Small-scale power plants – are they a solution?

Small-scale power plants – are they a solution?

by Paul Spare
article from Saturday 1, September, 2018

THERE HAVE BEEN regular warnings in recent years – especially during the drought – that the UK needs new policies to reduce carbon emissions – i.e. to decarbonise.  The sectors with greatest emissions are transport, domestic heating and electricity generation.  Schemes to increase the number of electric vehicles, to reduce coal and gas use and to increase the contribution from renewables are already being pursued.  Some environmental groups claim that retaining our 1000 MW scale power stations situated away from cities, is inconsistent with decarbonisation.  They claim that replacing them by many smaller-scale power plants will reduce emissions. The argument is extrapolated to claim that multiple plants could replace the national grid/electricity distribution network. Is this a viable and sensible policy? What evidence is there that a multiplicity of small units will reduce emissions?  There is after all a blatant incongruity in that, in parallel, green groups require more large wind farms to be built in the middle of the North Sea and in the Scottish Highlands.

There is no precise definition of small-scale in this context, but the government has referred to small-scale (less than 50 kW) equipment that can generate heat and electricity as micro-generation.   Such schemes may be able to export heat from the premises to other properties or electricity to the distribution system.   

Some history

The electricity supply in the UK expanded in a haphazard way in the early part of the 20thcentury, with local power stations in the main cities.  The 1926 Electricity Supply Act proposed a national “Grid” to bring economies of scale and many other advantages.  

The 132kV "Grid" system was largely completed by the end of 1935. By 1938 the proportion of spare generating plant had been reduced from 80 per cent to about 15 per cent and the resultant capital saving amounted to 75 per cent of the cost of building the Grid.  Generation costs fell by 24 per cent.  Similar policies have been developed by all the large industrial nations, using a small number of power plants in the 500 - 2000MW range.  These are powered by coal, oil, natural gas and nuclear fuel.   The present system works with HV transmission losses of only a few percent and provides supply security of about 99.98 per cent.  Any scheme that promises improvements over the present system has therefore to exceed some very high standards and would run entirely contrary to the developments of the last 140 years. 

Large plants have proved desirable for practical reasons - e.g. high reliability, 40+ years life, efficient spares holdings and ability to employ specialist staff. Standard generator units of 500 or 660 MW have been developed over many decades and now have very high reliability. Secondly their lifespan has been increasing progressively over the decades. The labour force employed/MW falls pro rata with plant scale. A 50 MW unit needs almost as many workers as a 500 MW, but with only a tenth of the income.   Thirdly, with an income of hundreds of millions of £ pa, an operator can afford to prepare for unexpected events and keep specialist staff and spares available. Professional engineers can co-ordinate their maintenance with similar generators to minimise power cuts.  Lastly, controllable turbo-alternators are needed to produce a stable 50Hz frequency that is essential for modern electronic equipment.

If a 500 MW plant were to be replaced by 10,000 plants of 50 kW size, there will inevitably be a multitude of plant types covering a wide range of designs and sizes, fuels, ages and efficiency. The risks that multiple small plants would introduce include:

·       Varying range of outputs

·       Poor Longevity/ Rapid Obsolescence

·       Low Reliability/Costly Maintenance and spares

·       Difficulty with plant co-ordination

·       Variety of fuel type and security – gas, solar, wind, wood-burning

·       Network disturbance – numbers connected and concentration

·       Poor control of emissions and environmental performance.

·       Poor response to lost generation

 Installation Range

The smaller the scale of operations, the more difficult it is to handle future demand, so the more likely that oversize equipment would be installed to accommodate growth in electricity demand.   Oversize plant is an especially tempting option if there is the prospect of extra sales of electricity or heat to the grid or the local community. Progressively therefore, the problem of redundancy that was eliminated 80 years ago would be re-introduced. Moreover, such supplies of electricity would have no guaranteed output and would not be under the control of the grid staff.

Longevity

The coal and nuclear plants that are gradually being retired have been operating for 40+ years. Any expanding market of small generators will attract new vendors, who will not all be competent and successful. Some of the manufacturers will inevitably go out of business or be taken over and their equipment will be discontinued.  Plants dedicated to a local group of houses, offices, a village or a school are very likely to be inadequate in ten or twenty years because local demand in years ahead is likely to increase. Working plant would therefore have to be replaced by a larger unit. This premature replacement would cut across the principle of sustainability and incur more costs and probably disputes about their attribution. The ownership and responsibilities could change many times in twenty years. 

Reliability/Maintenance/Spares

Larger plants have lower maintenance costs/ kWh through the economies of scale.  The management can afford to retain or manufacture rarely used components. They can organise other grid-connected plants to take over during maintenance or breakdowns. Customers of small plants will inevitably suffer serious problems when there are failures or forced maintenance/spares shortages. This will result either in long periods of shutdown or a poor quality supply.  Since domestic hot water and electricity are required almost continuously, stand-alone microgen will introduce a substantial new risk to consumers who have had 99.98 per cent continuity whilst connected to the grid. 

Grid Co-ordination

Since the uptake of the small-scale equipment may be decided by tens of thousands of households/offices/schools, the numbers of systems that might be installed in any area is unknown. Operators of the electricity networks may not even be told in advance that new micro generation systems are to be installed.   It will therefore be almost impossible for the District Network Operators and main energy providers to forecast the impact on their safety and control equipment.   The worst problem will arise if a large number of systems concentrate in a small area and try to export excess electricity to the network. The local electricity system is not designed to handle large flows back towards the grid end of the system.  This could increase faults and failures in the transformers and switchgear.  

Fuel Security

It is very likely that natural gas will be the preferred fuel for small generators. However there will also be some individuals committed to renewable powered plants i.e. – wood burning, solar, hydro or wind. 

Combined cycle gas turbine plants can generate electricity with an efficiency of over 50 per cent.  Domestic gas central heating systems have an efficiency of about 80 per cent. Although domestic CHP plants may use the low temperature heat that is wasted in CCGT plants, their small internal combustion engine that produces electricity will not operate with high efficiency under all conditions.  In the summer, there will be little use for the waste heat in our climate and the engine efficiency will fall to about 20 per cent.  This is much worse than conventional power stations.  

Micro generation proponents suggest that whatever fuel is used, there will be increased diversity/security of supply and that greenhouse gas production will be lower. It is a fraudulent argument that wind and solar plants increase the security of supply.  These provide their rated output for only 10–20 per cent of the time and so require grid backup to accommodate their unpredictable output variations.  As of today, wind power has fallen to only 0.3 per cent of our electricity.

In the winter at the time of peak demand (nights with freezing fog), renewable output is very low, so they contribute almost no extra security.  In the summer, there is low demand and hence plenty of excess conventional capacity so new diverse equipment brings no benefit. Additionally, in any local area, there will be the same microclimate.  All the wind or solar generators will therefore tend to turn on (and export) and off together (and import electricity).  This will add new cyclic demands on the network electricity flows.

Lost Generation

Power cuts can occur when a large power station  (500 – 2000MW plant) has an unplanned shutdown or fault or there is a sudden increase in demand. After the solar eclipse in August 1999, there was a demand surge of 3,000 MW in 30 minutes across the UK.  This was accommodated because the co-ordinated grid system could power up spinning reserve and black-starting static plant like gas turbines.  Small local systems will not be able to cope with large load changes or sudden demand in response to national events.   Hundreds of small renewable plants could not be co-ordinated, so blackouts, frequency excursions and disconnection would follow. 

Pollution

Instead of one large plant that may be amenable to carbon capture and storage (CCS) system, there will be a multitude of plants operating independently.  MicroCHP systems will often employ small internal combustion engines.  These will be producing greenhouse gases with no prospect of CCS.  Others may use wood fuel that is notoriously difficult to operate with consistent combustion conditions.  When not operating efficiently, they can produce carcinogens such as aromatic hydrocarbons, adding to urban pollution – as has recently been reported in London.  These would only be suitable for a minority of rural sites. Wind turbines are not suitable for urban environments and are visually intrusive.  Domestic scale solar panels have very modest output and ground installations take valuable farmland.  They will in addition require complex and costly recycling plants to separate the mixed materials and chemicals at the end of life.

Energy Storage 

Wind and solar power systems vary enormously and unpredictably over a few hours so don’t meet our need for continuous power.  If surplus power could be stored and released later, their value would be improved.  Unfortunately, the storage of electricity in large quantities is very difficult.  The only practical scheme that has proved workable on a large scale is pumped water storage in mountainous areas.  Batteries have many drawbacks.  They are bulky and heavy with lives as short as five years. They would be essential for anyone who chose to sever the mains electricity connection completely, but they are unlikely to appeal to many electricity customers.  An article in the Daily Telegraphon 25th August summarised the arrangements at an off-grid house in County Down, Northern Ireland.  Batteries from a forklift truck were used – weighing about one tonne. These cost around £2,000 – not a tempting financial proposition for most householders.

There will always be some eccentric individuals who wish to reject the arrangements that work well for the majority of consumers but as has been outlined above, small power plants will bring a range of problems but only minuscule benefits.

P H Spare CEng FIMechE FEI

30 August 2018

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