Monday, June 20, 2016

Diesel vs. Solar Generated Electricity in Botswana

Dumela*. As I have been travelling around Botswana visiting solar installations and chatting to solar system owners, I have encountered a lot of solar photovoltaic (PV) systems that have a diesel generator as back-up and so I have been fielding—and asking—a lot of questions about diesel versus solar. Most of my research has focused on solar systems with battery storage, but I decided that it was worth learning a little more about diesel generators: when and how they might be best combined with solar systems. In this blog, I share with you some of what I have learned.

Diesel (or petrol) generators have been around for a long time. They are robust and reliable, and there are numerous units from various manufacturers available in different sizes for different applications. They vary from small emergency generator units, which have a capacity of 2 to 5 kW, that you can purchase in a large supermarket along with your building supplies or groceries, to large units that serve as a back-up electricity supply for lodges, to even larger units that can power mining operations.

Diesel generators can even be used to add on-demand generating capacity to the electrical grid. For example, the Botswana Power Corporation (BPC)-owned 90 MW back-up generator operation in Orapa consists of two 45 MW GE LM 6000 turbine/generator units. Based on their specification data, when they are both running, these units can consume up to 22 000 liters of diesel per hour – that is almost one diesel tanker truck of fuel every sixty minutes! With diesel presently priced at about P 7.30/liter (US$2.52/gal), we can calculate that the Orapa station is, at capacity and just in terms of its fuel costs, generating electricity for ~P 1.78/kWh.

When considering off-grid electricity systems, the decision is usually made between selecting solar or a generator. In many respects, a diesel generator seems like a straightforward solution. The upfront costs are lower than those of a solar system; a generator is not dependent on sunlight and weather conditions; it does not need battery storage, and it is fairly easy to find technical assistance when the generator is not working. A small generator can even be loaded onto the back of a truck and transported to the repair shop.

On the down side, diesel generators are noisy, low-efficiency, highly polluting devices that require the transportation and storage of diesel. Typically, only about 25 to 37% of the energy in diesel is converted to electricity: the rest is lost as heat from the engine block and the hot exhaust gases.

Another complicating aspect of diesel generator operation is that generators burn a minimum amount of fuel, even at very low electrical demands. As Figure A shows, for a range of generator sizes, fuel consumption increases as the electrical load increases; however, at very low electricity demand, the consumption levels off. As a result, generators can be horribly inefficient (<15%) and wasteful devices at low loads. This escalates the cost of producing electricity at low demand levels, as shown in Figure B. If you need to turn on just a single light, you will need to fire up the generator–which will consume a minimum amount of fuel. It is for this reason that generators are often paired with some battery storage that can take care of minimum load requirements. Also bear in mind that these cost data were calculated using a relatively low diesel cost of P 7.30/liter: as recently as 2014, diesel was selling for over P 10/liter (see Figure C below).

Data Source: Able Sales
Data: World Bank and OANDA

Figure B indicates that the fuel costs of generating electricity from diesel generators is of the order of P 2.00 to P 2.20/kWh at demand levels above 50%, but this is only part of the story. To get a sense of the overall true costs of diesel generation, we need to include the purchase and maintenance costs of the generator.

To calculate the all-in lifecycle costs, we need to calculate the levelized cost of energy or LCOE. This parameter takes into account the initial cost of the unit, its electrical output, the annual cost of fuel, annual maintenance, and an interest cost (normally set by the minimum rate of return one would like to earn on a project and is often equivalent to the bank loan interest rate to purchase the equipment). These costs are then distributed evenly over the life of the project to yield a single “levelized” cost for energy. The attraction of the LCOE approach is that it allows a side-by-side comparison of different projects and energy sources that have very different financial requirements and expense flows. For example, it allows comparison of a solar system with high capital but low operating cost with a diesel generator project that has low capital but high operating costs. LCOE provides a means for determining which project is better from a lifecycle energy cost point of view.

Calculating the LCOE from scratch can be challenging as it is a rather involved financial calculation, but there is a great site, from the National Renewable Energy Laboratory in the US,  that can do the calculations for you. I used this to calculate the LCOE for a diesel generator. My set of assumptions included the following:

Capital cost: P 5000/kW ($ 450/kW @ P 11/US$)
Capacity factor: 90% (Generator runs most of the time)
Project lifetime: 10 years
Interest rates: 10% (Prime (6%) + 4%)
Diesel consumption rate: 0.27 L/kWh
Costs for diesel: P 7.30/L ($ 2.41/gal, June 2016)
Cost per kWh: P 1.71/kWh ($ 0.17/kWh)
Annual maintenance costs: P 330/kW ($ 30/kW)

With these assumptions, the calculator yielded the results in the figure above, indicating that the LCOE for a diesel generator is of the order of $ 0.183/kWh or P 2..01/kWh. I then computed the variability of the LCOE with high and low levels of some of the assumptions and generated the chart below. 

The data indicate that the cost of energy from diesel is most sensitive to the cost of diesel. Large increases in diesel prices lead to much higher costs for generated electricity; big increases in the cost of the generator do not significantly impact the electricity cost. The chart shows, within this span of assumptions, that the range of LCOE for diesel generators varies from P 1.86 to P 3.38/kWh, with a median value of P 2.62/kWh. To put these values into context, it should be noted that the average resident in Botswana, using more than 200 kWh/month, is paying P 0.88 per kWh (US$ 0.08/kWh @ P 11/US$) for electricity from the grid. In other words, within our chosen range of assumptions, the median cost of diesel-generated electricity is three times that of electricity purchased directly from BPC. So, if you have a grid connection and you install a diesel generator, do not consider the generator as a substitute for BPC: it is there to be used only when electricity is not available during load shedding.

Returning to the central theme of this blog (that of solar vs. diesel), let’s have a look at the LCOE for an off-grid solar installation with battery storage. The assumptions that I used for this calculation are as follows:

Capital cost: P 60 050/kW ($ 5500/kW @ P 11/US$)
Capacity factor: 90% (With battery storage, the system operates most of the time, like a diesel system)
Project lifetime: 10 years
Interest rate: 10%
Cost for fuel: Free!
Annual maintenance costs: P 55/kW ($ 5/kW)

I used local prices for the installed cost of a solar system with battery storage that I obtained from vendors here in Botswana. I also chose 10 years as a project lifetime to allow a direct comparison with the diesel project and because this is the typical lifetime of batteries under ideal conditions.  However, I will note that battery lifetime in Botswana can be a lot shorter as a consequence of the impact of high ambient temperatures.

Using the above assumptions, the LCOE for solar power calculates at P 1.25/kWh. This is still higher than the cost of grid-supplied electricity from BPC, but is almost half the cost of diesel-generated electricity.  The chart below shows how the LCOE changes with capital costs: as expected the costs for solar generated electricity are highly sensitive to the upfront costs.

LCOE comparisons for diesel vs. solar with battery storage are summarized in the table below.

It should be noted that I have included some pretty significant assumptions in my calculations, but the key points to note from these data are that (i) the capital cost of solar plus battery storage is 12 times that of a diesel generator with the equivalent capacity; (ii) the lifetime costs of electricity for diesel generators are twice that of solar. These options are both more expensive than electricity from the grid in Botswana.

When considering diesel vs. solar, the decision comes down to one of high upfront costs and low long-term operating costs compared with low upfront costs and high operating costs. It is clear from the financial data that solar, even with the costs of battery storage included, is the better option. The biggest obstacle to investing in solar is finding that substantially larger amount of money for the one-time installation of the solar and battery storage system.

However, (as these matters often are) it is sometimes not a black or white decision. Sometimes a hybrid solution, using a combination of solar and diesel, is the correct one. I noted in an earlier blog the disappointment many purchasers of solar have experienced due to poor battery and system performance. As I read and learn more, I am becoming convinced that the correct solution for off-grid systems is not diesel or PV: it is a hybrid of both. A long run of overcast days can really compromise the electricity output of a PV/battery storage system, so installing diesel generators alongside solar systems in a hybrid configuration can compensate for these weather shortcomings.

There are several other reasons why hybrid systems are a good choice. These include:

  • Improved reliability – crucial for service-oriented businesses, such as tourist lodges, which cannot afford to be without electricity;
  • Reduced capital costs – some diesel-generation capacity reduces the size and cost of the battery bank and solar array;
  • Better for short-term high-demand applications, such as stoves, welding machines, hairdryers, etc.;
  • Reduced emissions compared with a generator-only solution;
  • Reduced exposure to future diesel price increases;
  • Modular solutions are possible. As PV and battery prices decrease, more modules and/or batteries can be installed, leading to reduced fossil fuel use;
  • More system design options become available, for example, PV and no storage with generator or PV plus battery storage and a generator;
  • A hybrid solution is a partially “green” solution, but still allows optimization of capital expenditure. It avoids overspending on capital costs or underspending and then suffering from very high fuel costs.

Hybrid systems combining solar and diesel generators are growing in importance.  A number of island communities use them and there are a growing number of mining companies powered by diesel generators and using millions of liters of diesel fuel per year to power their operations that are finding big savings by introducing some solar generation into their energy mix. If you are interested in exploring hybrid options, I encourage you to take to look at the Homer Energy website and their Homer modelling tool. This enables you to investigate a range of hybrid options, all the way from 100% diesel to 100% solar and calculate the LCOE for each option. In this way, you can determine the optimal combination of solar and diesel for your application.

In conclusion, hybrid options seem to offer the best of both worlds. A hybrid system should have upfront capital costs somewhere between those of a diesel-only solution and a solar/battery storage system; the LCOE should similarly lie between those of both options. In turn, the resulting system has a higher degree of reliability, less weather sensitivity, and permits the installation of more solar and battery storage as time and finances allow. However, a combined system will no doubt be more complicated to operate, so if you have any direct experience with hybrid systems and their performance in the field, please share it with us in the comment box below.

In the meantime, 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, March 15, 2016

Off-Grid Solar Power in Botswana

Dumelang*. My previous post looked at the limited number of grid-connected PV systems in Botswana. There appear to be only three of noteworthy size (>10 kW) and a small number of lower power residential systems. In this post, I turn my attention to off-grid systems, of which there are many more throughout Botswana, but let’s start by reminding ourselves about the differences between grid-connected and off-grid systems.

In grid-connected systems, the electricity flow is one of three types:
  1. Outwards only, whereby power generated by the solar operation is fed into the electrical grid to supplement the power produced by other power plants, e.g., the Phakalane solar farm;
  2. Inwards only, where the grid just serves as a backup and feeds electricity into the system when insufficient solar energy is being produced, e.g., the 34 kW system at Scales Associates in Broadhurst, or
  3. Bidirectional, whereby excess power, over and above that used locally, is fed into the grid during the day, but during the night power is drawn from the grid, e.g., the University of Botswana experimental project in Mokolodi Village.
In off-grid solar systems there is no connection to the electrical grid and these installations are typically found on homes, on farms, in villages, or at tourist lodges in remote areas. Because these systems cannot use the grid for back-up electricity supply, they usually incorporate batteries so that any excess energy, produced during the day, can be stored for use during evening hours. Off-grid installations are sometimes combined with other means of electricity generation, such as diesel generators, that can provide backup power during cloudy conditions or when the batteries are depleted. These are referred to as hybrid systems.

Understanding these off-grid systems requires knowledge of the four key electrical components of the solar system: the panels, batteries, inverters, and charge controllers. In an earlier blog I provided some details on the functions of these components. The figure below shows key components in the basic layout of an off-grid solar system.

As I drive around Botswana, I see off-grid systems almost everywhere I go. They range from small single-panel systems providing a small amount of power for few lights or mobile phone charging, to multi-panel systems powering a home/ farm or a borehole pump, to large systems powering tourist lodges in the Okavango Delta. My research at the Clean Energy Research Centre at the University of Botswana is focused on these off-grid systems and particularly the battery-storage component. I am visiting these systems, chatting to the owners about the performance of the units, taking measurements, and collecting data.

As I have noted before, there has been a big push for installing solar systems in Europe, USA, and parts of Asia. These installations range from residential installations to large grid-scale systems of the order of 500 MW. Most folks in the US and Europe who are installing solar systems on their homes or business are installing grid -connected systems. The driver for the rapid rollout in these locations is purely financial. Electricity in these places is expensive and attractive subsidies for renewable energy are available. Only a small fraction of these new solar installations are off-grid systems and they tend to belong to hardy self-sufficient folks looking to live “off the grid” or owners of holiday homes and cottages located in rural areas that are far from the grid.

In the developing world, and here in Africa, off-grid solar is a whole other matter. In an isolated community where a grid connection is not possible due to distance or the cost of connection, energy options are limited. Many people live without power and rely on wood for light and cooking, or sometimes they use kerosene and candles. If they have a generator, they have to deal with the transportation and expense of diesel to keep the unit running as well as the maintenance of the engine. Off-grid solar systems are often the only viable way to bring electricity to remote homes, communities, businesses, and tourist facilities. Once solar is installed the fuel, sunshine, is free and maintenance is fairly simple and a lot less than that involved than that required for a diesel generator.

On the surface, off-grid systems appear to offer many advantages:
  1. The obvious one is that they are not grid-connected: they thus avoid long waits to get a grid connection and save on grid connection fees, which can range from as little as P5000 (~US$450) for residences located closed to the grid to hundreds of thousands of pula for locations far away from the main grid.
  2. Once the investment is made, the sunlight is free, which can significantly reduce the daily operating costs of the installation, However, as I will point out in a future blog, even though this is often a compelling reason for solar, it is a simplistic assumption. The correct analysis is to consider the upfront costs as well as the operating costs or the total lifetime costs.
  3. Solar is “green” and a more sustainable energy option and avoids the noise and pollution of a diesel generator as well as the logistics of trucking and transporting large quantities of diesel into remote areas. In fact, for many tourist camps and lodges this becomes an important part of the branding and selling point for their operations. Visitors can visit remote camps and enjoy the peace and quiet of the African bush, and still have access to their electronic devices and other modern conveniences, such as gourmet coffee, hairdryers, hot water, and air conditioning, without a noisy diesel generator puttering away and polluting the environment. I, for one, have stayed at one of these lodges and when it becomes necessary for an operator to turn on the diesel generator late at night when the batteries are depleted, the racket, after the quiet of solar power, can be quite jarring.

On the other hand, off grid-solar systems present a variety of problems:
  1. The upfront capital costs are high.  Solar systems are expensive, especially when compared with an equivalent diesel system. Moreover, the need for battery storage can often double the investment cost.
  2. Excessive heat is a problem for solar systems. In a previous blog, I noted that solar panel performance decreases as temperature rises, so the high ambient temperatures and the high panel surface temperatures can significantly decrease the expected output and even the life of solar panels. Similarly, battery lifetimes and performance are very temperature-dependent. Batteries do not like high temperatures and their lifetimes are considerably shortened in Botswana’s hot conditions.
  3. The output of a solar system is very dependent on weather conditions. The ideal conditions are cool sunny days; day after day of cloudy weather will seriously compromise the functioning of a solar system. After the sun goes down, the performance of the system is dependent on the installed battery capacity and operation of the battery bank.
  4. For large loads, large numbers of solar panels and batteries are needed, which means that a large amounts of space for their installation are required. In a space-constrained operation, say an island-based tourist camp in the Okavango Delta, this could present a problem.
  5. Unlike a diesel generator, every solar system is a custom built and the performance of the installation is very reliant on the competence and experience of the solar contractor.
  6. The lifetime costs of solar, which include the upfront investment cost, maintenance costs, and battery replacement, can be high especially when compared with grid-supplied electricity.
I have visited a good number of off- grid solar systems in Botswana, ranging from single-panel systems to large multi-panel installations with massive battery storage and have learned a great deal about the performance of these installations in general. Here are some of my findings:
  1. There are a lot of off-grid solar of different scales and sizes scattered through Botswana. These range from 50 W single-panel systems used for cell phone charging and a few lights to 100 kW multi-panel systems at lodges in the Okavango Delta.
  2. The solar portion and electronics are, for the most part, robust and work well. However, panel degradation has been seen in some cases. I have seen panels with cracks in the glass which have allowed in moisture and I have seen panels with a distinct browning of their appearance. See the photos below. The browning is due to the degradation of the organic material (ethylene vinyl acetate) that is used to encapsulate the individual solar cells in a panel. This degradation is accelerated at high temperatures and by high solar irradiation – both of which are plentiful in Botswana.
  3. I am surprised at how many non-functioning systems I have found. Often times, systems have been donated to communities that benefit initially from the installations. This is then followed by years of a lack of attention and post-installation follow up. As a result, three to five years after installation, the systems are no longer working. In some respects, this is a typical development problem – well-intentioned donors donate systems which are installed and are just left to community to run. Insufficient thought is given to developing local expertise or training nor is any provision made for follow up.  Years later, when problems occur, there is nowhere for the community to turn to for repairs and, as a result, systems, which initially provided a benefit, sit idle.
  4. There is a lack of branding with solar systems and little public knowledge as to what to constitutes quality for these installations. Solar systems are built from a number of components and purchasers are reliant on the design expertise of their solar contractor and the equipment choices those contractors make. It is unlike buying a car where, if you pay a lot of money for a Mercedes, you can be sure you are getting the benefits of great German engineering, quality components, and great after-market service. With solar power, there is no single Mercedes brand – you are reliant on your vendor making the correct design and component choices for you. I will note that many solar vendors, concerned about their reputation, will make the correct choices; however, as with cars, watches, and many other consumer goods, you get what you pay for. Buying cheap components or trying to save money by installing fewer panels or batteries will result in an inadequate solar system.
  5. Load creep is a problem. Often a system is designed for a specific load, say on a farm, to run lights, a fridge, a TV, and a borehole pump. Over the years, more electrical devices are purchased, such as a second TV, more lights, a freezer, a printer, etc. As result, the electrical load grows and, after a number of years, the system is inadequately sized for the demands placed on it.
  6. Many folks, ranging from individuals, to businesses, to high-end tourist lodges, are to varying degrees disappointed in their installed solar systems. The solar systems initially appeared to provide an elegant, sustainable, quiet, non-polluting alternative to a diesel generator. Invariably, the disappointment is associated with the inadequacy and shorter-than-expected lifetimes of the battery-storage systems.
  7. The Achilles heel of just about every system I have visited are the batteries. The battery life is generally half, or even less in some cases, of that expected. This seems to be result of:
      • Poor system design and insufficient solar panel output;
      • Lack of battery maintenance;
      • High operating temperatures;
      • Greater draw on batteries than originally planned.

Many of these problems can be overcome by the correct design, the right components, and spending oddles of money for both, which bring us to the fundamental problem that plagues solar systems: high upfront costs. In my next blog, I will take a closer look at this problem and dig into a comparison of the lifetime costs of solar vs. diesel.

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)

Sunday, February 14, 2016

Grid-Connected Solar Energy Projects in Botswana

Dumelang*. In my previous post, I took a good hard look at why there is so little photovoltaic (PV) solar power generation in Botswana and shared with you some of the complexities involved in implementing large-scale solar here. Botswana does have an impressive solar resource, but its exploitation requires a great number of tradeoffs. As a result, there are a limited number of larger-scale solar systems in the country. In this post, I focus on the grid-connected operations. These are systems that feed power back into the grid or that use the electrical grid as a backup, i.e., the grid makes up for any shortfalls in energy generation by the PV unit. I will look at off-grid systems in my next post.

There are presently three large grid-connected systems in Botswana: a single large-scale 1300 kW solar farm in Phakalane to the north of Gaborone; a recently constructed, but not yet operational, 20 kW EU-funded University of Botswana research system installed in Mokolodi village, just south of Gaborone; and a 34 kW system, owned by Scales Associates and located in the Broadhurst area of Gaborone near the Western Bypass. There are a number of other small-scale installations with similar configurations, i.e. grid connected but just using the grid as backup,  that have been installed on residences in Gaborone and surroundings. This post takes a close look at the first two of these grid-connected projects.
The Phakalane 1300 kW Solar Power Plant

On the southern outskirts of Phakalane, some 15 km north of the capital, near an industrial area and partially surrounded by undeveloped land, sits the 1300 kW project (see Google Earth view below). The facility is located on 2 hectares and is surrounded by a sturdy fence supplemented with electric fencing and rolls of razor wire and there is a guard permanently on duty at the gate. An informative signboard provides information about the project and a diagram shows the main components.

Phakalane Solar Farm Information Board

The project was completed in 2012 and electricity generation started in August that same year. The projected was largely funded by P80 million  from the Japanese government and some contributions from Botswana Power Corporation (BPC) and the Government of Botswana. The total cost of the project was approximately P106 million (approx. US$13.4 million). Even at the time that the facility was built, these project costs were on the high side. Using an average 2012 exchange rate of P7.90/US$, the cost comes to $10.30/watt (W). Average costs for solar projects in other parts of the world were of the order of $3/W at this time. (In 2013 and 2014, utility-scale PV projects in neighboring South Africa were being installed for ~$3/W.)  Now, normally the first major project in any country is an expensive one because everything is new and more ground work has to be undertaken, but a price greater than three times international averages seems exceptional. This most likely contributes to the prevailing perception in Botswana that solar PV is expensive.

The system contains 5920 panels, each with a 220-W DC rating, which gives 1 300 000 W or 1300 kW overall rating. The panels are wired in strings of 16 panels connected in series to provide a peak voltage of 470 V DC. These strings are then combined in parallel with other similar strings and fed into combiner boxes and then into inverters. The electrical output of solar panels is always direct current (DC), but what we transmit over power lines and use in our homes is alternating current (AC), so the function of the inverters is to convert DC to AC. These inverters output 380 V AC, which is then ramped up to 11 000 V through a transformer so that it can be fed directly into the BPC power grid.

I am always impressed that an array of solar panels, each with an output of ~30 V DC can be combined, inverted, and transformed into a grid-compatible AC voltage of 11 000 V. For electric power engineers this is old hat, but I am always impressed. I have visited numerous power plants and am used to seeing large machines thundering away, devouring large quantities of fuel, and spitting out large volumes of waste heat, pollutants, and carbon dioxide to produce electricity. It is a therefore a pleasure to see a field of solar panels, each quietly converting sunlight to electricity and then using technology to configure wiring, inverters, and transformers so that the electricity can be boosted to a form that can be transmitted over the grid and used to power homes in other parts of the country.

In the energy field, one needs to be sure to understand what is meant by the rating of a power plant. Most power plants, say the 600 MW coal-fired operation at Moropule B, refer to their output of AC electricity. In this case, it is easy to calculate how much energy a power plant would generate over a certain time period. For example, if all the generators at Moropule B were running at their rated output, uninterrupted for 24 hours, the output would be:

600 MW x 24 hours = 14 400 MWh = 14 400 000 kWh (1000 kWh = 1MWh)

However, with solar PV operations, the rating is most often given as the combined DC output capacity of the panels under the standard irradiation condition of 1000 W/m2 at 25oC – the conditions known as one peak sun (see an earlier post for an explanation of irradiation and the peak sun hour concept). Although the amount of sunlight in Botswana is high relative to other parts of the world, the irradiation levels are only close to one peak sun at around noontime. A solar panel will therefore only produce its rated output for a short while around midday; the rest of the time, the irradiation is lower and the output is commensurately lower. The figure below shows recent irradiation levels for Gaborone, as measured by a weather station on the roof of the Faculty of Engineering and Technology building at the University of Botswana.

Because irradiation conditions change during the day, we determine the energy that can be potentially harvested in a 24-hour period by calculating the area under the irradiance curve—the yellow area in the figure above. This measure of energy 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 a day. The number of 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. A typical insolation value for the Gaborone area is 6.09 peak sun hours. 

Cloud cover significantly affects the performance of solar panels: the figure below shows the contrast between a day with no cloud cover, February 1,  a day with lots of intermittent cloud cover, February 7 and Jan 14, which was overcast for the whole day: even with just passing clouds, the drop off in irradiation levels is significant.

In addition to a limited number of hours of peak sun per day and cloud cover, there are losses through the electrical system: in the wiring; in the conversion of DC to AC in the inverters; and through the transformers. Losses in PV systems are typically of the order of 20 to 30%.

There are also performance losses due to temperature. Intuitively, one would expect hot sunny days to be ideal for solar power generation, but one aspect of PV technology not often appreciated is that electricity output of PV panels actually decreases as the temperature increases by approximately 0.5%/ oC. Solar panels are rated for a temperature of 25oC. Panels operating in the Gaborone area, where the temperature on a summer’s day can be 35oC, can expect a 5% decrease in performance. Furthermore, when the clouds roll over the skies during the rainy season, we can also expect a big decrease in electricity production.

The European Commission’s Institute for Energy and Transport (IET)  has developed a great online calculation tool for estimating average losses and outputs for PV systems. One enters the geographic location, the rated power of the installation, and panel orientation and some very useful information on average irradiation, system losses, and PV output are generated. Alongside is the information I generated for the Phakalane location using this calculator.

As can be seen, the average losses were calculated to be 27%, the average daily peak hours were 6.49, and the average output was determined to be 6150 kWh/day, which totals 2240 MWh AC per year. With average systems losses of 27%, the output AC rating of the plant would be 819 kW.

During a recent (January 2016) tour of the plant, I was able to gather some information about its operation. It started up in August 2012 and since then has operated with only four months of downtime (June 2014, July 2014, Dec 2015, and Jan 2016), so the plant has operated for 36 months or 3 years. During this time, the plant has generated a total of 5286 MWh of electricity.

According to the IET calculator, the total electricity output over this time period should have been 3 years x 2240 MWh/yr = 6720 MWh, which means the plant has generated 79% of its expected output. The differences are due to: 1) lower solar irradiation levels during this period – data from the monitoring equipment at the Phakalane plant indicated average monthly irradiation levels of 161 kWh/m2 – rather than the 197 kWh/m2 used in the calculator; 2) the effects of cement dust from a nearby brick plant coating the panels and reducing their efficiency; and 3) a higher-than-estimated temperature effect on the panels. It always astounds me how hot solar panels get when they are exposed to sunlight: surface temperatures of solar modules can rise to greater than 70oC – much higher than ambient temperatures. At 70oC and with −0.5 %/oC performance losses, decreases in performance can reach as much as 23% at certain times during the day.

Another useful calculation is to determine the capacity factor, which is a measure of the proportion of its maximum output that a power plant actually delivers over a year. A capacity factor of 100% assumes that a power plant runs for 24 hours a day, 365 days a year. Coal-fired power plants have capacity factors of about 60%, nuclear plants 90%, wind plants about 35%, and solar PV plants are usually of the order of 26%. If we assume that the Phakalane plant has an output of 819 kW (assuming 27% system losses), we can calculate that over three years of operation:

Capacity Factor = 5 285 860 kWh / (3 yr x 365 d/yr x 24 h/d x 819 kW) = 24.6 %

This is in line with that expected for solar PV operations.

The Phakalane project is a well-designed operation and, like other PV plants, requires relatively little in the way of maintenance and operating costs. Costs include some electrical component maintenance, security services, cutting back of vegetation and weeds so that panels don’t get shaded, and the occasional washing of the panels. Unfortunately, it is located near a cement and brick factory and, as a result, there is a carryover of fine cement dust that settles on and coats the panels, especially those closer to the factory: a fair amount of effort is required to remove the encrusted coating on these panels.

The single biggest challenge for the Phakalane plant is vandalism and panel theft. There was recently intrusion at the site and large amounts of grounding cables were stolen for their copper value. Six panels were also stolen, despite the installation of special anti-theft fasteners on the panel attachments. As noted previously, the facility has been down since December 2015 and during my tour in mid-January I noted that repairs were underway. Hopefully the plant will be operational again before long.

As solar installations proliferate, panel theft is becoming a growing problem worldwide. Many installations are located in remote areas with limited security, so it is often easy for thieves to access the area and take their time to unbolt and load up the panels. Added to this, solar panel installations have large amounts of grounding wires, making them an inviting target for thieves who are looking to steal copper wire that can be sold to unscrupulous scrap dealers.

University of Botswana’s Mokolodi Village Project

To the south of Gaborone, and bordering the Mokolodi Game Reserve, is the small village of Mokolodi, which is home to about 600 people. This is a relatively new village by Botswanan standards and is surrounded by large properties and some high-end housing developments. Right in the heart of this village, the Clean Energy Research Centre at the University of Botswana has built, using money from an EU grant channeled through the African Union, a small, but in the history of solar power in Botswana—an important solar PV installation.

This will be the first of its type in Botswana to feed electricity produced in excess of that used onsite into the local low-voltage grid at 400 V. Phakalane is also a grid-scale project, but it is designed to feed all of its produced electricity into the high-voltage section of the national grid, whereas Mokolodi is designed first for local energy use and then for directing excess energy into the grid. It is important to appreciate that feeding excess solar power onto a grid is not a cutting-edge concept: in fact, it is the basis for most smaller solar projects installed in the US and Europe during the past decade or so. These systems are designed to generate electricity during the day to power the home or business; any excess generated is fed back into the grid and electricity is drawn back from the grid during the night hours as needed. Most of these systems are not designed with battery storage, so they are not self-contained off-grid systems: the grid is their backup system and they are very dependent on the grid to compensate for electricity needs at nighttime or for shortfalls during cloudy days.Another important aspect of these types of systems is that they are not a buffer against the effects of load shedding or other grid interruptions: when the grid goes down, they are designed to switch off immediately. This is to avoid little islands of solar electricity generation feeding electricity into a dormant grid and that could pose an electrical safety hazard for any electrical line workers who might be repairing lines in the area.

The Mokolodi system has a rating of 20 kW and, because it is an experimental system, consists of the following components:
  • A 5 kW system on the village clinic to provide daytime power for the clinic and to feed excess electricity into the grid;
  •  A 2 kW system on the home of the village Chief to provide daytime power and to feed excess electricity into the grid during the day;
  •  A 10 kW system at the Kgotla (meeting place), designed to power the Kgotla and some Village Development Committee owned homes during the day, with any excess fed into the grid;
  • A 3 kW experimental system that consists of three different types of silicon-based panels that their performance can be compared and contrasted;
  •  A small weather station to measure the solar irradiation, temperatures, and wind speed.

One of the more interesting aspects of this project is the direct comparison of the performance of the three types of panels (1 kW of monocrystalline silicon panels, 1 kW of polycrystalline panels, and 1 kW of amorphous silicon panels) under the energetic and harsh Botswana sunlight. UB researchers will be able to monitor their performances over the short and long term to determine which would be best for the high irradiation conditions here. The photographs below show some views of the Mokolodi project. 

13 kW University of Botswana Solar Installation at Mokolodi Kgotla

Once the project is up and running, the UB team is hopeful that the project will provide the following benefits:
  •  There are presently several non-electrified homes not connected to the grid. These homes will receive “free” solar power from the grid during the day; at night, they will draw power from the grid and pay normal BPC rates.
  • This is a demonstration installation for similar low-voltage grid-connected projects. It will allow BPC and the UB team to gather information and experience and open the way for the installation of similar systems throughout Botswana.
  • The generation of any extra power will be feed into the grid at no cost to BPC and, in a very small way, reduce the amount of coal that needs to be burnt.
  • It will be a valuable research tool for UB and its students and will allow the in-depth investigation of small-scale solar systems and their variability, their impact on the grid, and the performance of different panel types.
The UB team at the Clean Energy Research Centre will also be investigating the socioeconomic benefits to communities of free solar energy during the day and purchased electricity at night. The project is close to completion and hopefully will be commissioned in the near future. As the project moves forward and starts generating research results, I hope to share some details in a future blog.

This post has taken an in depth look at two important grid-connected PV systems in Botswana and in the next, I will be discussing the far more numerous off-grid systems that are scattered throughout the country. In the meantime, look out for solar projects and remember to turn off the lights when you leave the room.

Tsamayang Sentle**
Mike Mooiman

(*Greetings in Setswana)

(**Go well or Goodbye in Setswana)