Skip to main content
shopping_basket Basket 0

Energy Harvesting: The End of Fossil Fuel Power?

Bill Marshall

Heat pumps UK

The outside half of an Air-Source Heat Pump (ASHP) on the left, of the type soon to replace central heating gas boilers in UK homes over the next few years. On the right, part of a UK off-shore wind-farm that will see the end of fossil-fuel burning power stations. Image credit: Wikipedia

The phrase Energy Harvesting refers to the capture and use of surplus energy used in a process which would otherwise be wasted. In recent years it’s been used to describe grabbing microwatts for powering remote sensors on the Internet of Things; now we’re talking about megawatts for powering the national grid or kilowatts for individual homes. Worries about Global Warming caused by excessive CO2 emissions from burning fossil fuels have led to a rush to find renewable alternatives. Given all the massive hype surrounding wind energy as the replacement for coal, oil and gas when it comes to generating electricity, and more recently, UK government announcements that Heat Pumps will replace all domestic central heating boilers, I thought I’d see if all the excitement is justified.

Down on the Wind Farm

Although an old Windmill (which grinds corn or pumps water) is not the same thing as a modern Wind Turbine (which generates electricity), they both work in a remarkably similar way:

  1. Both use a multi-bladed fan arrangement to convert a linear flow of air to rotary motion of a shaft whose axis lies parallel to the airflow.
  2. The fan ‘blades’ or ‘sails’ are set in the central hub at an angle to the airflow. If this ‘Angle of Attack’ or Pitch is 45° then the force of the wind is split into two equal components at 90° to each other. One acts along the shaft axis and contributes nothing but wear in the shaft bearings; the other forces the blade sideways causing the whole fan to rotate.
  3. Both have mechanisms to stop the fan should the wind get too strong. In fact, both need to provide a constant-speed drive to grindstones or generators but can’t because the wind is fickle and uncontrollable.

The fact is, the industrial revolution was kickstarted when machines could be powered by releasing the stored energy in fossil fuels – ancient sunlight. But now we’re hoping to keep the revolution going by returning to the old ways which didn’t pollute the atmosphere. Unfortunately, point 3 above indicates that a technology originally developed to turn grain into flour doesn’t work so well when generating electricity.

Weather in the UK

It’s often said that we in the UK have a mild climate with a great deal of weather; meaning that we don’t suffer extremes of temperature, wind and precipitation, but what we have varies rapidly – sometimes by the hour. One year we might have on average, a miserable, damp summer; the next might see us basking in sunshine with attendant water shortages. It makes it difficult for a wind- or solar-driven power station to provide a guaranteed supply from one minute to the next. At least the designer of a solar power installation can assume there will be nothing generated at night! The old miller could handle this variability as long as windless days weren’t too frequent. If the wind speed increased to the point of causing damage, he could haul down the sail canvas and the sails would stop moving. A later development saw canvas being replaced by spring-loaded shutters which would open automatically with excessive wind speed. Small variations in wind speed affect the rotational speed of the grindstones which in turn affects the flour quality. This problem was solved by a Governor and Tentering mechanism which varied the gap between the grindstones with speed, so keeping the flour particle size and hence the quality constant. An early example of feedback control.

The variable nature of UK weather causes headaches for the designers of wind turbines even now because the final output, like the flour from the old windmill, must be of consistent quality. Unlike the lone windmill, wind turbines operate in big groups or ‘farms’ which are linked to other farms and other generators. Their outputs must all be precisely synchronised in voltage (UK 240Vac), frequency (UK 50Hz) and phase. The synchronisation problem was fixed long ago for ‘conventional’ power stations where the rotation of the turbine can be finely controlled with feedback thanks to the constant and consistent energy supply. No such luck with wind energy so instead it’s usual to generate dc power and electronically create ac with an inverter and control that instead. An advantage of this technique is that it allows a storage battery to be inserted between the dc output of the turbine and the inverter, which helps smooth out voltage dips that occur with small variations in the wind speed. But as yet we don’t have batteries big enough to keep the ac supply going if the wind drops altogether – which it does frequently.

Just like the frail wooden windmills of the past, modern wind turbines, despite their massive construction, are still vulnerable to high wind speeds. In fact, speeds exceeding about 55mph require them to be shut down; usually by ‘feathering’ the blades – changing the pitch of each blade until it’s parallel with the airflow. The stresses and strains on these huge structures are immense and one wonders about their safe working lifetime; especially for those off-shore constantly hammered by salty sea-spray. The size and weight of the blades means that they must face into the wind at all times, that is, not get ‘behind’ the mast. If they do then a cyclic stress is set up in the mast as it briefly shields a blade from the wind force as the blade moves past. Just the thing for fatigue fractures in the future….

The current ’solution’ to the problem of maintaining a constant 24/7, national mains electricity supply with an unpredictable energy input, is to have a surplus of turbines around the country sufficient to cope with localised wind variations. This might work on a continental scale, such as the whole of Europe or North America, but for the UK alone, without backup, no way. The UK is too small and a single weather feature such as an anti-cyclone can reduce wind energy output to almost zero – not just for minutes or hours, but days or weeks. It’s also said that anti-cyclonic weather means clear skies and lots of solar energy. Unfortunately, in the temperate climate of the UK the reverse is often true: no wind and grey skies. The condition even has a name – anticyclonic gloom. To get an almost real-time picture of what’s happening on the UK and French electricity grids, see the GRIDWATCH™ dashboard. Note how often the UK gas-fired stations have to be brought on line.

Gridwatch dashboard

Snapshot of the UK GRIDWATCH dashboard.

Heat-Pumps: More Old-Technology Repurposed

There are four laws of thermodynamics: the second states that heat does not spontaneously pass from a colder body to a warmer body. As a silly example, an ice-cube placed in a hot oven will not lose heat making itself colder and the oven hotter. Instead, heat flows one way from oven to ice-cube raising its temperature to melting point. A more realistic example is a car engine cooling system: Hot coolant from the engine flows through a radiator exposed to cool outside air. Heat is radiated and cooler liquid returns to the engine where it gets heated again. The domestic refrigerator seems to defy this law as heat appears to be moved from the cold interior to the warm room outside. This is, of course impossible without some clever tricks involving a compressor and an expansion valve. The key ingredient is a liquid coolant that boils (turns to gas) at a temperature a lot less than the freezing point of water. A fridge works on the principle of a Heat Pump (Fig.1), as does an Air-Conditioning Unit, the latter attempting to cool a whole roomful of air, not just that contained in a small insulated box.

  1. Coolant gas leaves the evaporator having picked up heat from the room.
  2. The gas is compressed raising its temperature still further, above that of the outside air.
  3. The high-pressure gas goes through the condenser, losing heat to the outside air and turning into a liquid.
  4. The hot liquid at high pressure passes through the expansion valve which massively reduces the pressure causing the hot gas to cool rapidly, partially returning to a liquid state.
  5. The level of the pressure release and hence the coolant temperature is controlled by a temperature sensor on the output pipe of the evaporator. In other words, the sensor/valve acts as a thermostat.
  6. The coolant enters the evaporator and being colder than the room air can pick up heat causing any remaining liquid to turn to gas. The cycle then repeats.

Air-source heat pump operation

The compressor and expansion valve ensure that the second law of thermodynamics is respected! Note the lack of a free lunch though – that compressor will consume a fair amount of electrical power. The really clever bit is that it’s possible to reverse the coolant cycle and turn the air-con into a room heater – even when it’s freezing outside. An air-con/heater installation like this will only serve one room however. This is where it gets messy. The single gas boiler in a house normally heats all the rooms via radiators and creates hot water for the basins and showers. The normal solution is to swap the condenser for a heat-exchanger which directly replaces the old boiler. The big problem is that the water temperature is only about +35⁰C instead of around +60⁰C. If you use the original radiators, the house will take a lot longer to warm up from cold, or react to a sudden cold snap. The total radiating surface area must be increased to get the same amount of heat energy into the house. This can be done in two ways:

  • Replace radiators with larger surface-area versions.
  • Replace radiators with under-floor heating pipes.

The second approach is best for a house under construction, but it would be hugely expensive as a retrofit. The problem of tepid showers is less easily solved. Solar thermal collectors driving a second coil in the hot water tank will provide gallons of very hot water after an hour or so of bright sunshine - even in winter. If hot water must always be on-tap, then in a country with unpredictable weather, there is of course, the (power-hungry) electric immersion heater as a last resort.

It is possible using this technology to extract heat from the ground with a collector buried several metres down under the garden (if you have one), or go a hundred metres or so further down and tap into geothermal energy. Both involve heavy capital expenditure.

The Bottom Line

The bottom line is this: For the UK it makes little difference if you cover the entire landscape in wind turbines, when the wind doesn’t blow, they’re nearly all at a standstill. We need either to replace thousand year-old windmill technology with new, or invent a practical Grid-scale energy storage system. Preferably before the lights go out and our new heat pumps stop working.

If you're stuck for something to do, follow my posts on Twitter. I link to interesting articles on new electronics and related technologies, retweeting posts I spot about robots, space exploration and other issues.

Engineer, PhD, lecturer, freelance technical writer, blogger & tweeter interested in robots, AI, planetary explorers and all things electronic. STEM ambassador. Designed, built and programmed my first microcomputer in 1976. Still learning, still building, still coding today.

Recommended Articles


November 9, 2021 08:26

I have a few oberservations fom personal experience that might be interesting. We used an air-sourced heat pump system for our space heating and domestic hot water for about ten years until it broke just about five years ago. Because that happened in the middle of November there was really no time to try to fix it and I was left with little choice but replace it with a new gas boiler (!). It was reasonable at the time, I think, to expect a much longer service life because, as Bill explained, it is essentially the same technology as a domestic refrigerator and a decent quality fridge will last forever - possibly because all the moving bits operate in an oil bath in a sealed system with no possibility for ingress of dirt or contaminants. But the design of our heat pump was much more complicated than this, with lots of electronics and moving parts, and I suspect that was its weakness. So if we are going to install lots of these systems then, based on my sample of just one, we need to be concerned about reliability.
A few points about performance: I added a little data-logger to our system to measure and record its performance - just because I am a geek and interested in that kind of stuff. Over its service life it achieved a coefficient of performance (heat out / electricity in) between about 2.5 and 3, depending on the outside air temperature, operating with an output flow temperature of about 35C. I believe that would be a realistic expectation for any air-sourced system. A ground-sourced system can achieve better performance because of the higher source temperature (typically around 10C, so a much lower temperature difference between input and output) but the cost of drilling bore holes could easily double the cost of an installation and would be difficult for most users to justify.
A final point about the installation: All the kit for our system was contained in a cabinet installed outside so there was no need for space indoors to accommodate equipment, other than the radiators and domestic hot water cylinder.
I hope these notes might be helpful and add a little to the discussion.

0 Votes

November 10, 2021 08:00

@Mike Baker Ten years ago a (wealthy) acquaintance of mine bought a newly refurbished weekend "cottage" on the south coast of England. Along with the multiple bathrooms, marble floors, satellite TVs in all rooms and all sorts of other toys, it has a ground source heat pump fed from an array of shallow pipes under the main lawn. Inside, the entire ground floor is heated by pipes under the floor, and upstairs by high-capacity radiators. A bank of solar collector panels on the roof provides hot-water and if all else fails, a very beefy electric immersion heater kicks in. The heat pump and all of its control systems is situated in a separate outhouse. This was state-of-the art at the time. Unfortunately, the instruction manual was inadequate to say the least, the original installer seemed to have disappeared and a number of other heating engineers contacted left hurriedly when they saw the 'ships engine room' in the outhouse. Soon after the owners took possession, they had become aware of enormous electricity bills. Neither radiators nor floors got warm, and after a period of being switched off during the winter, it took many days to bring the house back to a liveable temperature. Eventually it was discovered that some air-venting valves were not working and perhaps more importantly, that the heat-pump could not be operated as if it were a gas boiler. The water circulating in the underfloor pipes and radiators is only a few degrees above the selected temperature: nothing gets 'hot'. Essentially the system is designed to run continuously and not operate off a timer. It is economical to do this because there is no boiler. In other words you get the same total amount of heat over a long period of time as a boiler would deliver in minutes - but without burning any gas. Of course it only works if the house is very well insulated.

November 9, 2021 08:22

My air source heat pump for domestic hot water heats to 55C but it wouldn't maintain this if the water was being circulated.
The temperature can be achieved, just not fast enough for central heating.

First requirement for a warm home? Good insulation.
Not very exciting technically though!

0 Votes

November 9, 2021 08:21

What I do not understand is why local, decentralised Pump Storage is not used.
All industrialised countries have Thousands of former (mining) underground cavities at various depths.
My local example has tunnels (about 3M diameter) as deep as 690 Metres.
A quick spreadsheet using E=Mgh and asuming water (1000kg per cubic metre) shows the storage potential to be GAME CHANGING.
And the have excellent grid connections.
Mines where not "Tanked" when in use because they where constantly expanding.
Some such as the former salt mines are of course dry but need tanking to stop the cavities collapsing.

0 Votes

November 9, 2021 08:20

Directly harvesting second hand energy from the Sun is massively outgunned by the concentrated, low cost energy stored over millennia in fossil fuels, which our ability to harness efficiently has been responsible for the recent dramatic reduction of global poverty, starvation and disease. No, the future of continued evolution of mankind has got to be based on abundant, almost limitless, cheap energy that will be available in a few decades from nuclear fusion, the process that powers the Sun and other stars.

0 Votes

November 9, 2021 08:19

Although representing a Finnish point of view, I must comment on your statements in the article. We live in a house from the 70's. The heating system was converted from oil to heatpump about 8 years ago. Our heat is pumped from two deep wells drilled 300 m deep. The house and all tapwater etc. is heated by the heat pump. The working fluid is alcohol. The heatpump is set to produce 50° C water, but can reach even higher. I realize that the systems reviewed are air to water heat pumps, but they can also reach much higher temperatures than 35° C. The investment 8 years ago has already been recovered by the savings brought on by not burning oil. We still have about 12 years of expected lifespan, then the compressor must be replaced.

0 Votes

November 8, 2021 15:44

Not sure Heat Pumps are the solution for domestic settings. Hydrogen boilers may be the solution,no CO2 emissions, and can be supplied today as Hydrogen ready, and the converted with ~£100 components over to Hydrogen when(if) the gas infrastructure is changed over.

0 Votes

November 9, 2021 08:23

@britto_p Hydrogen might just be the answer to the problem of storing wind turbine energy for use when the wind drops. As for replacing domestic natural gas in our homes: don't hold your breath. The problem is, very little hydrogen gas exists on planet Earth. If you want it, you have to separate it from the Oxygen in water or from the carbon in Methane. Both methods require large amounts of energy and the Methane route generates volumes of...CO2. It's use in fuel cells is entirely practical for mobile applications. See:

November 8, 2021 12:27

Good article, but you missed another important point about converting from gas boilers to heat pumps. Most modern gas boilers are of the extremely compact 'combi' type with the result that everything can be installed in a small above-bench kitchen cupboard. This will not be the case with a heat pump. Finding space for a heat pump installation in a modern home will probably mean that the garage must be converted in a 'boiler house'. Modern homes are built for the masses by the likes of Persimmon probably wont last 25 years before falling down. There's not enough space for larger radiators

0 Votes

November 9, 2021 08:22

@alfredjones Quite agree. Before 'combi' boilers, houses were built with airing cupboards containing hot-water tanks. They are another example of technology pushed too far; being lamentably unreliable compared to a conventional condensing boiler + HW cylinder combination. What if you live in a flat with no garage? Might have to give over a bedroom to fit in the condenser, heat exchanger for the radiators, and probably very noisy compressor. Then there's the HW cylinder, solar collector panels and attendant valves, pump, pipework..... I doubt any government grant will cover more than half the multi-thousand pound cost either.

November 1, 2021 08:32

Very interesting, particularly regarding heat pumps. I had not realised they typically operate with the 'hot' water at 35C. That raises several issues, first as you say you need bigger surface area radiators, but what about microbe growth? I recall solar hot water systems having to regularly heated to 60C or more to ensure the water from the hot tank was safe.
I also wonder with a small garden how much cooler the air will get in winter when the heat extraction is at maximum!
Looking at adverts few mention the low 'hot' temperature and simply hint you may need larger surface area radiators. Not the great solution I was expecting.
One advert states the running cost could be £100 per year higher if you are replacing a modern gas boiler. I realise running cost is not the issue and it's reducing the carbon footprint, but your article has opened my eyes!
I haven't worked this bit out, but I assume the circulating pump is inside the house as that is where all the energy is being put into and the waste heat from the pump adds heat to the property?

0 Votes

November 1, 2021 08:33

@Boss I presume the circulating pump is a standard central heating pump so it won't contribute much heat! On the other hand the compressor which I imagine is much larger than a 'fridge unit, will get very hot. The big problem may be noise from both compressor and a large radiator fan - both located inside the house. And then there's the cost. I was one of the few who managed to secure a Green Homes grant this year and had a solar collector for hot water installed. It would have cost over £6000 without the grant. ASHPs will be well into five figures - how many can afford that nowadays? Now is the time for some real innovative thinking, not just dressing up ancient technology as new.

DesignSpark Electrical Logolinkedin