How to manage excess power

In a previous post on smart meters I raised the issue in passing of what to do with excess power output that may occur from very large penetration of  PV systems on the low voltage grid.  Germany is facing this issue now with wind and solar.  During the working week in summer this is not an issue as German industry (which moans about the cost of electricity) absorbs all the sun power produced. The problem arises at weekends when the 32GWp of installed capacity (at the end of 2012) is operating at anything like full capacity.  Last summer an estimate is that nearly 30GW of electricity was produced from the PV systems making up perhaps 50% of demand.  Surprising although it may seem this is could be a serious issue.  The problem is that if there is excess power on an electricity grid the voltage and grid frequency rise outside defined limits and the grid will shut down. So far the Germans seem to have managed the situation with few outages.  There are a number of solutions briefly covered below to this.  These are;

  • Use the excess power yourself.  Not an easy one this if lots of people are off work although as covered in the post linked to on this blog above smart grids and meters could help here by encouraging people to use washing machines and bread makers when electricity is cheap, which it will be when you have loads of it.
  • Switch off conventional power stations.  Obviously a whole heap of nuclear, coal and gas power stations are operating continuously providing baseload power.  These can be taken off line (not nuclear since they takes weeks to start back up again).  In fact recently RWE announced the permanent closure of a number of coal fired power stations as the wholesale power price had dropped too low to make them economically viable.  Of course if you go too far down this line you will have insufficient electricity and a grid outage for that reason.  Nevertheless its encouraging renewables are shutting coal down.
  • Sell your excess power to your neighbours.  Germany is doing this, despite the nuclear phaseout and the immediate shutting of a number of old reactors Germany is exporting large amounts of electricity and is a net exporter of electricity. However, there are problems with this solution.  First you may find they don’t need it.  Second, the Germans are selling expensively subsidised electricity at a loss to their neighbours.  It should be pointed out that these subsidies for new systems are much lower than they were and phase complete phase out of the FIT at 55GWp of installed capacity (which is only 2-3 years away) is planned.
  • Store the excess power.  Germany is building more pumped storage, but obviously this can only be a partial solution.
  • With PV systems being the problem, could they be the solution?  A number of solutions present themselves at the micro-generation scale.  Firstly, the individual householder could be encouraged to store electricity and use it at night.  The Germans are being encouraged to do this with a subsidy of some type, although uptake has been low.  Second, the PV inverters which convert DC from the PV to AC for home or grid use monitor the grid conditions.  In fact this is part of the problem.  Say its a dull day in July and the sun comes out and PV is suddenly producing 50% of the electrical supply.  The voltage rises and takes its outside the voltage limits set by the German government for PV inverters.  Millions of inverters shut down but not before you have taken base coal fired power baseload generation off line.  You go from feast to famine in minutes and have an outage.  However, you can by regulation use this grid monitoring facility to aid this control of the grid in a number of different ways.  You can set the inverter to trim its power output (lower its efficiency) when the grid conditions reach a certain voltage or frequency and switch off at a higher frequency (50.2Hz).  This is what the Germans have done.  Another thing they are doing I understand is trying to encourage PV system installation on west facing roofs to spread the peak output over the day.  Another strategy I would suggest is that inverters have different cut-off settings.  So for example in the north of Germany the voltage and frequency settings are set higher than say in Bavaria where its sunnier.  This means not all inverters cut power or cut at the same point and people in Northern Germany are not losing as much money.

There is no doubt that excessive renewables production is a challenge, but one that is not unmanageable with probably all the strategies above required in combination.  Since conventional fuels are finite we will have to cope with this problem at some point in the future. Other countries need to think about this.  There is an estimate that in July the UK got 2.5% of its electricity from PV over 24 hours and about 6% at around midday.  We are way behind Germany but we don’t need as much PV installed for us to catch up.


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