One of the biggest problems about renewables is that they are not a constant source of energy.

Before looking at possible solutions to cope with the intermittency of renewable energy sources, I'll start analysing the energy demand fluctuation of the commercial building I described in previous articles. The fluctuation of both electricity consumption and production are so tightly bounded together that we can't talk about one without taking the other into account.

Figure 1 - Electricity consumption fluctuation

As it is clearly visible in figure 1, the energy consumption of the zero emission workspace is not constant throughout the day. The consumption curve has been worked out making the following assumptions:

  • the whole office equipment is used only during working hours (8am-6pm) with the only exception of the network hub/router which is kept switched on 24h
  • the kitchen appliances are used only during working hours (8am-6pm) with the only exception of the fridge/freezer which works 24h
  • the ventilation system and the CCTV system are in operation 24h
  • the lightning is mostly used during working hours (8am-6pm) but 20% of the total lighting consumption is overnight (6pm-8am)
  • The lift, the conference audio system and the hand driers are used only during working hours (8am-6pm)
  • the vacuum cleaner is used only 3 hours a day from 7pm to 10pm
  • the electric vehicles are charged overnight over a period of 8 hours, 10pm-6am

The diagram in figure 1 shows that the majority of the electricity demand is concentrated during working hours (8am-6pm) and that the main contributors to this energy demand are the lift, the lighting, the LCD screens and the laptops. Overnight the electric vehicle chargers are the main responsible for electricity demand.

From the diagram it comes out that if we had an electricity generator which pumps a steady 25 KW power 8am-6pm and a steady 10 KW power overnight, we would satisfy the energy demand every day. However, this would be possible only if we could turn on and off the generator whenever we need and if the generator was a constant source of electricity, like a fuel power plant.

Unfortunately, this is not the case with renewable energy sources. Wind and solar power generators are not constant sources of electricity: their daily production depends on how much wind or sunshine the nature provides and the size of the daily energy yield is quite difficult to forecast.

Figure 2 - Power production vs power consumption in an average scenario

In fact, we could have the situation depicted in figure 2. Wind turbines and PV solar panels could deliver a constant 20 KW power all day which would be more than enough overnight but not sufficient during the day. Or we could have an extremely good situation, like the one depicted in figure 3, in which we have a variable power oscillating around 90 KW: that day the energy provided would be a lot more than what we need! Or we may encounter a cloudy day with no wind, when only very little electricity would be produced by the PV solar panels around midday, as shown in figure 4.

Figure 3 - Power production vs power consumption in a best case scenario

Figure 4 - Power production vs power consumption in a worst case scenario

Now we need to answer a question: what should we do with the surplus energy produced during the day, as in the case of figure 2 and figure 3? Of course we can't just simply waste it. One option is to feed all that electricity into the grid and sell it to a network utility operator. This solution is remunerative but this way we would leave unresolved another main problem: where do we get the electricity that is missing when there is a slew or lull of on-site production?

Obviously, the simplest option is to buy it from the grid. That would be perfectly acceptable if our goal was not to build a Level 6 building according to the Energy Saving Trust "Code for Sustainable Homes". A Level 6 building needs to achieve a 100% reduction in CO2. To reach zero carbon, all emissions produced for any energy need must be zero or negative. This can be achieved with zero carbon micro-generation, other onsite generation or offsite generation provided that this is connected directly to the development via a private wire arrangement.

It comes easy to understand that if we simply buy electricity from the grid, there is no guarantee that that electricity has been generated by zero carbon generators. Therefore we could certainly sell a part of the surplus to the grid, but this can be done only once we make sure that we can satisfy the energy demand, ideally with electricity generated on-site.

In a day like the one described in figure 3, the amount of electricity generated is so much that we would be tempted to sell part of that electricity to the grid and, at the same time, find a feasible way to store part of it to be used in days when there is lack of production.

That would be the breakthrough: if we found a way to set apart the surplus of electricity when there is abundance of energy so that the stored electricity can be re-used when there is not enough production in days with no wind or sunshine, we would be eventually off-grid.

How much energy should we store in order to cope with long lulls of wind or sunshine?

Figure 4 - Total output of all wind farms of the Republic of Ireland, from 15th January to 31st January 2010

Figure 4 shows the total output, in MW, of all wind farms of the Republic of Ireland, from 15th January 2010 to 31st January 2010 (source Eirgrid). I marked a 4 day period of time when there was a low level of electricity production, precisely from 22nd January to 26th January. Is this the longest country-wide lull ever measured in Ireland? 2 to 5 day lulls happen quite often but the longest lull ever measured in Ireland was in February 2007 when the wind was very weak for 5 consecutive days. Of course there can be months with very low total electricity production: in the UK, between October 2006 and February 2007 there were 17 non consecutive days when the output from Britain’s 1632 windmills was less than 10% of their capacity.

In our case the use of PV solar panels in conjunction with wind turbines may help: usually when there is less wind there is more sun (even if this is not always true).

Let's assume we want to store as much energy as we could consume over five days so that we would be ready to cope with a five day zero electricity production from both wind turbines and PV solar panels. The daily energy consumption of the zero emission commercial building under analysis is 325 KWh, which means 1625 KWh over five days. As usual, let's exaggerate this number and set it to 2 MWh.

From now on, our goal will be to find a viable way to store up 2 MWh of electricity to use when renewable energy sources have gone to zero.

But is that actually possible? Do we have the technology required to achieve this outstanding result? This will be the topic of the next article.