Memorandum 9
Submission from Alan Shaw, Retired Chartered
Engineer
TECHNICAL AND COMMERCIAL COMPATIBILITY WITH
THE NATIONAL ELECTRICITY GRID AND ITS BALANCING MECHANISM
EXECUTIVE SUMMARY
This brief paper summarises the UK context for
introduction of new electricity generation sources based only
on the Great Britain (GB) sector of the UK public electricity
supply system, being some 97% of the UK total.
The second part of the paper discusses the fundamental
differences between the pattern of GB electricity demand as shown
in the National Grid Seven Year Statement. This pattern arises
from British habits of life and work combined with seasonal and
climatic influences, The largely predictable historic shape of
this pattern, day by day and year by year, is not matched in achievable
supply patterns from most forms of renewable energy. Instead lunar,
solar and other cyclic bases are typically the the governing factors.The
need to safeguard the integrity of National Grid's demand Balancing
Mechanism (BM) is underlined.
PAPER
1. Introduction
1.1 Successful large scale development of
any renewable energy depends on its ability to fit in controllably
to the overall demand pattern of the national electricity supply
system , hour by hour, day by day and annually. The overall pattern
of demand varies with the season of the year, weather and special
events but by and large is predictable from past records and the
expert knowledge of daily events of the Great Britain System Operator
(GBSO)—National Grid Electricity Transmission plc.(NGET)
1.2 The UK electricity system in England,
Scotland and Wales (GB) together forms a system separate from
Northern Ireland (NI) but interconnected with the NI system by
the Moyle high voltage direct current (HVDC) submarine interconnector
between Northern Ireland and south west Scotland The direct current
feature of the Moyle interconnector means that the two systems,
which can exchange power in either direction by planned mutual
consent and are both of 50 cycle alternating current frequency,
are not synchronised with each other.
1.3 The NI system maximum power demand MW
(megawatts) and its total distributed energy MWh (megawatt hours)
are similar in demand profile to, but less than 3% of, the GB
system. I will for simplicity therefore take the GB system operated
by NGET as representative, bearing in mind that, based mainly
on 2005-06 figures * it is around 97% of the UK total.
1.4 It should be noted that although revenues
from electricity ( and fuel or renewable energy used in generation)
are energy (MWh) based, the continuous balancing of demand with
supply is carried out entirely by frequency sensitive control
of the overall generation power rate (MW). Excess of demand over
generation causes frequency to fall, excess of generation causes
it to rise. Frequency maintenance at an average of 50 cycles per
second is a statutory requirement for system stability. Instantaneously
demanded power in MW must be continuously matched by instantaneously
supplied generation in MW. In practice the balance is recorded
half hourly. Electricity on a national scale can NOT be stored.
The four pumped storage stations in Wales and Scotland have a
total capacity of only 2,290 MW or about 2.7% of current annual
maximum demand **. Not all of this may be available at any given
moment as it is subject to normal pumping/generation profiles
imposed by system requirements.
1.5 Until the recognition some 10 years
ago of the necessity for greenhouse gas (GHG) control the entire
electricity system was supplied by fully controllable forms of
energy generation—coal, oil, natural gas, nuclear power
and a small percentage (about 1%) of hydro-electric generation.
Hydro power is of course a renewable energy but dependent on a
variable rainfall. As rainfall is to some extent predictable,
visible once it falls, and some can be stored, the small and variable
percentage of total generation it represented at any given time
is normally able to be accommodated by the national grid system,
but not always. In 1955 the North of Scotland Hydro-Electric Board
(NSHEB) contracted to supply annually to the then South of Scotland
Electricity Board 280 GWh. In the event, in that year an unprecedented
and prolonged drought reduced the figure to 167.5GWh or only 60%
of the contractual amount. The shortfall had to be made good from
the England and Wales system.*** Although such extreme shortfalls
are rare this event was a sharp reminder that hydro power under
UK weather conditions is not completely predictable.
2. Fundamental differences between pattern
of UK electricity demand and various renewable energies ability
to match with supply
2.1 The following Figure 2.2 and explanations
extracted from NatGrid GB Seven Year Statement 2007 shows how
the seasonal demand profiles follow a characteristic shape determined
entirely by the British habits of life and work, some determined
by the weather and climate.
Figure 2.2 below presents demand profiles for
the days of maximum and minimum demand on the GB transmission
system in 2006-07 and for days of typical winter and summer weekday
demand. These demands are shown exclusive of station transformer,
pumping demand and interconnector exports.
Figure 2.2
GB SUMMER AND WINTER DAILY DEMAND PROFILES
IN 2006-07
Key points of interest are:
(i) Maximum & Typical Winter Profiles
(Weekday)
(ii) Typical Summer Profile (Weekday)
(iii) Minimum Summer Profile (Sunday)
2.2 The various types of renewable energies
produce annual daily and annual availability profiles quite unrelated
to the electricity demand pattern produced by the British way
of life and work—tidal power is governed by sea level which
varies approximately with a 12.4 hour period, the diurnal ebb
and flow cycle, superimposed upon a longer sinusoid with a period
of 353 hours, the springs-neap cycle. The largest tidal barrage
in operation is the Rance estuary scheme in France. The tides
follow a two week cycle throughout the year. The Rance output
is computer controlled and optimised to match the needs of the
French grid. The nominal average output of this 240 MW project
is between 50 and 65 MW and is thus not the maximum that could
be obtained, but it contributes maximum savings to the grid. While
La Rance electricity is the cheapest electricity on the French
national grid Electricity de France say that it would be too expensive
to build any further power stations. Studies have shown that the
method of operation that results in the lowest unit cost of energy
is either simple ebb generation, or ebb generation with pumping
at high tide. As the generation period is about an hour later
each day the generation (and pumping if used) needs to be planned
in advance to integrate with the needs of the French national
grid.
2.3 Studies have shown that the method of
operation that results in the lowest unit cost of energy is either
simple ebb generation, or ebb generation with pumping at high
tide. ****
2.4 Solar energy in the UK is of course dependent
on time of day, season and cloud cover. Wave energy is affected
by "fetch" ie, distance of wave travel, on strength
and direction of wind and in some cases tidal conditions.
2.5 Such influences tend to produce renewable
energies which are intermittent, uncontrollable and unsuitable
for the national grid's continual need for firm, responsively
controllable power. This is the function of the NGET's "Balancing
Mechanism" (BM). From the point of view of economic electricity
generation the most valuable sources of energy are those which,
in instantaneous rate of electrical production are "firm
" ie, reliable, fully controllable and quickly responsive.
*****
2.6 To have large MW capacities of uncontrollable
non-firm power running loose risks the stability of the entire
national grid system and can greatly increase the stress under
which grid controllers work. Also of growing concern are the costs
of generation coupled with the annual capital charges of Supergrid
transmission reinforcements to generate and convey the renewably
sourced electricity from the favoured generation sites (in the
Highlands and Islands of Scotland and offshore) to satisfy competitively
the dominant demands in the Midlands and south of England. These
must be very carefully considered before even greater expenditures
are incurred, all of which must eventually percolate down to consumers
and taxpayers.
2.7 The full extent of the potential problems
which would be presented to central grid control by, for example
, the realisation of leading Scottish politicians' aspirations
in past months, quoting 40% and even as high as 100% of Scottish
electricity MWh from renewable energy is obviously politically
uncomprehended. The basic reason is the uncontrollability and
unpredictable intermittency of wind energy together with its overall
average annual load factor making both its generation and Supergrid
high voltage transmission to its supposed markets in the Midlands
and South of England economically unattractive except for the
entrepeneurial purpose of earning quite unjustified subsidies.
2.8 Even at present levels of installed
windpower MW capacity the growing total UK burden of fully controllable
standby plant capacity is not publicly understood. To bring up
from near zero load to full load on-line standby plant at the
MW per minute rate ("response time") at which large
scale windpower can disappear only to unload it similarly rapidly
risks damage to high temperature thermal plant such as gas and
steam turbines. In extreme circumstances only large pumped storage
hydro turbines can start up "from dry" and pick up load
shed by renewable energy sources rapidly and safely enough. As
footnoted in ** below the existence of such plant nationally is
very limited and largely already spoken for by normal operational
contingencies.
2.9 I would earnestly recommend the Select
Committee to study, with NGET assistance, the latter's excellent
Seven Year Statement 2007 (and previous years) produced annually
as a condition of its Transmission Licence and downloadable on
the internet.
I am sirs, yours most faithfully,
Alan Shaw BSc CEng MIET
(Retired ex National Nuclear Corporation Limited
(1955-81) and author and co-author of energy papers to World Power,
United Nations and other international engineering conferences.)
FOOTNOTES:
* Electricity Industry Review 11 (EIR11)
June 2007 pps 7, 9 and 10.(published by Electrica Services Limited
and sponsored by NationalGrid).
** Dinorwig 2,200MW, Festiniog 350MW, Foyers
300MW, Cruachan 440MW ( Source: EIR 11).
*** "The Hydro" by Professor
Peter L Payne pub Aberdeen University Press 1988.
**** Section 21 of "Kelvin to Weir and
on to GB SYS 2005" by Alan Shaw: Royal Society of Edinburgh
Inquiry into Scotland's Energy Issues 2005.
***** Please note that Capacity Factor, a partial
synonym for Load Factor often appearing in the press nowadays,
is an Americanism and a term not recognised by the IEC/ International
Electrotechnical Vocabulary (see "Electropedia" on the
internet.)
July 2007
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