Last edited 20 Jul 2021

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Electricity supply


[edit] Introduction

Most buildings [1] in the UK are connected to a world class, albeit ageing, electricity generation and supply network that has benefitted from immense investment over the years.

In 1925 Lord Weir was asked by the UK Government to solve the issue of a fragmented electricity grid that up until then consisted of a myriad of independent producers all with local networks using different voltages and frequencies.

In 1926, the Electricity Supply Act [2] created the Central Electricity Board that oversaw the development of the UK’s first nationwide AC grid in 1933. Since its introduction the grid has been founded on large scale centralised AC generation primarily based on fossil fuels with the introduction of nuclear electricity in the last sixty years.

Power stations are normally located away from centres of population where fossil fuels are abundant or good transport links exist. Many of these locations are well away from the towns and cities where the electricity is used and hence there is a need for electricity transmission and distribution. To do this efficiently the voltage at which electricity is generated is stepped up for efficient transmission and distribution and then stepped back down for safe usage.

At the time of the grid being established this could only be achieved by the use of linear transformers and these only work on AC [3]. As a result AC grids now dominate throughout the world [4].

[edit] The electricity supply network

The UK’s electricity transmission network is based on a 400 kV AC super grid and a 275 kV transmission network. The local distribution network steps this down through a number of stages from 132 kV to 11 kV although some big industrial users will be supplied with 33 kV or higher. The voltage is then reduced further to 415 V three-phase for small/medium sized commercial and industrial users and finally it is supplied to domestic dwellings at 230 V single- phase (the voltage between one of the three-phases and neutral).

Conversion is by way of linear transformers but unlike some of their smaller counterparts the ones used in the supply of electricity can be extremely efficient, in the region of 99.8 per cent [5], however reactive loads and their non-zero imaginary impedance can reduce this figure under normal operating conditions.

Since de-regulation of the electricity sector the supply network, from generation through to the consumer, is managed by four separate organisations that fulfil very different functions [6]:

The national grid transmission network is on average 93 per cent efficient and is one of the most reliable in the world with an operational reliability of 99.99998 per cent [7] although these figures only apply to the main transmission network. Reliability and efficiency figures for the local distribution networks are more difficult to come by due to individual network characteristics and estimated billing.

However, with the introduction of smart meters electricity consumption and availability will become a lot clearer allowing local distribution network performance to be better characterised. Overall the conversion of energy from primary fuel at the power station to usable electricity in the home is only in the region of 35 per cent for coal fired power stations and 45 per cent for the most modern Combined Cycle Gas Turbine (CCGT) power stations [8].

[edit] Decarbonising the grid

Peak demand for electricity across all sectors on an Average Cold Spell (ACS) in Great Britain is approximately 60 GW (2013/14). In 2013/14 approximately 350 TWh of electricity was generated and consumed, the majority of which was produced by burning coal and gas, and by nuclear power stations.

In 2035/36, total electricity generation is expected to be over 365 TWh with a peak demand of 68 GW (National Grid, Gone Green scenario). This will rise still further to approximately 600 TWh/yr by 2050 primarily driven by increased electricity exports and the electrification of transport and domestic heating using heat pumps [9].

Domestic electricity consumption has increased by approximately 40 per cent since 1970 although it peaked in 2005/6 and has fallen slightly to 118 TWh in 2013/14. Under the National Grid Gone Green scenario this is expected to fall further to just over 100 TWh by 2025/26 and then rise to over 125 TWh by 2035/36 [9] (note; see reference [9] for other ‘less greenscenarios). To achieve these modest growth figures requires the domestic sector to meet challenging energy efficiency targets over the next 20 years.

The UK Government has set challenging targets for carbon dioxide reduction and together with the EU’s Large Combustion Plant Directive [10] and the Industrial Emissions Directive it is having a big impact on the UK’s electricity generation capacity. The UK has committed to a 34 per cent reduction in carbon dioxide emissions by 2020 (over 1990 levels) and an 80 per cent reduction by 2050 and to help meet these targets the national electricity supply will need to be more-or- less decarbonised.

In the short term approximately 20 per cent of the existing power plants (coal and nuclear) are due to close in the next five years. This shortfall calls for over £110 bn of new investment in the next decade [11], [12]. To meet carbon dioxide targets the new capacity will be more intermittent and inflexible as a result of renewable generation (primarily wind) and less flexible as a result of nuclear generation.

Due to the intermittency of renewable electricity generation it has a load factor, the estimated contribution as opposed to the maximum potential, of between 30 and 40 per cent for wind, onshore and offshore respectively, and just over 10 per cent for PV. As a result, renewable generation is driving the need for a near doubling of installed capacity over that of today from 91 GW to over 163 GW in 2035 despite only a slight increase in peak demand assuming energy efficiency targets are met [9].

In the shorter term however, at periods of peak demand the loss of generation capacity will have an impact on the headroom available between supply and demand. The prediction is that in high demand periods supply may only just exceed demand by a few per cent, probably around 4% or less. In the past this has typically been held between 10 and 20% so it represents a significant drop in headroom. As a result, the probability of a large shortfall in electricity requiring the controlled disconnection of consumers increases from around 1 in 47 years in the winter of 2013/14 to 1 in 12 years in 2015/16 or lower if energy efficiency measures don’t materialise.

In terms of security of supply two probabilistic measures are used, Loss of Load Expectation (LOLE) and Expected Energy Unserved (EEU). LOLE estimates for the next few years show that demand may exceed supply for more than the target of 3 hours and that this shortfall may be made up of a number of relatively frequent small events or infrequent larger events.

However, the National Grid power supply background has been developed so as not to exceed the 3-hour LOLE threshold from 2018/19 onwards. Before then, while the shortfall is of concern, system operators have some control over the network by for example, reducing electricity exports or selectively disconnecting industrial users, so there may be little or no significant impact on domestic consumers [13].

In general terms the total energy consumption in the domestic sector has seen a change in makeup over the past 40 years with an increasing use of electricity which is likely to continue in the future. Coal has been substituted by natural gas and as the grid becomes decarbonised natural gas will be slowly displaced by electricity.

This article was created by --BRE. It is taken from The future of electricity in domestic buildings, a review, by Andrew Williams, published in November 2014.

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[edit] References


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