• Ian Sutton

Renewables: Supply and Demand

Updated: Mar 18


Batteries used to back up intermittent power source such as solar or wind.
Credit: Energy and Resources Institute

Modern society is utterly dependent on carbon-based/fossil fuels (coal, oil and gas). Yet our consumption of these fuels means that we dump billions of tons of CO2 and other greenhouse gases into the atmosphere every year. If we do not change course the consequences will be catastrophic. Therefore, at the heart of virtually all climate change proposals such as ‘Net Zero by 2050’ is a plan to switch from fossil fuels to alternative sources of energy that do not emit greenhouse gases. These alternatives include solar, wind, hydro-electric and ocean (wave and tidal) power.


In this series of posts we consider some of the engineering, project management and financial realities to do with switching from carbon-based fuels to alternative energy sources.

The first two posts in the series were:

  1. Renewables: The Paradox In this post we look at the apparent paradox that renewable energy sources are growing much more quickly than carbon-based fuels, yet their share of the energy supply is declining. (in it we saw that alternative energy sources have not been replacing fossil fuels; instead, these new forms of energy have merely provided a large fraction of the additional energy that we are using.)

  2. Renewables: Not Fast Enough In spite of the fact that renewable energy sources (principally wind and solar) are growing fast, their growth is not fast enough if they are to carry the bulk of society’s energy load just three decades from now.

In this third post we look at the difficulties associated with the intermittent nature of wind and solar power. (Wave and tidal power are partially dispatchable. The supply, particularly of tidal power, is predictable. However, the fluctuations in supply do not match the fluctuations in demand.)

Dispatchable Energy


A fundamental feature of our current energy supply is that it is “dispatchable”, which means that supply and demand can be balanced at all times. With respect to the electrical grid, conventional power plants (nuclear, gas, coal and hydro) are “always on”. As the customers increase or decrease their demand for energy during the course of a normal day, so the power plants can ramp up and down to meet that demand. Transportation fuels are also dispatchable. When an automobile pulls into a gas station the driver can assume that the gasoline that he or she needs is available on demand. Fluctuations in demand are handled by the fact that the gas station has an inventory of fuel in its storage tanks.


Solar and wind power are not dispatchable. On the supply side, the power is only available either when the sun shines or when the wind blows. Overall, this means that these energy sources are only supplying power about 35% of the time, depending on location. On the demand side, the need for power will vary during the course of the day. The fluctuations in demand will not match the availability of power. Therefore, even if the nameplate capacity of an intermittent energy source matches the nominal demand for power, the reality is that supply and demand will rarely match one another.


There are three ways of resolving this dilemma without needing to build an enormous amount of over-capacity. They are,

  1. Provide a backup power supply for the wind or solar farm. If demand for power is higher than can be delivered by the renewable source, then the “always on/always available” energy source would fill the gap.

  2. Provide energy storage capacity adjacent to the solar/wind farm. When more energy is being created than the customers call for, then the surplus energy would be stored. That same energy could then be released in the opposite situation.

  3. Develop smart grids that can balance supply and demand over a wide area.

1. Backup Power

The simplest way of handling the intermittent power supply from solar and wind is to have sufficient conventional power available to fill in the gaps. This is mostly what happens now. For example, if a person has a grid-connected solar panel on the roof of his or her house and the sun is not shining then the conventional power plant down the street increases its output to make sure that the customers have what they need. Then, when the sun comes out, the conventional plant backs off a small amount to allow the solar panels to provide power to the grid.


When renewables are just a small fraction of the overall power supply as they are now, (as shown in the following chart that is taken from the BP World Energy Outlook 2020 report), then this approach works.

BP chart shows growth of all energy types, declining percentage for renewables
Credit: BP

However, if renewables provide say 60-80% of the total energy that society needs then a massive amount of backup capacity will be needed. Such an approach fundamentally defeats the object of the exercise: to run society on renewable energy only. It would also be a very expensive way of meeting society’s energy needs. The rate structure for electricity would have to reflect the fact that the power company has installed substantial extra capacity that is not being used much of the time.


2. Storage

The second option would be to store surplus renewable energy as it was created, and to release it on demand. Options for doing this include pumped-hydro, batteries, hydrogen, compressed air, liquid air, gravity, thermal energy and thermochemical energy. We will look at each of these in more detail in future posts. Suffice to say that each has the same problem as installing a backup “always on” power plant, and that problem is cost. If each solar or wind farm requires a large investment in a matching energy storage facility then the rates would have to be increased by a large amount to pay for this storage capacity. This can only be justified in special cases, say when it comes to maintaining power to critical institutions such as hospitals.


There is also a common-sense conundrum. The reason we install solar/wind facilities is to avoid the need for conventional power plants. But if conventional plants are needed on an on-demand basis, then the point of the exercise has been defeated.


Moreover, some of these options are technically limited. Pumped-hydro, for example, sounds attractive, but there are not nearly enough suitable locations available to implement this option at scale.


3. Smart Grid

One option that is receiving considerable attention, particularly in Europe, is the concept of a “smart” or “super” grid. A smart grid connects areas that have surplus power to other areas that need more power at that particular point in time, thus helping to balance supply and demand. A smart grid can also organize the input for different, variable sources that provide power at different times. Their implementation involves the building of many more long transmission lines, which is both expensive and would run into many property rights and environmental road blocks. There would also be substantial transmission losses.


Conclusions

As already mentioned, we will look at some of the technical issues discussed here in greater detail in future posts. But one conclusion is already clear, and it is part of the overall theme of this site. It may not be realistic to expect alternative, carbon-free energy sources to totally replace what we have now because they will be too costly. We will not be able to maintain our current, energy-intensive lifestyle. We will have to learn to live more simply.

For example, there is currently a good deal of publicity to do with the development of electrically-powered vehicles (EVs) from Tesla and many other companies. There are two assumptions baked into most of these discussions. The first is that the power for the EV can be provided by solar panels installed adjacent to the vehicle’s normal overnight location. In fact, the solar panels will have to be part of the grid, and, as we have seen, that will lead to a substantial increase in electricity prices.


The second (unspoken) assumption is that that we can maintain our existing life style, that we can own a personal vehicle that we can use at any time to take us anywhere we want to go. We also assume that the government will provide the roads and other infrastructure needed for us to drive our new EV.


But, if the above discussions to do with the price of electricity are correct, then the only sensible way of applying renewable energy sources is to use them to power public transport. In other words, we will have to give up some of our current standard of living. But having to use public transport may not be as convenient as having one’s own EV, but it’s the way we used to live, and we got along fine. Here is a picture of a trolley bus taken in the 1970s (note the absence of private vehicles on this major highway).


Bradford trolley bus illustrates use of electrification for public transport

And here is a picture of a sensible use of the next generation of EV technology.

High speed train illustrates best use of electrification of transportation
Eurostar. Credit: Pixabay


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