Wednesday, November 4, 2015

History of Electrical Energy Generation in Botswana

Dumelang*. In this post, I take a look at historical trends to see how electricity generation in Botswana has changed over time. I noted in an earlier post about Sankey diagrams that Botswana generated only 7% of its electricity needs in 2012. This seemed an extraordinarily low number, so, in my research for this post, I also took a close look at the source data and uncovered some interesting discrepancies.

Utility-scale electricity generation in Botswana began in 1970 with the commissioning of a small oil-fired station in Gaborone. This provided for the needs of the capital and surrounding areas until its decommissioning in 1989. In 1974 a 65 MW coal-fired operation was built in Selebi-Pikwe to service the mining industry. 1985 saw the start-up of a large coal-fired plant at Morupule near Palapye and the shutdown of the Selebi-Pikwe plant a few years later in 1989.  The Morupule operation consisted of four 33 MW air-cooled units, providing a total generation capacity of 132 MW. This operation, known as the Morupule A plant, served Botswana’s needs well for a time. However, with increasing population, electrification of the country, and industrialization, demand rose and increasing quantities of electricity had to be imported from neighboring countries. The construction of the Morupule B coal-fired plant, consisting of four 150 MW air-cooled units, commenced in 2009. The deadline for completion was 2012, but many startup and operational problems have been encountered and today—three years past the scheduled completion date—the new plant has not lived up to its promise. There have been frequent breakdowns and, as a result, a great deal of Botswana’s electricity is still sourced from the Southern African Power Pool (SAPP).

Location of Morupule power plant near Palapye

In the figure below, I have plotted two sets of data for the past 18 years. The orange line shows the percent of electricity generated in-country by the Botswana Power Corporation (BPC), largely from their Morupule operations. The blue bars show the total annual amount of electricity (generated locally and imported) supplied in gigawatt hours (GWh). Data for 2015 are for the first three quarters only.

Data Source: 1997 to 2004; Adjusted BPC annual report results. 2004-2015; Statistics Botswana

An examination of the data shows that, from 1997 to 2008, the amount of electricity supplied steadily increased, with a compounded annual growth rate of 5.7%. During this same period, the proportion of local generation, which was as high as 60% in the late 1990s, decreased. This is to be expected from a fixed source of local generation in the face of increasing demand. However, the decline in local generation accelerated because of operational problems at the aging Morupule A power station. In 2011, an extreme situation was reached, when only 9% of the electricity supply was locally generated.

The difference in supply was made up of imports from the SAPP. However, since late 2008, supply shortages in the entire SAPP region have become acute and electricity imports to Botswana were reduced. As a result, rolling blackouts, also known locally as load shedding, were introduced across the country to curtail demand. Since then, the annual electricity supply has been up and down and there have been some years (specifically 2009, 2011, and 2013) in which energy supply was actually lower than the previous year. Load shedding, combined with the introduction of demand reduction initiatives, prepaid electricity, smart metering, and hot water load control, has slowed the rate of electricity supply growth since 2008 to a compounded rate of ~3% per year.

Since 2012, with the startup of the Morupule B operation, the local supply situation has improved and there have been year-on-year increases in electricity output from this operation. In 2014, Botswana again generated 60% of its electricity demand, significantly reducing the amount of electricity that it needed to import. However, the first quarter numbers for 2015 are disappointing because they indicate a similar level to that of 2014. It was anticipated that this value would be higher, but the performance of the Morupule B plant has been problematic, with only 300 MW of the specified 600 MW generation capacity currently available. We are hopeful for an improvement by the end of the year.

(In my post, The Big Picture, I noted that Botswana only generated 7% of its electricity needs in 2012. However, the historical data in the figure above indicate that this low actually occurred in 2011. My earlier statement was based on data from the International Energy Agency, IEA. A careful analysis of the IEA data led me to conclude that they were using information from the BPC annual reports. Because the BPC financial year ends in March, use of their data requires adjustments to take into account that their annual numbers actually incorporate the last three quarters of the previous year’s data. As a result, IEA reports of low levels of energy generation in 2012 actually correspond largely to the previous year, 2011.)

In my last post, I noted that energy students sometimes confuse capacity factor with conversion efficiency. The concepts are quite different: capacity factor is a measure of how much of the theoretical capacity of an energy-generating device was utilized over a time period (typically one year), whereas conversion efficiency is a measure of the effectiveness of the conversion of one form of energy to another, such as the conversion of energy in coal to electricity. To get a sense of conversion efficiencies, let’s take a closer look at the Morupule coal-fired plant.

According to the data provided by the IEA, the Morupule operation burnt 169 kilotons (kt) of coal and generated 250 GWh of electricity in 2012. To calculate conversion efficiency, we need to compare the input energy in coal and output energy as electricity in the same energy units by converting the energy in coal to GWh equivalents. If we assume that the Morupule plant burns coal with a heat content of 24 MJ/kg (a typical value for Botswana coal), we can calculate that 169 kt of coal contains 7.1 billion MJ of energy. Considering that there are 3.6 MJ in a kWh, we can calculate that the energy input was equivalent to 1126 GWh, which generated 250 GWh of electricity.

With this common set of units, we can now calculate that the conversion efficiency is Input/Output x100 = 250/1126 x 100 = 22%.

In fossil fuel plants, a different measure of conversion efficiency is often applied. This is heat rate, which is the amount of input energy (usually measured in kilojoules (kJ)) that is needed to produce one kilowatt hour (kWh) of electricity. One kWh of energy is equivalent to 3600 kJ, so if a fossil fuel plant is 100% efficient, it would have a heat rate of 3600 kJ/kWh, i.e., the plant would take 3600 KJ or 1 kWh of coal-based energy and convert it into 1 kWh of electricity. A plant operating with 50% efficiency would have double this heat rate or 7200 kJ/kWh (= 3200/0.5). At 22% conversion efficiency, the heat rate would be 16,363 kJ/kWh (= 3200/0.22).

Compared with other coal-fired plants around the world, which have conversion efficiencies of 30 to 40%, this value of 22% seemed extraordinarily low. To understand this discrepancy, I undertook a careful review of the IEA data and determined that they used a standard conversion efficiency of 22% every year to back calculate the amount of coal burnt in producing electricity in Botswana, i.e., their coal consumption numbers are not based on actual coal consumption data! The correct way to do this calculation would be to use the actual coal consumption numbers for the Morupule coal-fired operations. Unfortunately, I have not yet been able to source this data.

Further research led to some old 2006 data from SAPP which indicates that conversion efficiencies for the old Morupule A plant were actually more of the order of 30%. Additional evidence for higher conversion efficiencies was provided by data from the World Bank, which indicate that the heat rates at the Morupule plant were 11,621 kJ/kWh. This is equivalent to an efficiency of 31% (= 3600/11,621 x 100). As a result of this analysis, I am forced to conclude the IEA numbers for coal consumption in Botswana have a high bias.

A conversion efficiency of 30% (although significantly better than a value of 22%) still means that only 30% of the energy in coal ends up as useful electricity. The remaining 70% is lost due to process inefficiencies and heat losses.

To understand why this occurs, we need a better understanding of how the Moropule coal plant works. Coal is burned and the heat produced is used to boil water to generate steam. The steam is used to drive a turbine which then drives an electrical generator. In the process, we have the conversion of the chemical energy in coal, to thermal energy in the steam, to the kinetic energy of the turbine, to the electrical energy leaving the generator. In this process, there are heat losses in the hot off-gases that leave the combustion chamber and then exit those tall stacks at the Morupule operation. Another problem with a steam plant is that steam can only be used once to turn the turbine: it then needs to be condensed into water so that the water can be again be boiled to generate steam. The Morupule plants are air-cooled so the energy that was in the hot steam is lost to the atmosphere during the cooling process. The challenge with air cooling is that this is an energy-intensive operation itself, because large air fans need to be powered to drive air past heat exchangers. A significant portion of the energy generated by an air-cooled plant, typically 10%, is used to run the cooling units, which reduces the amount available for distribution. 

There are certainly coal-fired steam plants that have higher conversion efficiencies. Many modern coal-fired power plants have conversion efficiencies greater than 35% and water-cooled plants, such as those located near rivers or oceans, have even higher efficiencies. The world’s most efficient coal plants have conversion efficiencies as high as 47%. One hopes that, as the operational problems are solved at the Morupule plant, attention will be focused on improving its conversion efficiency to above 30%.

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

Tsamayang Sentle**

Mike Mooiman

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(*Greetings in Setswana)
(**Go well or Goodbye in Setswana)

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