Solar Power in Botswana – Photovoltaics – The Technology

Dumelang*. My last few posts have discussed the potential for harnessing energy from the sun in Botswana. Various ways of doing this include:

• Concentrating solar power, where the energy of sunlight is focused by mirrors onto a focal point. The focused sunlight heats a fluid that is used to generate steam, which then turns a turbine to generate electricity.
• Photovoltaic generation of electricity through the use of solar panels.
• Solar thermal, which 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.

In this post, I take a look at photovoltaic (PV) technology in general; the next blog will focus specifically on its application in Botswana. PVs generate electricity directly from sunlight using semiconductor technology, which is built into the PV panels. The ever-increasing scope of PV applications ranges from small devices that generate tiny amounts of electricity used to power calculators (outputs in the milliwatt (mW) range), to one- or two-panel systems generating 100 to 300 watts (W) to charge cell phones and provide light, to 2 to 50 kilowatt (kW) systems that power homes and businesses, all the way to grid-scale solar farms with outputs as high as 550 megawatts (MW). Today, we see PV applications all over the place; below are photographs of some solar installations I have observed in Botswana.

We will return to specific solar installations in Botswana later, but first let’s turn our attention to  looking at photovoltaic technology and the main components of a solar PV system.

PV panels produce electricity by the use of semiconductor technology. Most PVs are based on silicon semiconductors, but there are some newer panels that use non-silicon semiconductors. Silicon-based solar panels use bilayer materials of n- and p-type semiconductors. The n-type contains a small amounts of phosphorus in the silicon matrix which creates extra electrons in this layer and the p-type contains a small amount of boron which creates electron shortages or holes. At the junction of these two layers, the electron imbalance creates an electric field that can be used to control the flow of electrons. When panels are exposed to light, the photons in sunlight knock electrons from their positions in the silicon lattice and allows them to flow through an external electric circuit connecting the two types of semiconductor. This generates a small direct current which can then be harnessed. (For a more detailed explanation, here is a great YouTube video worth watching.)

Assembling many of these cells into a solar panel (modern panels typically contain 60 individual cells) allows their flows to be combined and permits larger flows of electricity. The first power solar cell was developed in 1954 and, since then, the key driver in solar cell research has been to improve the efficiency of these devices. Overall, solar panels are still low-efficiency devices with average efficiencies of converting sunlight to electricity of about 15%, although newer panels are now reaching efficiencies of the order of 22%.

The components of a solar system include the following:

• Solar panels – Most commercially available panels measure ~1.6 m x 1 m and produce 150 to 250 W with a direct current (DC) output that can range from 15 to 60 volts and 3 to 7 amperes. The outputs of the individual panels are combined by wiring them in series or parallel configurations: connecting them in series boosts the output voltage, whereas parallel connections provide a larger current. The combination of these two wiring modes provides a range of voltage and current outputs that can be tailored to meet the requirements for a specific application.
• Inverter – Most household and business appliances operate on an alternating current (AC) supply, so the direct current (DC) electricity that is generated from the panel must be converted to AC, which can then be feed directly into the grid or used locally. This is done by the inverter. Today, some DC appliances and lighting are available, which can eliminate the need for an inverter.
• Charge controller – This is an electrical interface between the solar panels and the batteries that is used to ensure that the batteries receive the correct charging voltage and current. The electrical outputs of the panels vary during the day with changes in the sunlight intensity, so these devices prevent the over- and undercharging of the batteries. Charge controllers and inverters are often combined into a single multipurpose electrical component.
• Batteries – These are usually lead acid batteries specifically designed for deep-discharge applications. They are quite different from car batteries. Car batteries are designed to put out a lot of power for just a short period of time to turn over the car engine when it is first switched on. Deep-discharge batteries are designed for cycling applications where there is moderate power draw over a long period of time, say during the evening, and recharging every day.  The two are not interchangeable. Not all solar systems include batteries, but battery storage systems are becoming an increasingly important. Many people have chosen to just install battery storage systems in their homes without solar panels to have some electricity available for when the electric grid is down during load-shedding periods. In a future blog, I will be taking a much closer look at battery storage systems.

There are two kinds of PV systems:  grid-connected and off-grid systems. In grid-connected systems, the AC output of a solar operation is fed into the electrical grid to supplement the power produced by other power plants. These operations usually do not include any storage so they can only generate and supply power to the grid during daylight hours. The supply from these operations is therefore highly variable: low in the mornings and afternoons, high at midday, and cloud cover significantly reduces their output. The electrical grid needs to be managed to adjust to this variable output. Most systems in Europe and the US are grid-connected and range from large utility-scale systems to smaller home-based units in which electricity produced during the day in excess of that used by the homeowner is fed back into the grid. These systems are often bidirectional: during the day, electricity is supplied to the grid; during the night, when no solar electricity is produced, power is drawn from the main electrical grid.

The other type of solar system is not connected to the main electrical grid. These are known as off-grid systems and are typically found on homes, on farms, in villages, or at tourist lodges in remote areas. These usually incorporate batteries so that any excess energy can be stored for use during evening hours. During the day, the sun generates electricity that is used to power the site, while excess electricity is stored in batteries to provide power for the evenings. Off-grid systems 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.

Some solar systems combine grid-connected and off-grid systems. These have battery storage, but are also connected to the grid. These operations generate some or all of the electricity needed by the homeowner or business during the day and any excess is stored in the batteries (as opposed to sending it out to the grid); however, the grid connection is there to provide any shortfalls in power production from the solar panels or when the batteries are depleted. These systems offer the best of both worlds – they produce and use renewable energy so their electricity purchases from the grid are reduced, but the electrical grid is there as a standby to cover any shortfalls in energy production.

Although PV technology has been around for a long time and its applications have been expanding, it is only recently that we have seen significant growth: in fact, the roll out of electrical power generation from PV panels during the past decade has been quite phenomenal. The figure below shows the exponential growth. It was forecast that that there would be over 200 gigawatts (GW) of installed solar capacity by 2015—some 1% of the world’s total installed generating capacity—and that this would double by 2019 to 400 GW.

Source: Wikipedia

This growth has been driven by two factors:

• Prices of solar systems have dropped, caused by improvements in PV technology, improved manufacturing processes, accelerated Chinese production, and, in some cases, the overproduction of solar panels. In 1977, the price of solar modules was $ 77/W; by 2013, the price had dropped 100-fold (!) to $ 0.74/W. This astounding price reduction is charted in the figure below. Today’s solar module pricing is now of the order of $ 0.50/W.
• The implementation of renewable energy programs in Europe, Asia, and the US that offer large subsidies or feed-in-tariffs (FIT) has made the installation of solar power attractive for homeowners, businesses, and independent power producers (IPP).

 

Source: BNEF

The massive rollout of PV in these regions has made its way down to parts of Africa, notably South Africa. As part of its Renewable Energy Independent Power Producers Programme (REIPPP), South Africa has implemented 1059 MW of grid-scale PV solar projects, with an additional 1255 MW under construction or in development. This does not even include all the small-scale solar projects that businesses, homeowners, and farmers in South Africa have implemented.

Despite the large solar resource available in Botswana, this country has not been part of this rollout. In my next blog, we will look at some of the reasons for this. In the meantime, remember to turn off the lights when you leave the room.

Tsamayang Sentle**

Mike Mooiman

mooimanm@franklinpierce.edu

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(*Greetings in Setswana)

(**Go well or Goodbye in Setswana)