Energy Harvesting: The End of Fossil Fuel Power?Follow article
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
- 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.
- 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.
- 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.
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.
- Coolant gas leaves the evaporator having picked up heat from the room.
- The gas is compressed raising its temperature still further, above that of the outside air.
- The high-pressure gas goes through the condenser, losing heat to the outside air and turning into a liquid.
- 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.
- 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.
- 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.
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.
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