Can we use lakes to heat our homes?

Having made some light-hearted comments about the effect of insecurity in Russia on commercial interest in renewable energy in a recent post (“The truth is sometimes stranger than fiction …”), I was intrigued by an article in this week’s Independent on Sunday: “Renewable energy from rivers and lakes could replace gas in homes”. The premise is that the high specific heat capacity of water means that large bodies of water store a huge amount of thermal energy acquired from solar heating. An engineering company has now come up with an ingenious device, based on similar principles to refrigerators and air conditioners, which extracts this heat and stores it in a form that can be used to heat homes. The headline figure, probably widely optimistic, is that this could reduce household bills by 20 per cent.

But, hang on, what are the environmental consequences of this? Lakes, in particular, depend upon solar heating in many ways. What might happen if large quantities of the thermal energy in lakes are removed? Could it, for example, affect the ecology of species that depend upon particular ranges of temperature? Might this shorten the potential growth season for some species? Might it, even, counteract some of the consequences of climate change?

One reassuring aspect is that the most plausible locations for such schemes are lowland areas where rivers are already heavily modified and the standing water bodies are artificial reservoirs. The Independent on Sunday article described a scheme on the River Thames for example. Managers of lowland reservoirs, in particular, might find unexpected benefits: solar energy is responsible for the stratification of lakes in summer, when a warm surface layer sits over cooler water in the depths. When stratification is combined with abundant nutrients, as is often the case in lowland reservoirs, you have ideal conditions for toxic blue-green algae to thrive. Toxic algal blooms are neither natural nor desirable, as they pose health risks and incur higher treatment costs. Reducing nutrient concentrations is desirable but difficult. Maybe this type of energy-harvesting scheme would have unexpected spin-offs in terms of making conditions less favourable for toxic algae?

So, whilst I am not wholly convinced by the hyperbole in some of the statements in the article, I find myself intrigued to know exactly what the environmental costs and benefits are. And I wonder if, just for once, this might be a renewable energy scheme that does have some very positive spin offs too.