Environmental Engineering Reference
In-Depth Information
in turn relies on rainfall. Reservoir storage will help to mitigate fl uctuations, but a prolonged
drought will cause generation output to cease. Biomass generation, though controllable, is
dependent on the biomass feedstock, which may be seasonally dependent. Photovoltaic power
can be reasonably predictable in a warm climate with little cloud, but the movement of clouds
can cause signifi cant fl uctuations in output. Wave power generation is reliant on wind to
create waves. Changes in the energy in waves tend to be smoother than changes in wind
energy as the waves tend to 'integrate' the energy that the wind imparts. Tidal power relies
on the relative phases of the moon and sun and as such is very changeable on a daily basis
but is reasonably predictable. Changes in wind speed/direction and pressure can modify the
expected tidal range.
The topic of variability was discussed at length in Chapters 2 and 3 and the summary above
highlights that all renewable energy sources have issues relating to variability to varying
degrees and on different timescales. The ability to forecast changes also varies from one
renewable energy generation source to another. If an intermittent renewable energy generator
contracts bilaterally for a given amount of energy with another party, e.g. a supplier, any
difference between what the generator is contracted to supply and what the generator actually
supplies will be 'cashed-out' at top-up and spill prices emerging from a balancing market.
These prices are generally unfavourable compared with average bilateral prices. In this
type of market, renewable generators that cannot accurately predict their output are
disadvantaged.
Case Study: The Impact on a Small Hydro Generator of an Electricity Market with
Bilateral Trading and a Balancing Market
A small hydro generator wishes to trade its power output into an hourly electricity market
where participants are able to trade bilaterally and cash-out imbalances in a balancing market
at asymmetric top-up and spill prices, i.e. top-up and spill are charged/paid at different rates
refl ecting bids and offers from fl exible generators/consumers who are able to adjust their
output at short notice if called upon by the system operator. The hydro generator has entered
into a day-ahead contract with a supplier to provide a fi xed output of 1.9 MW every hour
over a 24 hour period at a price of
15/MW h. What will be the generator's average revenue
€
in
/MWh over this 24 hour period?
To answer this question, the actual hourly output of the hydro generator and the hourly
top-up and spill prices over the 24 hour period need to be known. Table 7.9 illustrates
the required calculation. It can be seen that the hydro generator's output varies throughout
the day above and below the 1.9 MW value for which it was contracted. As far as the
supplier is concerned, the 1.9 MW is supplied as contracted and the supplier will pay the
generator a total of
€
684.00 for the energy supplied during the day. This contract is notifi ed
in advance to the market operator. However, the hydro generator's output is metered
hourly and the market operator can see that output does not match the contract at each hour.
The generator is thus obliged to make up shortfalls at the top-up unit price (TUP) and is
paid the spill unit price (SUP) for any excess. In this market, as with most markets having
asymmetric balancing prices, the SUP is rather less than the prevailing market price and the
TUP is rather more than the prevailing market price. The net value of energy defi cits and
surpluses then has a value of cost to the hydro generator of
€
53.40. If this is added to the
€
