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.


The truth is sometimes stranger than fiction …

Just before Christmas I had an idea for a story: a group of campaigners battling to save the last polluted river in the country before the evil utility company ceased to pour in their effluents and a unique and unusual ecosystem was lost forever.  It was, obviously, not a very serious topic but there were serious ideas behind it.  There are also precedents, with some former industrial land now protected as Sites of Special Scientific Interest because of its distinctive flora.

The thinking behind the story was that the factor most likely to lead to widespread reduction in pollution is not better regulation but the profit motive.   A few months earlier, I had watched a TV news story whilst in a hotel room, describing how Thames Water was able to extract phosphorus from sewage, process it into fertiliser then sell it to farmers.   Much of my professional work addresses the better regulation of phosphorus in the environment but I also know that there is a global shortage of phosphorus, which has stimulated considerable commercial interest in recovering phosphorus from sewage effluent.   The market, in other words, may ultimately play as large – or even a larger – role than legislation in controlling phosphorus releases to the environment.

I ran with this idea a little further: suppose utility companies found other ways of making money from sewage?   This is already happening on a small scale, with capacity to store and use methane released during the decomposition of sewage.  The limiting factor, as in most aspects of waste disposal, is economics.  Imagine, however, that the costs of energy were to shift dramatically … suddenly the opportunities presented by the huge quantities of sewage – which is just a semi-liquid form of the cow pats that half a billion Indian farmers traditionally used as fuel – look more attractive.  How might utility companies react?

So I needed a plot device that pushed up the price of energy and, in the process, stimulated utility companies to invest in energy production on sewage treatment plants, along with the infrastructure to connect this to the grid.   Suppose, I speculated, relations between Russia and the West deteriorated, threatening the huge natural gas supplies on which central European countries such as Germany depends?   This, in turn, would create greater demand for other sources of energy and push up prices to the extent that alternative sources of fuel might look more attractive.

All I needed was a geopolitical scenario that would create this east-west tension and my plot synopsis would be complete.   On cue, the crowds gathered in Kiev to overthrow Viktor Yanukovych and suddenly my bright idea for a work of fiction looked a whole lot more plausible …

About crackers, peanut butter and marmite …

I’m a sucker for good metaphors and analogies when I’m teaching. These are great for linking the ideas that I am trying to communicate with things with which the students are already familiar. One of my favourite analogies for stream ecology comes from a 1974 review paper by the US ecologist Kenneth Cummins. He was describing the process by which leaves which fall into streams at this time of year are broken down by the organisms that live in the stream in order to release their energy. There are a number of aquatic invertebrates, termed “shredders”, whose mouthpieces are specially adapted to tearing apart these leaves. They gain their nutrition from the leaves, so the theory goes, with the partially-digested leaf material emerging from their intestines, in due course, as “fine particulate organic matter”. That itself is a euphemism. Go figure.
But leaves alone do not make a particularly nutritious diet. In fact, the shredders are not living solely on these leaves. As soon as a leaf falls from the tree it is vulnerable to attack from bacteria and fungi. Like the invertebrates (like humans eating spinach, too), they can gain nutrition from this leaf, and the enzymes they produce help to soften up the tissues making it easier for the shredders to tear apart. Once in the water, the dead leaf will also be colonised by algae whose photosynthesis will produce oxygen which will replace that used by the various bugs as they break the leaf down. The combination of fungi, bacteria and algae also add to the nutritional content of the leaf. Cummin’s great analogy was that the leaf was akin to a ‘cracker’ whilst the microbial life was akin to ‘peanut butter’. A single cracker, as you know, is not itself greatly nutritious, but we tend to use crackers as ‘carriers’ for protein- and energy-rich foods such as cheese or, in Cummin’s example, peanut butter. An even better analogy for a UK reader is a cracker spread with Marmite which really is microbial-based nutrition.
Metaphor and analogies have their limitations, of course. But in an age where science is increasingly quantitative, the importance of having strong mental images of systems before you start taking them apart and counting and measuring the various components must be emphasised. It is a tradition that goes back at least as far as Leonardo da Vinci, and possibly further.


Cummins, K.W. (1974). Structure and function of stream ecosystems. Bioscience 24: 631-641.