Opportunities for solar energy

In 2019, the year before the restrictions of the covid-19 pandemic, the world energy consumption was slightly higher at around 170 000 TWh.

In this post I will talk about some of the science behind this amazing fact and discuss the  challenge of getting solar energy from where it is plentiful to where it is needed and storing it for future use.

The facts behind this statistic

The Sun generates energy by nuclear reactions which occur at its dense hot core .It produces a massive 382.8 trillion trillion (3.828  x  10²⁶ ) watts of electromagnetic radiation (Williams 2018) – mostly in the form of visible light, infrared and ultraviolet. As you get further from the Sun, the intensity, which is the power falling on a unit area declines as the square of the distance

The solar constant is the average intensity of the Sun’s radiation at the Earth’s distance from the Sun. It has a value of 1361 watts per square metre (W/m²). (In fact,  the output of the Sun is variable and fluctuates by 0.1% around this value). Using this number, a simple calculation tells us that the total solar energy hitting the Earth in one hour (in watt-hours) is

solar constant  x    area of an Earth-sized disc

         1 361 W/m²         x     1.2748 x 10¹⁴ m²  = 1.73 x  10¹⁷ watt-hours.

 This  can be expressed as 173,000 terawatt hours (TWh), where one terawatt is one trillion (1,000,000,000,000) watts

The total energy consumed by humanity in 2020 is slightly lower than this at 163,000 TWh (Enerdata 2021). This figure includes not just energy used to generate electricity, but also energy used:

• directly for heating (for example by burning firewood, coal, oil or gas), 
• for transport (mainly petrol, diesel and aviation fuel) and
• energy used in industrial processes.

The total amount of electricity consumed in 2020 was approximately 27,000 TWh.

The potential for solar energy

There are two different methods of generating electricity from sunlight.

• One way is to concentrate the Sun’s energy using mirrors onto a small area and use the heat generated to produce steam to turn a turbine which generates electricity.
• The other way uses arrays of photovoltaic cells (more commonly known as solar panels) to generate electricity directly from sunlight. The vast majority of solar electricity is produced this way, much of it in solar farms like the one in California shown below. As the cost of solar panels continues to fall and their efficiency increases the amount of electricity generated this way will continue to go up.

A photo taken from space of the Topaz solar farm in California. It covers an area of nineteen km² (not all of which is covered with solar panels) and generates around 1.25 TWh of electricity per annum.

The growth of solar energy (Our world in data 2021)

A key advantage that solar energy has over other forms of green energy is that it has an almost unlimited potential. In the idealised case, where solar energy could be transferred from where it is generated to where it is needed and stored without loss, it is necessary to cover  only 0.12% of the Earth’s surface with solar panels to meet all of humanity’s energy needs. (The details of this calculation are in the appendix at the end of this article.)

The challenges of supply, demand and storage

However, things are not that simple in the real world. The countries which could generate the most solar energy (particularly those in Africa) actually have modest energy consumption and many densely populated countries, particularly those in Northern Europe, have high energy consumption and receive relatively little sunlight.

For example, the UK has a small surface area, receives less sunlight than the world average, and is densely populated with a high energy consumption. Because of its high latitude there is great variation between the solar electricity which could be generated in the sunniest month which on average is July (although June has more hours of daylight it has more cloud cover than July) and the least sunny month, December.

In July, the UK would need to cover 2.3% of its area with solar panels (roughly one and half times the area covered by buildings) to generate enough solar electricity to meet its  energy needs for that month, which is a sizeable fraction. However, to meet the UK’s  December energy needs, using the meagre amount of sunlight available, we would need to cover 23% of its surface area with solar panels. This would clearly not be practical.

In reality, it is exceedingly unlikely UK-generated solar power could generate most of the UK’s energy needs. To meet the UK energy needs, averaged out over a 12 month period, it would be necessary to cover 5% of its land area with solar panels – a massive amount. However there a huge fluctuation in solar energy supply and total energy demand between the winter and summer months, If the UK were to rely totally on solar energy, an enormous hurdle which would need to be overcome, would be the need for a long-term storage facility, so that the excess energy generated in the sunny months could be accumulated and used in the less sunny months. The amount of energy needed to be stored would be immense, around 500 TWh. It would be impractical to store this amount of energy in batteries.

In theory the solar electricity generated in the more sunny months could be stockpiled by splitting water to make hydrogen which would be stored and used to generate clean energy, or to power hydrogen-fuelled vehicles. Hydrogen is the most energy dense chemical fuel known. One kg of hydrogen when burnt releases 33 kWh of energy. So, to store 500 TWh of energy in the form of hydrogen would require (a relatively modest!) 15 million tonnes of hydrogen. In reality, more would be needed since the conversion to and from hydrogen is not 100% efficient.

Another possibility, if the UK wanted to generate most of its energy from solar, would be to build high voltage, high-capacity transmission lines to import solar electricity into the UK. Solar energy could be generated in North Africa, where the solar irradiance is greater and, being closer to the equator, there is a smaller variation throughout the year.  It could be transmitted to northern Europe over high voltage DC power lines which typically have a loss of around 3.5% per 1000 km. However, it is unlikely that the UK would want to be dependent on North Africa for the bulk on its energy supplies.

Greater role for wind power                                        

Solar energy plays a significant role in the UK energy generation and will place an increasing role in the future. However, the  UK generates most of its renewable energy by wind and with its long windy coastline it is certain that offshore wind generation will play a greater role in the coming decades as it moves toward becoming an economy which contributes zero carbon dioxide to the atmosphere by the year 2050.

Taken from https://www.nationalgrideso.com/news/record-breaking-2020-becomes-greenest-year-britains-electricity

Small scale generation for consumers off grid

A key advantage of solar power is its ability to generate electricity on pretty much any scale. A single solar panel has exactly the same efficiency as a large array of a million panels. A panel one metre square will generate up to 250 watts of electricity, if connected to a rechargeable battery it  can provide a cheap and reliable source of electricity for off grid customers. This is particularly useful in the world’s poorest countries which are mostly situated at sunnier latitudes and have a more modest demand for electricity compared to richer countries. Once the initial cost of installation has been paid the running costs of a solar array are very low.

In contrast,  small wind turbines are not as efficient as larger turbines and so need to be situated in an area of above average wind in order to generate reasonable amounts of power. They also require a smooth airflow: the smaller turbines are very susceptible to turbulence – so if you live near trees, or in a built-up area, a wind turbine is unlikely to be efficient. Small scale hydroelectric plants which generate less than 5 kW are known as pico hydro systems and although they are relatively cheap to build, they need a constant supply of water running downhill and have moving parts which need to be serviced and maintained.

Appendix

How much of the Earth’s surface would need to be covered to meet humanity’s energy needs?

The first thing we need to consider is the amount of solar radiation reaching the Earth’s surface. Although the solar constant is 1,361 W/m², this is the intensity of the radiation which hits the top of the Earth’s atmosphere. Even on a cloudless day not all this radiation reaches the ground. Some is reflected back into space, and some is absorbed by the atmosphere.

On a clear day, if the Sun is directly overhead, the intensity of the radiation hitting the ground direct from the Sun is around 1,050 W/m². On top of this a further 70 W/m² comes from the bright blue sky, giving a total of 1,120 W/m². (If it is cloudy this figure will be lower.)

In fact, the Sun can only be directly overhead at tropical latitudes.  When the Sun is lower in the sky, the intensity of the solar radiation is reduced because its rays are spread out over a larger area and pass through more atmosphere before they hit the ground.

The variation in the solar intensity at the equator, at an equinox. The time axis is in solar time where the Sun rises at 0600, is at its highest at 1200 and sets at 1800. A cloudless day is assumed.

In any large structure which generates solar electricity there must be gaps between the solar panels. In these calculations I have assumed that one sixth of the area of a solar farm/large array of solar panels is not covered by panels.
 
Calculation for the UK
For the UK, since it is a long way from the equator,  there is larger difference in the solar irradiance between the winter and the summer months.  For a location near Manchester (in the middle of the UK) daily average solar irradiance is around 200 W/m² in  July but in December it is ten times lower. 

Data from Science Direct (2014) 

Therefore, there is a massive difference between the calculation for July compared to December. In July we would only need to cover 2.3% of the UKs surface with solar panels to generate all its energy needs. Whereas if we run the same calculation for December, the solar irradiance is only 20 W/m2 and we would need to cover 23% of the UK area with solar panels to generate all its energy needs

Data from Science Direct (2014) and Enerdata (2018)

In this table, for simplicity,  I’ve assumed that the energy consumption for  December and July is the same at 150 TWh. This is the annual energy consumption of the UK in 2020 divided by twelve. Clearly more energy is used for heating in December, particularly in people’s homes but these figures refer to total energy consumption and
·       to some extent, less energy is used for  transportation in the cooler months (people travel less in winter)
·       the amount of energy used for industrial processes is the same all year round
·       in the summer months a significant amount of energy is spent on air conditioning particularly in shops and offices – a figure which is likely to increase over the coming years.

Even so, in reality more energy is needed in the winter months in which case we would need to cover even more than 23% of the UK’s surface to meet its winter energy needs.

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References

Enerdata (2021) Global energy statistical yearbook 2020, Available at: https://yearbook.enerdata.net/total-energy/world-consumption-statistics.html (Accessed: 13 October 2021).
Our world in data (2020) Global renewable energy consumption over the long-run, Available at: https://ourworldindata.org/renewable-energy (Accessed: 13 October 2021).
Science Direct (2014) The UK solar energy resource and the impact of climate change, Available at: https://www.sciencedirect.com/science/article/pii/S0960148114002857(Accessed: 13 October 2021).
Williams, D. R. (2018) NASA Sun fact sheet, Available at: https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html (Accessed: 13 October 2021).