Myths and Facts about Heat Systems

hot water heater

As with any profession, engineers can experience misconceptions about key matters relating to their area of expertise amongst members of the public.

When such misconceptions occur, the profession has a leading role to play to educate the public and promote informed decision making and debate.

In the United Kingdom, one such area of particular relevance at the moment relates to heat energy and heating systems. In The Future of Heating: A strategic framework for low carbon heat in the UK, published earlier this month by the UK’s Department of Energy and Climate Change (DECC), the department’s Chief Scientific Advisor, David Mackay, sets out to clarify what he sees as four common misconceptions about heat and heat energy.

Clarifying such misconceptions is particularly important to the UK right now as the country aims to lay down a framework for heating systems in order to achieve its goal of zero or near carbon emitting buildings by 2050.

Debunking common misconceptions

Mackay says four common misconceptions regarding heat and heat energy are as follows.

(1) A unit of heat is as useful as a unit of chemical or electrical energy.

Mackay says that whilst it is true that electrical and chemical energy can be converted into heat, it does not follow that a unit of heat is as useful as a unit of chemical or electrical energy. The laws of thermodynamics, Mackay says, dictate that heat cannot be converted back into electrical or chemical energy with one-hundred percent efficiency, and that there is an ‘irreversible’ reduction in the capacity of energy to do useful things as it is converted between different forms.

One unit of heat energy, therefore, is intrinsically less useful that a unit of chemical or electrical energy.

(2) A good way to measure the value of some heat is by the quantity of energy in that heat.

Mackay says a further misconception revolves around the notion that a good way to measure the value of a given amount of heat is by the quanity of energy in that heat.

Wrong, he says. Assuming a surrounding air temperature of 10 degrees celcius for example, one could obtain the same ‘quantity’ of heat energy by either heating one cup of water to 100 degreses or heating three cups of water to 40 degrees. However, one could not make a cup of coffee with the heat in the lesser cups because the water is not boiling. Thus the three lesser heated cups contain the same total quantity of heat as the one boiling cup, but the heat in the lesser cups is not as useful or does not have the same intrinsic value as that in the boiling cup.

In short, the quality of heat matters as well as the quantity.

hot water heater system

(3) Power stations are wasting energy if they send 50% of their heat into cooling towers.

Mackay says a third misconception exists that power stations are ‘wasting energy’ if they turn only 50% of the heat in their boilers into electricity and send the other 50% into air conditioning towers as opposed to putting the ‘waste heat’ into better use.

He says that such ‘waste heat’ is an essential feature of an efficient thermal power station and that this near-ambient temperature heat must be delivered to a near-ambient temperature place for the power station to work.

Whilst it is not necessary for such heat to be delivered to a cooling tower (radiators in a greenhouse or building would be fine), this ‘waste heat’ have little value for alternative uses because it is near ambient temperature. To make any use of waste heat, Mackay says, power stations would need to send piped heat at a higher temperature, which would mean that they would inevitably produce less energy compared with an optimised electricity only power station.

(4) Boilers that turn gas heat into heat with 90% efficiency and electric heaters that do so with 100% efficiency are as good as it gets.

A final misconception, Mackay says, is that boilers which turn gas into heat with 90% efficiency and electric heaters that produce heat from electrical energy at 100% efficiency are unbeatably efficient ways of delivering heat.

Wrong, he says. Heat pumps, can deliver three times as much heat as the energy it uses whilst power stations (‘virtual heat pumps’) can deliver heat at between three and seven times each unit of energy foregone.

Furthermore, he says, the closer the heat’s temperature is to ambient, the better the ability of heat pumps and power stations to deliver more heat.

“So for domestic heating, there is a common message: both heat pumps and CHP-driven district heating work more efficiently with low temperature heat emitters; the key steps that enable central heating systems to be run at lower temperatures are to improve building insulation, and to ensure that the radiators are large enough” Mackay says.

Educating the public

Misconceptions about particular areas of expertise on the part of policy makers or the general public are a common feature of any profession.

Still, in order to assist policy-makers in effective decision making regarding town planning and infrastructure and households and commercial building owners and managers in understanding the implications of their choice of heating systems, engineers have a leading role to play in educating the public about heat energy and its uses.

 

 By Andrew Heaton
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