Friday, October 23, 2015

Electricity Production in Botswana

Dumelang*. My post this week is part informative and part instructional. When we discuss energy issues, a couple of key concepts come up time after time and, to be a contributor to an energy discussion, we have to know, or familiarize ourselves with, some technology and terminology. In this post, I want to explain two fundamental energy concepts. The first is the difference between energy and power, and the second is capacity factor. I will then show how they can be applied to electricity production in Botswana. 

Let's start with the difference between energy and power. These terms are often used interchangeably. This is okay in a general conversation, but in an energy-related discussion, it can lead to confusion, misunderstanding, errors and bad decisions. It is essential to be specific about which term you are discussing, so let’s take a look at distinguishing between the two.

The standard scientific definition is that Energy is the ability of a system to do work. It is the quantity which we need to get something to move, heat up, light up, burn, explode, etc. Energy is not just one thing, however – it comes in different forms, for example, electrical energy, chemical energy, nuclear energy, kinetic energy, etc.: much of energy technology deals with converting one form of energy to another in the most efficient manner. For example, converting the chemical energy in petrol into the kinetic energy of a moving car.  Some of the more common units of measurement for energy are kilowatt hours (kWh), megawatt hours (MWh), megajoules (MJ), and terajoules (TJ).

Power, on the other hand, is the ratio of energy per unit of time or the rate at which energy is produced from a fuel source or is converted from one energy type into another. Units of measure for power include kilowatts (kW), megawatts (MW), joules/second or horsepower (HP).

The confusion between these two often stems from the similarity of the units like kilowatt hours (which is an energy unit), and kilowatts (which is a power unit). However, it is necessary to understand that, even though the units seem similar, there is a world of difference between them. This difference stems from the simple mathematical relationship between energy and power:

Energy = Power x time.

One my students in the Energy and Sustainability program at Franklin Pierce University  noted that energy and power are analogous to distance and speed. Energy, like distance, is a quantity, whereas power is a rate like speed. Like the relationship between energy and power, the relation between distance and speed is written as:

Distance = Speed x time.

Let's consider a simple backup generator that I have been looking at in the Game Store in Gaborone. 

This unit is rated at 5500 Watts or 5.5 kilowatts (kW) under long-term running conditions, so the power of the unit is 5.5 kW. If I were to run this unit for 1 hour, I would produce:

5.5 kilowatts (kW) x 1 hour = 5.5 kilowatt hours (kWh)

of electrical energy that I could use to run my home. Running it for 24 hours would produce 5.5 kW x 24 h = 132 kWh of electrical energy. The power rating of 5.5 kW is a measure of the rate at which the backup generator can take the chemical energy in the diesel fuel and convert it to electrical energy that I can use to keep my home running during load shedding. The larger the motor on the generator, i.e., the greater the power, the faster is the rate of energy conversion.

Let’s take a look at another example. In a car, we convert the chemical energy in petrol into forward kinetic motion to get us from point A to B. Again, the greater the power of the engine, the faster will be the rate of energy conversion. The pictures below illustrate this point.

The Mercedes S500 sedan has a high-powered 5 liter, 302 HP motor that can more rapidly convert the energy in the petrol tank into forward kinetic motion than my rental Toyota with its  1.5 liter, 89 HP motor. These two automobile engines, under specific circumstances, can produce the same amount of energy, however, the Mercedes can do so in substantially less time. It is likely that the Mercedes will do so a lot less efficiently than the Toyota—but with a whole lot more fun.
Let's go back to the Ryobi generator unit so that we can discuss the second fundamental concept for this post – capacity factor. If I could run the generator solidly for 24 hours a day for an entire year, I theoretically could produce:

5.5 kW x 24 h/day x 365 day/year = 48 180 kW of electrical energy.

However, if I were to use the generator only for 1 day per month during the year, say during a load shedding, I would produce:

5.5 kW x 24 h/day x 12 days = 924 kWh of electrical energy.

Dividing actual produced energy by the maximum that theoretically could have been generated in a 24/365 scenario produces a ratio called the capacity factor. In my example above, we would divide 924 by 48 180 to produce a figure of 0.019, which converts to a percentage of 1.9%: this would be the capacity factor of my generator for that year. In other words, my generator only ran at 1.9% of its maximum potential output. Students in the energy field often confuse capacity factor with conversion efficiency and it is important to appreciate that they are very different concepts. The 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. On the other hand, conversion efficiency is a measure of the effectiveness of the conversion of one form of energy, say that in coal, to another form of energy, for example, electricity. We will be taking a look at conversion efficiencies in a future post.

With these basic terms—energy, power and capacity factor—under our belts, let's turn back Botswana energy issues and particularly electricity generation.

I have examined the 2013 electricity generation figures for Botswana that were published in the Botswana Power Corporation (BPC) 2013 Annual Report and have combined, in one table, the  generating units, their combined power, the energy produced from these units, the  calculated capacity factors, and the overall capacity factor for the combined generators in Botswana.

In 2013, there were five energy-generating operations, including the new (and trouble-prone) generators at Moropule B, the aging and largely shutdown Moropule A operations, two large-scale diesel generating operations, one near Orapa and the other near Francistown, and the 1.3 MW solar plant in Phakalane. The combined nameplate capacity of the generating units was 743 MW and they generated just over 877 000 MWh of electrical energy for an overall capacity factor of 13%. (Note that there are 1000 kilowatts in a megawatt, similarly there are 1000 kilowatt hours in a megawatt hour.)

On examining the capacity factors, it is interesting to note how far they are from 100%. The only way a generating device can run at a capacity factor of 100% is by running 24 hours 365 days a year—which is simply not practical or realistic. Equipment breaks down and has to be repaired or has to be shut down for maintenance. Moreover, operators of power plants make operating choices, based on the cost of coal and diesel compared with that of imported electricity, as well as demand to throttle back their units from their rated or name plate capability. This reduces the amount of electricity produced which, in turn, reduces the capacity factor. However, the low capacities of the coal-fired units are of concern. Typically, coal-fired plants have capacity factors that range from 50 to 80%, depending on location and demand. Low capacity factors for coal-fired plants result from either low demand or operational issues. In the case or Moropule B, the problems that have plagued the startup of new generators have been extensively reported on in the media.

Generating electricity using diesel is an expensive proposition and therefore the low capacity factors of diesel plants are not surprising. These units are seldom used and they function as back-up generators and are only used in an emergency. In many respects, they are just like the Ryobi generator I presently have my eye on.

Even though the Botswana-based generating units are operated separately with different technical and economic considerations, it is useful to consider their aggregated capacity. As noted above, the combined nameplate capacity of the generating units is 743 MW. This combined capacity in a single unit would be one mammoth-sized generator – we could call it the "Botswana Megarac 743" – which is almost a 140,000 times larger than the Ryobi unit I am eyeing at the Game Store in Gaborone. Looking at this another way, if Botswana bought 140 000 of these units, it would match the country’s present generating capacity.

Based on 2013 financial year data, this Botswana Megarac 743 was operated at a capacity factor 0.13 which means the combined Botswana facilities only generated 13% of the electricity that was theoretically possible. Over time, it is hoped that the start-up problems at Moropule B will be solved and the overall capacity factor for Botswana’s generating facilities will increase. However, we need to keep in mind that practical considerations, such as cost and availability of imported electricity, will also have to be factored in and that Botswana electricity generation is not an island unto itself. Botswana draws electricity from the Southern African Power Pool (SAPP) power generation and transmission system which coordinates electricity supply and demand throughout Southern Africa.  The SAPP system has a combined capacity of about 55 000 MW of electrical generating capacity.

Hopefully, this has been an informative and instructional post and you now know the difference between energy and power and you have an appreciation for capacity factors. As you can see, capacity utilization of generating facilities is low in Botswana: hopefully this will improve in the future, but it is crucial to appreciate that not all of this capacity can be tapped at any one time. Running these generators depends on complex issues, which include demand, cost and availability of fuel, maintenance shutdowns and financial considerations.

In the meantime, if you see me in the parking lot at the Game Store in Gaborone trying to load up that Ryobi generator into my rental Toyota, stop and give me a hand. Until next time, remember to turn off those lights when you leave the room.

Tsamayang Sentle**
Mike Mooiman

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

1 comment:

  1. Thanks Mike, well written post. Very interested in diesel capacity factors in Africa.


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