Renew On Line (UK) 30

Extracts from the March-April 2001 edition of Renew
These extracts only represent about 25% of it

FORUM

An extract from the Forum section of Renew 130

Small is beautiful again?

Everyone's is raving about Micropower. But it isn’t the complete answer says David Milborrow.

Thank goodness you put a question mark on the front cover of Renew 129. Micropower enthusiasm is indeed spreading and small power systems, renewables and storage all have enormous potential. The electricity system of the future, it is claimed, will use decentralised power systems; these will reduce emissions and the days of large centralised generating plant are numbered.

Well, maybe, but on closer analysis there seems to be an alarming absence of sound technical and economic arguments to justify many of the hand-waving claims which are made. The arguments in favour of wholesale decentralization generally overlook three key, interrelated issues: they relate to efficiency and economics, the benefits of aggregation and reliability.

Economies of scale

Micropower enthusiasts appear to believe that "economy of scale" is an old-fashioned idea, which can cheerfully be disregarded. Renew picked up this point. Where is the evidence? The facts are that the prices per kW of small gas turbines - 1 MW or less, say - are about four times those of the largest machines and efficiencies are around 30 percentage points lower. That pushes up generating costs rather sharply.

Even the most enthusiastic champions of small gas turbines do not expect their efficiency to climb much higher than 40%. That is nearly 20% less than the levels achieved by the large machines and their champions are aiming for 70%. As transmission and distribution losses in the developed world are usually around 10%, at most, this means that modern combined-cycle gas turbines feeding into "conventional" electricity networks can deliver power to consumers with an overall efficiency around 50%. Since the efficiency of small-scale micro turbines and reciprocating engines is much lower, any wholesale replacement of large power stations by numerous smaller units will increase, rather than reduce, greenhouse gas emissions.

Aggregation

The benefits of aggregation seem to be overlooked by the micropower enthusiasts and the concept of power supplies for individual dwellings is not seen as extreme. If every house had its own micro-turbine, the power output of that turbine would need to be sufficient to cover the maximum demand of the dwelling. Allowing for kettles, freezers, televisions and lights, that demand will be at least 4kW - 6 to 10 if it uses electricity for cooking and heating. The average electrical load of a typical house, however, is around half a kW. So, for most of the time, a micro turbine will be operating at well below peak output. Part load operation - at any scale - means lower efficiency. So the average efficiency will not be the peak value, discussed above, but an even lower value, which means even higher emissions.

Similar arguments apply when it comes to sizing the power supplies for commercial or industrial premises. There are also cost implications. The price of generating plant is related to its power output and it makes no sense for householders, commercial or industrial premises to pay for power which they are only going to use for a relatively short time. That pushes up the price of electricity even more steeply. If, on the other hand, householders, commercial premises and industry all link together they find that their aggregated maximum demand is much less than the sum of their individual maximum demands, simply because the peak demands come at different times.

So less power plant is needed, bigger, more efficient units can be used and they will benefit from "quantity discounts" for the fuel. This, of course, is why integrated electricity systems came into being. In Britain, for example, the ratio between average and peak demand is around 66% and the average load factor of all the generating plant is a little over 50%. These figures would be very difficult to match in a disaggregated system.

Reliability

Reliability also deserves more careful analysis. Integrated electricity systems deliver very high levels of reliability, simply because there are many interconnections and for most of the time, every consumer has access to most of the operational plant. A householder or industrial plant who chose to be reliant only on his own source of supply would find it very difficult to match these levels of reliability without providing 100% backup which, of course, would push costs up even more.

Renewables, storage and CHP

Renewables are often bracketed with small-scale thermal power sources which is, perhaps, misleading. Their efficiency is of little relevance in the context of emission savings and what matters is whether they are cost-competitive with other options at the point of connection. However, they function best in integrated electricity systems, as backup for the intermittent sources is readily available if needed at low cost. The notion that storage can "transform the economics of clean sources of power" is correct, but misleading. It is only true if the cost of providing electricity from the storage device is less than the added value.

Combined heat and power also figures prominently in the propaganda for micropower. Rarely is there any discussion as to whether the heat and electrical demands can be matched. If this is not the case, it is difficult to achieve high efficiency.

The real future?

There is, of course, a great future for the renewables and for small-scale generation sources. In the developed world, however, that potential is likely to be realised alongside rather then instead of conventional power systems. Wind and solar, in particular, need the benefits of aggregation if they are to function at their best.

In the developing world, renewables and small-scale power systems have an equally bright future. As they are probably not competing with the generation costs of large centralised plant, the economics are completely different. The intermittent sources such as solar and wind can, and do, provide power in remote areas or isolated systems- provided some backup, possibly storage, is available. Although backup and storage cost money, the extra costs may well be affordable when compared with alternatives such as high-cost imported diesel fuel.

David Milborrow.