The Potential for Solar Energy in Botswana

Dumelang*. In my past few posts, I have focused on Botswana’s main energy resource: coal. However, Botswana has another very important energy resource that is presently poorly utilized: that of sunlight. Anyone who spends even just a little time in Botswana always marvels at the sunshine and the long days of clear skies that roll one into the next for weeks at a time. Combine this with vast expanses of low-population-density, semi-arid, flat countryside and the potential for tapping the solar resource of Botswana becomes obvious. In many countries, there is a remarkable roll-out of solar energy generation operations underway, both large and small scale, driven by improvements in technology and the falling prices of solar panels.  In some respects, Botswana is arriving late to the game, but, as I will highlight in future posts, there are positive steps being taken. In next series of posts, I will discuss various aspects of solar energy, how Botswana is benefitting from its ~3200 hours of sunshine per year, and how the country could further tap into this solar energy potential. 

There are several ways to harness the energy of the sun:

• Solar thermal uses the heat of the sun to warm up water so that it can be used for showers and other hot-water applications like washing.
• Concentrating solar power concentrates the energy of sunlight by mirrors onto a focal point. The focused sunlight heats a fluid, which is used to generate steam, which then turns a turbine to generate electricity.
• Photovoltaic generation of electricity through the use of solar panels is the most widely used and most promising approach to tap into the sun’s energy. Its application is growing exponentially in many countries and it represents one of the most significant resources of renewable energy.

Each of these applications requires sunny days and the direct radiation of the sun, so let’s start with some measures of solar radiation. Botswana has about 300 clear days annually and, as noted above, about 3200 hours of sunshine. In comparison, the state of New Hampshire in the US, where my home university of Franklin Pierce University is located, has ~2500 hours of sunshine and only 90 clear days per year. For a more exact determination of the power available from the sun, we use the concept of irradiance, which is a measure of sunlight intensity on one square meter of horizontal surface at a point in time. This, of course, changes during the day: it is low in the early morning; at a maximum at noon when the sun is directly overhead; and then it tapers off towards evening. It also changes depending on the location, time of year, and the amount of cloud cover. The typical average peak irradiance at noontime is 1000 watts per square meter (W/m²) for the planet. This value is referred to as the peak sun value. Measurements of irradiance taken over one summer day at the University of Botswana in Gaborone 20 years ago appear in the figure below. It is notable that there are times when the peak irradiation is greater than 1000 W/m², which is indicative of the solar potential available for harnessing.

 Source: Univ. of Botswana

Irradiance is a measure of the intensity or power of sunlight at a point in time, but what we are really interested in is the energy that we can harvest over a period of time. In an earlier post, I explained the difference between power and energy. Energy is the amount of power expended over a period of time (an hour or a day). The mathematical relationship between Energy and Power is given by the simple formula:

Energy = Power x Time

In the solar field, we determine the energy that can be potentially harvested over a day by calculating the area under the irradiance curve, such as the yellow area in the figure above. This measure of energy over a day is termed insolation; it is measured in kilowatt hours per square meter (kWh/m²). Another measure of insolation is to calculate how many hours of peak sun (with a fixed irradiance of 1000 W/m²) will deliver the same energy as the sum of the varying (irradiation x time) values over the day. The hours of peak sun per day is a particularly useful measure and is extensively used in the solar energy field to determine the daily output of electricity from solar panels. Insolation data are often available in tables of peak sun hours for different times of the year and different locations around the world. For example, the figure below shows peak sun hours for Gaborone at different times of the year and compares it with Manchester, New Hampshire, in the US (the location of Franklin Pierce University’s graduate campus). The differences are stark: the lows in the winter months in Gaborone are not that much lower than the highest value for Manchester in the summer! On average, the annual peak sun value for Gaborone s is 5.64 hours, while for Manchester, it is only 3.48 hours. This is to be expected, because the latitude of Manchester, NH, is further north (43^o) than Gaborone is south (25^o) and the former also has far more cloudy days.

Source: Solar Electricity Handbook

Now these insolation measures are for a panel lying flat on the ground, but that is not the normal orientation for most solar projects. In the southern hemisphere, solar panels are angled towards the north, so as to capture as much sunlight as possible as the sun rises and sets in the northern skies. In the northern hemisphere, panels are angled towards the south. Typically, the mounting angle of the panels is equivalent to the latitude of the location: panels in Gaborone are often oriented at an angle of 25^o from the horizontal. With the correct mounting angle, the average annual insolation value increases to 6.07 peak sun hours, an 8% improvement over a horizontally mounted panel. This is the average improvement for a fixed-angle array, but further improvements can be achieved by adjusting the orientation of the panels during the year. In winter, panels should have a higher mounting angle to capture the sunlight from the sun sitting low in the northern skies and, in summertime, the panels should lie flatter to catch the rays of the sun that sits high in the skies during the day. Some solar arrays are designed to allow for manual adjustment of the angle of the array during the year. If optimized monthly, a further 5% increase in the average annual insolation value over the fixed-angle array can be achieved.

Some solar arrays are very sophisticated, with intricate motor drives and control systems that can follow the sun from east to west during the day and also make small daily adjustments in mounting angle to follow the sun’s seasonal orientation.  Dual-axis systems, such as these, can boost the insolation by about 30% or more over a fixed-angle array. These dual-axis systems are expensive and maintenance issues with the drives and controllers often occur. Solar panels are pretty cheap these days, so a 30% gain in efficiency can rather be captured by simply adding more panels and avoiding the maintenance headaches. For this reason, most solar systems are simple fixed-angle systems.

Let’s return for a moment to measures of solar insolation. We have been using units of peak hours, which are particularly useful when calculating the energy that a photovoltaic (PV) panel will generate. Another useful unit is the direct measure of energy per square meter, i.e., kilowatt hours per square meter, kWh/m². We determined above that the average peak hours for a horizontal array for Gaborone was 5.6. So, at a peak sun value of 1000 W/ m², we can calculate the average annual insolation value as:

                5.6 hours/day x 1 kW/m² x 365 day/year = 2044 kWh/year.

Of course, higher irradiation values created by improved mounting angles lead to higher annual insolation values. To gauge the amount of solar energy that can be harvested, maps of annual solar insolation have been prepared for the whole planet. Those below show the average annual insolation, in kWh/m², for Africa and Botswana. An examination of the African map shows areas of very high annual insolations, >2200 kWh, particularly in the desert areas of North Africa, but also in areas covering a good part of Namibia and Botswana. The Botswana map shows that the best areas for high solar insolation lie in the western and northern parts of the country, particularly the Ghanzi and Maun areas.

It is clear that Botswana has large areas that are subject to high-intensity solar irradiation that can be used to generate electricity. In an earlier post, I noted that annual electricity consumption for Botswana in 2014 was ~ 4000 gigawatt hours/year (GWh/y) (one GWh is equal to one million kWh). Using a value of 2200 kW/m² (such as that seen in the western and northern parts of the country), we can calculate roughly how much land area would be needed to generate Botswana’s annual electricity demand using the following assumptions:

PV panel efficiency: 15% (a typical value for modern panels)

Electrical and storage system losses: 50%

Panel coverage of land area: 50%

Based on these assumptions, we can determine that an area of approximately 50 square kilometers would be needed to generate sufficient energy to meet Botswana’s annual electricity needs. This hardly seems much in a country with a footprint of 566 730 km². This suggests that it would not require much land area to generate all of Botswana’s electricity needs. However, it is important to put these fun-to-do order-of-magnitude calculations into perspective and to consider technical feasibility of these ideas.  

Let’s start with the size of the plant. Very large solar plants are being built today. There are several in the 550 MW range in the US and some larger ones are being planned. As of the date of this post, the largest solar power plant in the world is the BHE Renewables Solar Star operation in Antelope Valley, California. This is a 579 MW AC output plant, capable of generating ~1785 GWh of electricity per year or about 45% of Botswana’s needs. It uses 1.7 million solar panels and is located on 13 km² of land. Technically, large-scale solar plants are feasible, but the costs are high. The cost for a 550 MW plant in California was reported to be $2.4 billion, which yields an installed cost of $ 4400/kW. Coal-fired power plants have lower installed costs (especially if pollution control equipment is not included)—of the order of $ 1300 to $ 2300/kW. However, one should not just consider capital costs, but operating costs as well. Taking into account all costs for a power plant is a complicated topic and it is a subject I will tackle in a future post.

Cost is an issue, but the bigger problem associated with solar power is that it is an intermittent and variable resource. Unlike a traditional power plant that can vary its output day or night (within a certain range), a solar resource can only produce energy during the daylight hours and is subject to the whims of cloud cover and passing storms – during which output can drop considerably. To fully utilize solar energy, we need electricity storage in batteries to provide power for the nighttime and when it is cloudy. Unlike grid-scale electricity generation from large PV plants, grid-scale battery storage is still in its infancy. It is complicated and very expensive. To date, the largest grid-scale battery-based storage operation is in Japan – a 40 MW unit with storage of 20 MWh. To store just three days’ worth of electricity for Botswana, would require some 40 GWh of storage. This is 2000 times the largest plant storage plant at the moment and is simply not feasible at this time. In many respects, generation of the electricity from sunlight these days is fairly straightforward; the storage aspect is the bigger challenge that we face this century.

This is the first in a series of posts about solar power and its potential for Botswana. I trust that I have given you a sense of the tremendous resource available and the scale that is needed to harness it, but also a sense of the technical challenges, complexities, and costs involved in developing this resource.

Until next time, remember to turn off the lights when you leave the room. 

Tsamayang Sentle**

Mike Mooiman

mooimanm@franklinpierce.edu

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(*Greetings in Setswana)

(**Go well or Goodbye in Setswana)