Monday, December 21, 2015

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.

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 (43o) than Gaborone is south (25o) and the former also has far more cloudy days.

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 25o 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

(*Greetings in Setswana)

(**Go well or Goodbye in Setswana)

Tuesday, December 8, 2015

Coal Based Energy in Botswana: Coal-to-Liquids, Coal-Bed Methane, and Underground Coal Gasification

Dumelang.* There is more to coal utilization than just burning it to make steam to generate electricity. In my previous post, I carried out a general overview of coal projects in Botswana, but it was heavily weighted towards coal mining and coal-fired electricity projects. In this post, I take a close look non-coal-fired electricity projects, such as coal-to-liquids, coal-bed methane, and underground coal gasification, to get a better understanding of the benefits and challenges associated with these other ways of drawing out the energy stored in coal. Even though some appear to offer advantages over traditional coal mining and coal-fired electricity, it is always important to understand the shortcomings associated with these alternatives. Various proposals for implementing these technologies in Botswana have been made, there has been a lot of information about these projects in the Botswana press over the past few years and some exploratory work has been carried out but, as yet, no commercial operations have been implemented. 

Coal to Liquids (CTL)

Let’s start with coal-to-liquids technology. A large CTL operation producing 20 000 barrels per day of petroleum liquids has been proposed for Botswana. The proposal incorporates a 300 MW coal-fired power plant, a fertilizer plant that will produce 300 600 tonnes/year of ammonium nitrate, and 15 200 tons of sulfur as a byproduct. The budget for this project is $4.6 billion and construction is supposed to start in 2016. When I read about this project, its size and scope surprised me because it is beyond the price and scale of anything that has ever been built in Botswana.

Let’s look first at the technology. The CTL process, also known as coal liquefaction, involves taking the carbon in coal and converting it into hydrocarbon molecules that can be used for a variety of purposes, including liquid fuels, such as diesel or petrol.

This is carried out in three ways:
  • Coal is heated in an oxygen-free atmosphere to distill off coal tars and oils (also known as pyrolysis).
  • Coal is treated at high temperature and high pressure in the presence of solvents, hydrogen gas, and catalysts to hydrogenate the coal and form hydrocarbons. 
  • Coal is treated in a three-stage process, which first involves its high-pressure reaction with steam to form syngas – a mixture of carbon monoxide (CO) and hydrogen (H2). Syngas is treated at high temperatures in the presence of a catalyst to form hydrocarbons, which then need to be refined and separated to produce individual hydrocarbon products.

This series of steps will usually convert one tonne of coal into one to two barrels of hydrocarbon fuel. If we consider that the heat content of one tonne of coal is ~24 000 megajoules (MJ) and that of one barrel of hydrocarbon fuel is 6120 MJ, you can see that the process is not very energy efficient: only ~26% of the energy in coal is converted and distilled into one barrel of fuel. That being said, the convenience of liquid fuels cannot be overstated – our transportation system is now dependent on liquid fuels like petrol and diesel and not, as at the start of the industrial age, on solid fuels like coal.

All these CTL options are technically complicated, capital-intensive, highly polluting operations with high operating costs. They involve sophisticated and large-scale high-pressure, high-temperature chemical processing equipment. Depending on their size, location, and coal supply, these operations only tend to be profitable in a world where crude oil costs more than $50 - $110 a barrel. The price of crude oil is presently around $40/barrel. As a result, there are a limited number of these operations in the world. The largest is the Sasol complex in Secunda, South Africa. There are some operations in China and several smaller operations scattered around the world. To give some sense of the polluting nature of these operations, Sasol’s Secunda operation, which produces ~150 000 barrels per day of synfuels, is the largest point source for CO2 emissions on the planet!

Implementing the technology successfully involves requires large numbers of skilled, well-trained, and experienced personnel, and—in the case of Sasol—years and years of government subsidies. In a country like Botswana, there are insufficient trained personnel for such an operation and there will be a heavy reliance on expatriate personnel at the start. The water demand for these operations is also substantial; in Botswana, reliable, high-volume water supply is presently a difficult resource to come by. Moreover, these operations are very energy-intensive and require a reliable high-voltage electric grid. Reliable power supply can be difficult to achieve in Botswana at this time, so the incorporation of a 300 MW coal-fired power plant into the operation appears to be a wise, albeit expensive, way to guarantee reliable power.

Although the promises of local infrastructure, industrial development and employment opportunities are alluring, the scale of this project, as well as the demand for energy, water, and personnel resources required for a project of this size, suggests that caution and extensive scrutiny is warranted.

One of the axioms of the energy business is: It takes energy to get energy. We have to expend energy drilling into oil deposits, pumping, refining, and transporting the oil before we can use it as petrol to power our cars; we have to expend energy mining, processing, and transporting coal before we can burn it; even in the renewable energy field, where we have free fuel sources, such as sunlight, we have to expend energy creating the components of solar panels, transporting, and installing them. Every energy source requires an investment of a certain amount of energy to recover energy that we can use somewhere else. The trick is to be sure that the energy invested is less than the energy produced. This concept called EROI – energy return on investment (also referred to as EROEI – energy recovered on energy invested). The formula for EROI is straightforward:

EROI = Energy recovered/Energy Invested = Energy output/Energy input

One always wants the EROI to be greater than 1, i.e., one wants to recover more output energy than input energy. There are many ways to calculate the inputs and outputs, but the most rigorous way is to undertake a life-cycle analysis and compare all the inputs and outputs over the whole life of the project. The range of EROI values in the energy field is wide: for hydropower, it is of the order of 80; for ethanol from biomass, it is of the order of 5. The figure below provides ranges of EROI for some important energy sources. It is interesting to observe the wide range. The reason I have raised this concept is that questions have been raised regarding the EROI of coal to liquids technology. As I noted above, this is an energy- (and water-) intensive technology and there are concerns that the input energy might be greater than the output energy. There may well be other compelling reasons to consider CTL processes, such as job creation, access to and the convenience of liquid fuels – it is difficult to fill up a car with coal – but, nevertheless, the EROI is important factor to consider, particularly in a country like Botswana that is already energy-challenged.

Coal-Bed Methane (CBM)

A number of CBM methane projects have been proposed for Botswana. Methane is the simplest hydrocarbon. It is a gas that contains one carbon and four hydrogen atoms and it is a very useful chemical. It can be used as a starting block to build longer chain hydrocarbons, such as the molecules that are found in fuels like petrol and diesel, and it can be burned directly to generate electricity. One of the great advantages of methane, also called natural gas, is that natural gas-fired power plants have higher conversion efficiencies than coal plants and are far less polluting. Coal-fired plants typically have conversion efficiencies of 30% to 35%, whereas modern natural gas-fired plants have efficiencies of 50% or higher. On a per kilowatt hour of electricity basis, coal plants tend to generate 40 to 50% more carbon dioxide emissions than natural gas operations. As a result, methane is a highly desirable fuel: the large volumes of natural gas that have been released from shale deposits in the USA via fracking technology have had a profound impact on US energy markets and a depressing effect on their coal mines and coal-generated electricity industries.

Methane is found in underground deposits, such as underground reservoirs and gas-rich shales. It is also often a byproduct of crude oil production as well. Methane is recovered by drilling down into these deposits and pumping out the gas. In shale deposits, horizontal drilling is utilized, followed by fracturing the deposit with high-pressure water in a process known as “fracking” to create a web of small channels that release the natural gas locked up in the shale.

Methane is also found associated with coal deposits, where it is a byproduct of the process that led to the formation of coal. In fact, methane is often the cause for the terrible explosions that occur in coal mines that can lead to the deaths of many miners.

Coal-bed methane technology involves drilling holes into coal deposits and then pumping out water to lower the pressure. This creates the conditions that lead to release of methane which can then be pumped out of the coal bed. However, the methane produced by this process often needs to cleaned up to remove water, carbon dioxide, and nitrogen, which reduce the heating value of natural gas. Once methane is treated, it then needs to be transported to its point of utilization. This requires a pipeline network with compressor stations every 80 to 100 km. Pipelines are expensive, they need to run parallel to an electric grid to power the compressor stations, they run across peoples’ property, they require maintenance to run, and are vulnerable to damage. That is why it might be best to build a generating plant close to the source of the coal-bed methane, provided the source of methane is large enough to support such an investment.

The figure above shows the basic operation of a CBM operation. One of its drawbacks is the water issue. Water has to be pumped out from the coal bed to create the low-pressure conditions that lead to the release of methane. The removal of large amounts of water from a coal deposit can result in the lowering of water tables in local underground reservoirs if there is a linkage. At first one might think that water is not necessary a problematic byproduct, especially in an arid country like Botswana, but the snag is that the water is often saline, which leads to disposal challenges.

CBM is an interesting technology and might have applicability in Botswana, however the transportation of gas across hundreds of kilometers of non-electrified areas and the water disposal issue need to be carefully considered.

Underground Coal Gasification (UCG)

Another way to exploit coal resources without direct mining involves a combination of the CBM and CTL approaches, known as underground coal gasification and there has been some interest in applying this technology in Botswana. UCG involves the direct controlled burning of coal in an underground deposit and then using the generated heat and injected air, oxygen, and steam to generate syngas (that mixture of carbon monoxide and hydrogen gas referred to in CTL technology). The syngas is then drawn to the surface where it is dried, cleaned, and then can be combusted to turn a gas turbine to generate electricity or it can be transported in pipelines elsewhere where it can be used to produce liquid hydrocarbon fuels.

An underground gasification operation usually involves the drilling of two wells into an underground coal deposit. One is the injection well where air, oxygen, and or steam are injected into the burning coal bed and the other is the production well where the produced syngas is drawn out from underground. The key is obtaining a linkage between the two wells so that heat, steam, and the syngas produced in-situ can move from one well to another. Once the coal bed is ignited, the extent of combustion is controlled by air or oxygen injection. In the process, the combustion zone grows and moves towards the production well.

One of the attractive features of the UG process is that the coal ash is left underground and this removes a disposal problem associated with aboveground coal gasification. The process also allows exploitation of low-value and difficult-to-mine coal deposits and, because no direct mining is involved, it is far more energy efficient than aboveground gasification. Compared with traditional coal mining and CTL technology, the capital and operating costs are lower and the water consumption is relatively low. Because no mining is undertaken, the surface disturbances are much less and there are no open shafts or depleted mine works to deal with once the coal has been mined. The emissions from UCG, such as carbon dioxide, metals, particulates, sulfur, etc., are reported to be less as well.

However, and this is a big however, it needs to be understood that UCG involves setting fire to underground coal deposits. There is a possibility of runaway or poorly controlled combustion that could result in disastrous consequences, such as uncontrolled emissions of carbon dioxide, carbon monoxide, methane, and other hydrocarbon gases, contamination of local water tables, and uncontrolled subsidence into the burnt-out chambers. The very high temperatures of combustion could also affect the local geology, with the formation of cracks and fissures that result in uncontrolled flow of water and emissions into areas where control is not possible. There are thousands of uncontrolled coal seam fires presently burning in countries across the planet: just across the border in South Africa in the coal mining area of eMalahleni, abandoned coal mines have been burning for years. In the US, the town of Centralia in Pennsylvania had to be abandoned in 1984 due to an uncontrolled underground coal fire that made living and safety conditions unbearable. Once they start, these fires are difficult to reach and almost impossible to extinguish.

Another disadvantage associated with UCG is that produced syngas has to be transported to its point of use so a surface pipeline network has to be built to transport it. Additional complications are introduced by the nature of the syngas produced from UCG: it contains a great deal of nitrogen from the air used in burning and therefore has lower heating value than syngas produced above ground. Depending on the final application of the syngas, i.e., direct burning or hydrocarbon production, the nitrogen might have to be removed.

UCG is an intriguing and attractive concept and numerous pilot-scale tests have been carried out around the planet with mixed success: despite the promise of UCG, it still has to take off. Problems of uncontrolled gas emissions, pollution of farmland, and acidification of water supplies have occurred. The longest running underground gasification plant is the Linc Energy Yerostigaz plant in Uzbekistan. This has been running since 1961 and produces 1 million cubic meters per day of syngas that is used to power a nearby power station. There have been recent reports of commercial-scale UCG operations in China.

This post has covered a lot of ground from coal-to-liquids technology, to coal-bed methane, to underground coal gasification. In all cases, these operations appear to provide some advantages over traditional coal mining, but their complexity, costs, and potential environmental impacts create challenges that could be difficult for a resource-constrained country like Botswana to deal with. Some of these approaches have promise, but, in every case, require caution and extensive scrutiny so that we clearly understand and appreciate the tradeoffs between energy production and energy input, water, expertise required, and long-term environmental impact. These tradeoffs can be difficult and, at times, unpopular to consider in the face of demands for natural resource utilization, industrial development, and job creation, but prudent, unbiased, and in-depth evaluation of these interesting approaches is warranted.

Until next time, remember to turn the lights out when you leave the room so that less coal has to be burnt.

Tsamayang Sentle**
Mike Mooiman

(*Greetings in Setswana)

(**Go well or Goodbye in Setswana)