Low Grade Heat

Much of the debate on sustainability focuses on electricity, partly because wind and solar technologies are sufficiently developed to make a significant contribution.  However, whilst a typical household in the UK might consume 4,000 kwh or electricity per year, it’s consumption of gas for space heating and hot water might be 15,000 kwh, the hot water component might be 3,000 kwh.

Gas boilers are a reliable way of heating water, typically the feed water temperature is in the range 10 – 15 deg. C and the final temperature is around 50 deg. C.  Typical uses for hot water are bathing, laundry and rinsing pots and pans.  After use, the water goes into the drain at something like 20 – 25 deg. C.  Maybe some form of heat recovery device which raises the feed water temperature by 5 – 10 degrees might save 500 kwh/year in a typical household.

Sadly, there is little economic incentive to investigate such a scheme.  Gas currently costs 6p/kwh, so a saving of 500 kwh of energy, represents a cost reduction of £30/year which does not buy a lot of plumbing.  The simplest scheme I can think of is a holding tank for warm waste water, at the top of this tank is a heat exchanger coil which is coupled to a lower coil in a domestic hot water cylinder (similar to those used in solar thermal water heaters), this setup might rely on gravity feed to save the cost of a pump.  It is difficult to see how this could be produced for an installed cost which is within sight of £150.  Such a device might look like this:

An idea for a domestic hot water economizer

Technology is an important element in sustainability, but so to is culture and behavior.  From the mid-19th to the mid-20th century steam was extensively used for motive power (steam trains), static power (pumping, electrical generation, factories etc) and heating.  Many steam installations had some form of economizer, the purpose of which was to recover “lost” heat.  Sometime back in a restored steam pumping station I saw something that looked like this.

Chain Economizer

A series of cast iron block were attached to a chain, this chain was hung on a wheel located in a chimney where it was heated by the flu gases from the boiler.  As the blocks rotated, they passed through the feed water tank where they raised the temperature of the water.  The drawing is from memory and may not be accurate.  Handbooks on the operation of steam plant usually have a section on economizers.

The quest for fuel economy some times took some strange turns.  In the inter-war period, both naval and merchant ships often had steam driven ancillaries, so even a ship was in port or at anchor it was consuming energy.  Competitions devised by admirals and owners in wood paneled London offices  awarded prizes to ships with the lowest energy consumption.  I’m guessing, but the value of the prize was much lower than the reduction in fuel bills.


About SolarBucket

I trained as a mechanical engineer in the 1970's and then spent most of the following 25 years doing sums and software for Oil and Gas Exploration. Current interests are the study of wind and solar resources.
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