Renewable Energy Sources
The 100% renewable energy system
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A vision for a 100% renewable energy system is shown in Figure 1.2. The renewable energy can be in the form of wind or solar power, but also as small- or large-scale hydropower installations. Also, certain types of biomass are considered renewable energy sources energy.
What is the main discussion?
For this chapter, the discussion about what exactly constitutes renewable energy is not relevant. Important is only that there will be a mix of energy sources of different sizes and at different locations, a lot of which will be constrained in time and in location.When available solar and wind power?
Solar and wind power will only be available when the sun is shining or when the wind is blowing. A lot has been written about this already, where the somewhat inappropriate and confusing term ‘intermittency’ is typically used. But the actual situation is much more complicated. Next to the variations in availability, the availability of sun and wind is also difficult to predict renewable energy can be in the form of wind or solar power,.
Figure 1.2 A vision of a 100% renewable energy system; the arrows indicate the different energy conversions
This holds especially for regions with fast-changing weather patterns, like northwestern Europe. The availability of hydropower is easier to predict, at least up to a few months in advance, and at least where it concerns large-scale reservoir-based hydropower.Unfortunately, this is also the type that is most difficult to build in many parts of the world, not only technically but also politically.
At timescales longer than a few months, also hydropower becomes less predictable. The amount of water in the reservoirs depends on the amount of rain or snow in the preceding weeks or months.Countries with large amounts of hydropower (like Spain and Sweden) are very much aware of the existence of ‘wet years’ and ‘dry years’.
To what extent there is a correlation between dry years, years with little sun, and years with little wind are not known at the moment. But the overall impression from various studies is that the probability of lack of energy will reduce by mixing different sources of renewable energy, for example, combining wind power, solar power, and hydropower.
Use of renewable energy :
The renewable energy will be used for a range of applications. An important part of the transition to a 100% renewable energy system will be an overall shift from energy-intensive activities to non-energy-intensive processes as well as an overall increase in the efficiency of the energy-intensive processes. Some of the ongoing or heavily discussed changes are:- Insulation of buildings to reduce the amount of energy needed for space heating or cooling;
- A shift from gas heating to electric heating and from direct electric heating (resistive heating) to heat pumps;
- A shift to more energy-efficient (electric) heating, ventilation, and air conditioning;
- The shift from incandescent lamps to LED lamps;
- The shift from gasoline-powered cars to electric cars and the continued electrification of the railways.
But a certain energy demand will remain, which is the one indicated in Figure 1.2. Energy efficiency is beyond the scope of this book; it is assumed that the energy demand is the one that is necessary and that is the most efficient one.
For simplicity, a distinction is made in Figure 1.2 between three types of energy consumption: ‘heating and cooling’; ‘transport’; and ‘other consumption’.
Note that:
We use the physically incorrect expression ‘energy consumption’ when referring to an energy conversion process where the entropy increases. In almost all cases it is obvious to the reader what is meant by energy consumption, despite the term being physically incorrect.Note also that:
The vision in the figure does not include a 100% electrical transport system. Instead, some of the transport will use fuel. But this fuel will not come from fossil fuels but from renewable sources, like biogas. Part of the fuel will also be produced from electricity through processes that go under the terms ‘Power2Gas’ and ‘Power2Liquid’.(We will use the general term ‘Power2Fuel’ where needed.)
During periods with a surplus of fuel and/or a shortage of electricity, either locally or globally, the fuel can be turned back into electricity.
Heating and cooling will be partly using renewable energy directly, e.g., solar panels for water heating, partly based on electricity, and partly using the fuel from renewable sources or from Power2Fuel. In the latter case, this will most likely be in the form of combined heat and power (CHP, co-generation). Burning fuel just for space heating is rather inefficient compared to modern heat pumps and CHP installations.
Energy conversions:
The arrows in Figure 1.2, numbered in Figure 1.3, are an important part of the energy system. It is these transitions that allow electricity to become the heart of the energy system. But these conversions are also, in many cases, where the losses are.
The lower the losses, the sooner a 100% renewable energy system with sufficient reliability can be reached. The amount of conversions thus has to be limited to get an efficient energy system.
Increasing the efficiency of the various conversions is an important development as well, but also this is beyond the scope of this book. It should be noted however that the conversion technology to and from electricity can impact the electric power system as well.
Examples are harmonics from adjustable-speed drives and short-term voltage collapse due to airconditioners based on direct-driven induction motors.
Figure 1.3 Different energy conversion processes in a 100% renewable energy system
The following conversion processes can be identified from Figure 1.3:
1. Using renewable energy directly:
For heating and cooling; for example, solar panels for water heating;2. Renewable electricity production:
Hydropower; wind power; solar power and other future sources like wave or tidal power. The increase in efficiency of, for example, solar cells is important here;3. Fuel from renewable sources:
Like biomass;4. Using electricity for heating and cooling:
In an efficient energy system, this is accomplished using heat pumps. In cold climates resistive heating or fuel-based heating may be use as an additional source during very cold periods.5.Other use of electricity:
Like lighting, washing, computing, or industrial processes;6. Use of electricity for transport:
This can be the direct use of electricity like electric trains, trams, and subways. For electric cars, the users will more likely be in the form of batteries that will be charged from the electric power grid. The transport system will be hybrid but most or all vehicles may be based on one single source.7. Power to Fuel:
Using electricity to produce gasses or liquids with high-energy contents. This could become an important form of storage ensuring the stability of the electric power grid.8. Burning fuel to produce electricity:
This can be in the form of the conventional Carnot cycle, as part of CHP installations or by means of fuel cells. There are high expectations for the latter, but for the time being the first and second ones remain the ones with the highest efficiency.9. Burning of fuel for space heating:
Burning of fuel for space heating or for other heating processing like in industrial production;10. The use of fuel:
The use of fuel for powering vehicles like cars, boats, or planes.- 1.4 Flexibility
Many textbooks on the electric power system and many texts on the integration of renewable electricity production in the electric power system contain one or both of the following two statements:
‘The operation of the electric power system requires that production and consumption of electricity have to be always equal to each other’; ‘The reason for this is that electricity cannot be stored’. Both statements are commonly made and are strictly speaking incorrect.
When looking at a timescale of milliseconds:
When looking at a timescale of milliseconds, there is a continuous conversion between electrical and magnetic energy in the electric power system, both of which are stored for a few milliseconds. This is related to the concept of ‘reactive power’.At timescales of seconds:
At timescales of seconds, there are subtle differences between production and consumption that are visible in the form of frequency variations. They are these small differences, and the associated frequency variations, that allow the balance between production and consumption to be kept at longer timescales, through what is referred to as ‘power frequency control’.Electricity or electric energy (electromagnetic energy would be the correct physical term) is a kind of energy and as such, it can be stored.
However, there are no known processes to store electricity in a way useful for large-scale use. For the purpose of most of this book, we can therefore assume that electricity cannot be stored and that the production and consumption of electricity have to be equal to each other at all times.
In a renewable energy source electric power system, the consumption varies with time over a range of timescales. As production and consumption have to be equal, the production has to vary in exactly the same way as consumption.
In the existing power system, the variations are set by the consumption and various control systems ensure that the production follows the consumption.
From a consumer viewpoint, this means that electricity is available whenever wanted.
With conventional production units
This can be achieved, and actually, it has for many years been the aim of the power system planning and operation to ensure that this is always the case. An alternative way of looking at this is that the power system is dimensioned in such a way that it can cope with the worst-case; i.e., the highest amount of consumption. It is obvious that this can easily result in overinvestment, but electricity is such an important commodity for a society that this was considered a price worth paying.This general planning approach is on the way out, however, for a number of reasons. One reason is that overinvestment has become less acceptable in society. Another reason is the introduction of free competition between companies producing electricity.
The transport part of the electric power system (the wires) remains in the hands of a monopoly company. But even in the transport part of the power system, there is less willingness to overinvest.
An important driving force:
An important driving force here is the availability of technology to shift some of the flexibility from production to consumption.Flexibility on the consumption side is not completely new and some applications have been around for many years. For example, many countries used to have time-of-use tariffs where electricity was more expensive during peak hours.
Also some countries, France being one example, have a tariff option where electricity was more expensive on days with high expected demand in relation to the available capacity. Several of those schemes disappeared with the restructuring of the electricity market in the 1990s but have attained renewed attention rather recently in renewable energy sources energy.
Figure 1.4 Sources of flexibility for the electricity grid in the 100% renewable energy system
In a 100% renewable energy system, many of the sources of electricity production cannot be easily controlled so as to follow the consumption. This will either require a very large installed capacity of renewable electricity production or alternative sources of flexibility. The latter is the most likely solution and the following sources of flexibility are indicated in Figure 1.4.
Conversion 2:
Curtailing the sources of renewable energy. The main source of flexibility is formed by reservoir-based hydropower. Already in the conventional power system, in many countries, hydropower is an important source of flexibility. But also solar and wind power can contribute to flexibility by occasionally being curtailed.
Conversion 4:
Electricity for heating and cooling. The long thermal time constant of most buildings (several hours to days) makes it possible to adjust the electricity consumption to the availability of RES.
Conversion 6:
Electricity for use in transport. This concerns especially the use of electricity for charging batteries for electric vehicles. With direct-driven transport like electrified railways, the flexibility is much less.
Conversion 8:
Burning fuel for producing electricity. This is equivalent to the use of conventional fossil fuel-based production units.
Production and consumption should be equal to each other at timescales from a few seconds and upwards. At the shortest timescale, the power-frequency control takes care of the balancing. Differences between production and consumption result in frequency changes that are detected by the power-frequency control.
As the frequency is the same throughout the power system, each device (production or consumption) can contribute to the balancing at this shortest timescale.
How can production units increase?
Production units can increase their production when the frequency drops; heating, cooling, and battery charging for electric vehicles can reduce their consumption when the frequency drops. This so-called primary control takes place at a timescale of seconds.
In the conventional power system, an unbalance between production Pprod and consumption Pcons does not immediately result in instability, because of the availability of energy in the rotating mass of the generators. Instead, the generators change speed, which is visible throughout the system as a change in frequency,
around a frequency f0:
The kinetic energy Ekin of the rotating mass:
The kinetic energy Ekin of the rotating mass in the generators is important to limit the frequency change and therewith give the control system time to react.
Modern types of renewable energy:
Modern types of renewable energy are connected to the grid through a power electronics interface that does not contribute to the kinetic energy available for system stability.
Keeping the system stable at these timescales is a major power system challenge for the 100% renewable energy sources energy system.
At longer timescales, beyond the power-frequency control, the issue is not so much the actual balancing but ensuring that there is sufficient margin available with production and/or consumption to allow them to be brought in balance with each other.
The term ‘operating reserve’ is used to describe this in the conventional electricity grid. Maintaining sufficient operating reserve is an important task of the system operator in the conventional grid and it is likely to be an important task in the 100% renewable energy system as well.
In conventional power systems:
In conventional power systems, there are strict rules for the minimum amount of operating reserve that shall be available up to several hours ahead of time.
The further ahead of time, the more reserves should be available as the uncertainty in both production and consumption increases. Those rules do however only consider the availability of power production capacity.
Those rules do not consider the energy aspect of the reserves. In the 100% renewable energy system, the limitations may be set by the available energy instead of by the available power.
Several of the storage options:
Several of the storage options, to be discussed in Section 1.5, are limited in time by up to a few hours. When planning more than a few hours ahead, those cannot always be relied upon.
Alternative ways of scheduling reserves are needed, where the time element becomes a very important renewable energy source.
At longer timescales, weeks through years, we enter the realm of ‘operational planning’ and ‘planning’. For these timescales, the expected maximum consumption is compared with the amount of production capacity that is available with high reliability.
In a system with a large amount of hydropower, somewhat different long-term planning methods would be needed.
In the 100% renewable energy system, such alternative methods are likely needed throughout.
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