Few of us are at their most efficient when the temperature rises. Just as human beings can find it harder to get to grips with their activities on a hot day, the efficiency of many electrical assets changes with the ambient temperature. Most temperature effects are relatively unimportant in the UK because our summer temperatures seldom exceed the mid-thirties Celsius, but they can be much more significant in some other countries.
What’s going on?
When the temperature rises in the summer, people often say that solar power must be working at full strength. In fact the opposite is true. For PV it is light – not heat – that is the source of power, so PV panels are at their most efficient when the ambient temperature is low. So a cool day with many hours of sunlight striking at the most beneficial angle works best.
Large arrays are at risk of overheating on a hot day. How much efficiency is lost? It’s not a huge effect – typically PV panels lose about half a percentage point of efficiency for each degree of ambient heat rise. Standard information from the manufacturer will give the PV panel output at 25°C and the ‘power temperature coefficient’, which describes how that figure changes with temperature. Equally, efficiency improves on the nameplate rating as the temperature drops below 25°C.
The effect is not restricted to the PV panels: efficiency is also lost in the inverters. Researchers found that inverter efficiency falls by about 2.5% when the ambient temperature approaches 40°C.
As with photovoltaics, gas turbine efficiency is affected by the ambient temperature. Gas turbines rely on a temperature differential between the inlet and the exhaust, and the cooler the inlet temperature the more power a gas turbine can generate. Turbines are expected to be at their most efficient when the inlet is at about 15°C.
The numbers vary for different types of gas turbine, but increasing the inlet air temperature to the typical temperature of a hot summer day – say 37°C – can reduce the output of an aeroderivative turbine to about 73% of what would be expected on a cool day. Temperatures rarely get that high in the UK, but even in the type of temperatures that apply in a British summer, a gas turbine might lose 10% of its capacity or more.
The UK sees lower power demand on a hot day than on a cold winter’s day, but that’s not the case in countries where there is a much larger air conditioning load on a hot day. In areas like the Middle East, where the summer temperature is much higher, gas turbine operators lose a third of their potential export (due to lower plant efficiency) just when the potential requirement for air conditioning is at its highest. In that situation, companies may decide to install turbine inlet cooling to get back some lost efficiency.
All thermal plant has to be cooled. Generally river or seawater is abstracted for this purpose and discharged back into the river or sea. Discharge water is usually at a slightly higher temperature than the water where it is discharged, and permission to discharge is governed by strict controls over both the discharge water temperature and the temperature of the water in the river. This is intended to ensure that the warm water that is discharged does not raise river temperatures above limits.
River conditions can bring plant operation to a complete halt. That may be because dry conditions mean the river is too low to abstract the necessary water at all. It may be because water temperature limits are breached at the discharge point, or because river water has already been warmed by the sun – especially when river levels are low – and the water is too warm to allow it to be abstracted for use in the power plant.
Several nuclear plants in France have suffered shutdowns during past summers for this reason.
The question for hydropower is whether there is enough water to drive its turbines and maintain river conditions. In the summer, rivers can run much lower and in some cases dry out. Hydro plants may operate under restrictions that stop them from using water to generate at times when river water levels are low.
It’s not just power generation assets that are affected by ambient heat. The conductivity of metals varies depending on how hot they are. So the actual amount of power that can be carried on transmission and distribution lines can vary according to the ambient temperature.
It can also cause network cables to expand so transmission cables ‘sag’ further between transmission towers.
Limits are set for all of these physical parameters, and they determine how much power can be transmitted.
In the UK, of course, periods of hot weather tend to coincide with lower demand and, as we have seen, the situation is different in countries where there is a large air-conditioning demand in hot weather. That is a relatively small part of UK demand at the moment, but it could grow. Air conditioning use may coincide with peak generation periods from solar PV panels.
National Grid’s Future Energy Scenarios include two scenarios – Slow Progression and No Progression – in which air conditioning use remains at its current level. But the system operator says that could change dramatically after 2040.
The Gone Green scenario envisages 10GW of air conditioner demand by 2050, and the Consumer Power scenario suggests that it could reach as much as 18GW by that date.