs and carbon emissions. Without this recovery, the heat energy is simply wasted.

In the urban environment, electrical generators, industrial processes and heat lost through building envelopes are the biggest sources of waste heat. The burning of transport fuels is also a major contributor.

Typical applications which generate waste heat and from which recovery is possible include:

• Boiler combustion gases;
• Domestic, commercial and industrial extract air;
• Hot water discharge (domestic, commercial and industrial);
• Refrigeration plant;
• Power generating plant;
• Lighting systems;
• Kilns;
• Combustion engines;
• Sewage.

The uses to which recovered heat can be put include:

• Space heating;
• Water heating;
• Pre-heating of combustion air for boilers;
• Ovens and furnaces;
• Pre-heating fresh air in building ventilation systems;
• Drying processes;
• Power generation;
• Heating greenhouses in colder climates.

[edit] Recovering waste heat

In order to reclaim waste heat energy, it has to pass through a lower temperature heat sink, i.e some sort of physical collector which will absorb as much heat as possible from the recovery source and from which the waste energy can be taken. An example is waste heat from air conditioning units stored in a buffer tank to be used for night-time heating.

Ideally, the waste heat source and the sink into which it can be stored temporarily are physically in close proximity. If this is not the case, there are some systems that can be used at a different location or at a different time.

More than half of the input energy used in almost all industrial processes becomes waste heat. Some of this may be converted into electrical energy by a range of methods, one of which involves the use of a thermoelectric device: this works when a change in temperature across a semi-conductor material creates a voltage that causes a flow of electricity.

Waste heat forms part of the equation for the conservation of energy:

• QH = QL + W

So, for example, in a gas boiler:

• QH is the heat input to the system (e.g that which may be derived from burning gas)
• W is the useful heat (or work) produced by the system, and
• QL is the useful waste heat.

By the nature of the laws of thermodynamics, waste energy will have a lower utility (or exergy) than the original energy source.

[edit] Related articles on Designing Buildings Wiki

• Air conditioning.
• Air handling unit.
• Chiller unit.
• Coefficient of Performance CoP.
• District energy.
• Exhaust air heat pump.
• Geothermal pile foundations.
• Gross calorific value.
• Heat exchanger.
• Heat pump.
• Heat recovery ventilation.
• Heating.
• HVAC.
• Mechanical ventilation.
• Refrigerants.
• Types of heating system.
• Variable refrigerant flow.
• Zero Bills Home.

[edit] External sources

• Carbon Trust: Heat recovery checklist and briefing note