Concentrating Solar Power in Botswana

Dumelang*. In my previous post, I discussed the potential for harnessing the energy from the sun in Botswana. I also pointed out the various ways of harnessing this potential:

• 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, such as washing;
• Concentrating solar power, where the energy of sunlight is focused by mirrors onto a focal point: the focused sunlight heats a fluid, which generates steam, which then turns a turbine to generate electricity;
•  Photovoltaic (PV) generation of electricity by the use of solar panels.

In this post, I focus on concentrating solar power (CSP). This option has been under consideration for several years in Botswana and may see implementation in the future.

Let’s begin by taking a closer look at the technology. There are two main types of CSP units: point-focus and line-focus units. Point-focus CSPs comprise an array of mirrors that reflect and focus the light of the sun onto a single point – normally a central tower. The mirrors, known as heliostats, track the sun’s path and the concentrated sunlight is used to heat a heat-transfer fluid, normally an organic liquid that can withstand very high temperatures (max. 400^oC) or a salt mixture that melts at low temperatures.  This heated fluid, which can reach temperatures of 250 – 550^oC, is then used to boil water, thereby generating steam that is used to rotate a turbine and generator to produce electricity.

A variant of the point-focus system involves a satellite dish-shaped mirror that focuses the energy of the sun onto a single point. This allows the operation of a Stirling engine, a special type of engine that uses heated air to generate electricity. Photos of these two point-focus systems are shown below.

  

Source: seia.org

Source: Wikipedia

In a line-focus system, sunlight is focused onto a pipe running down the focal line of a parabolic trough. In this approach, the focusing of sunlight and heating of the fluid is done locally within the trough, as opposed to a central tower. During the day, the troughs pivo to track the sun across the sky and all that is required is that the heated fluid be pumped through the pipes to a central steam boiler and generator unit. Some 80% of installed CSP systems are parabolic-trough systems because these lend themselves to offsite modular construction of the mirror and receiver components and because it is a more mature technology.  The troughs can be arranged in series and/or parallel configurations, which creates versatility by allowing a range of heat-transfer fluid temperatures and energy outputs tailored for different steam boiler and generation units. The components of a parabolic-trough system are shown in the figures below.

Source: US DOE

Source: cspworld.org

One of the most important advantages of CSP systems is that thermal energy can be stored by holding the hot thermal fluid in a tank. This provides a thermal reservoir that can be tapped to continue generating steam and electricity after the sun has set. CSP plants can be designed for up to 12 hours of thermal storage; storage for four to six hours of operation after sunset is normally considered sufficient. This represents a major improvement over utility-scale PV operations, which do not have a storage component. Typical output profiles of PV vs. CSP electricity production are shown below. The extended window of electricity production, especially during the high-demand evening hours, makes CSP systems more like traditional fossil-fuel systems and less like many of the other renewable energy systems (such as wind and PV) that are highly variable in output. Energy storage is a major benefit, but thermal storage adds considerable costs to the construction and operation of a CSP unit.

Source: CSP Alliance.org

Most modern CSP systems have the following key components:

• A solar field, which is the array of mirrors, heliostats, or parabolic troughs;
• A receiver that absorbs the focused sunlight and heats up a heat-transfer fluid: this is the central tower (also called a power tower) for  point-focus systems  or a specially configured pipe with a coating designed for energy absorption for parabolic-trough systems;
• A heat-storage system, which is series of tanks or containers to hold the warm heat-transfer fluid;
• A power plant, which includes a steam boiler, turbine, and generator to convert the thermal energy into electrical energy;
• A steam-cooling system to condense the steam back into water to allow for its reuse.

CSP systems are often compared with PVs in terms of technical complexity, energy output, capital cost, and the cost of electricity produced. It is clear that CSP systems are far more complicated operations than utility-scale PV systems. Like PV systems, they have large solar fields to harvest the sunlight, but they also require several additional expensive operations to transfer, store, and convert the captured thermal energy into electricity. CSP systems are therefore more expensive to install and operate. PV panels produce electricity directly and so capital and operating costs are lower. Generally speaking, kWh for kWh, PV is cheaper than CSP and prices for PV systems continue to fall.  

Moreover, CSP systems face the following additional challenges:

• Water is needed to generate steam for the power plant and cooling systems are needed to condense the steam back into water for reuse. Because CSP systems are often located in desert or semi-arid areas, cooling systems involving water cannot be used and so forced-air cooling systems are required. These use large electrically driven fans, which consume a great deal of energy and therefore reduce the net electrical output.
• Because of their location in arid environments, dust is a big problem for CSP systems. It coats the mirrors and reduces effectiveness of the system. In some areas, mirrors are cleaned – with water – every day.
• Direct sunlight is required: cloud cover will shut down the operation of a CSP system. In contrast, PV systems can still produce electricity (although reduced amounts) on cloudy days from the diffuse sunlight.
• Like other renewable energy systems, connection to the existing power grid is an important technical and cost factor. The choice of location is critical to avoid large expenses associated with building long transmission lines.

Notwithstanding these challenges, there are several CSPs, either proposed or under construction, in the Southern African region. Most notably, South Africa has instituted the Renewable Energy-Independent Power Producer Programme (REIPPP) and, since 2011, there have been four opportunities for companies to bid for wind, solar PV, and CSP projects as Independent Power Producers (IPPs). An impressive amount of interest has been shown and, so far, awards for over 3000 MW of renewable energy generation, involving $10 billion of investment, have been made. Even though the bulk of the awards have been for wind and PV, this program has been particularly positive for CSP projects. The first CSP project, a 100 MW parabolic trough system, the KaXu operation in Pofadder in the Northern Cape, operated by Abengoa Solar, came online last year. There is an additional 250 MW of CSP projects under construction—a 100 MW parabolic trough and a 150 MW power tower—and a further 250 MW under development. The REIPP program has been so successful that the bid price of electricity from CSP operations decreased by ~50%  over the first three bid windows.

A CSP project has also been proposed for Namibia and a feasibility study was recently completed. In Botswana, the implementation of CSP has been under consideration and study for a number of years. The review process has gone through the following steps:

• In 2009, a pre- feasibility study that assessed the potential of a 200 MW parabolic-trough installation was completed. Five locations considered, including Selebi Phikwe, Maun, Letlhakane, Serowe, and Jwaneng.
• In 2013, a feasibility study for a 100 MW power tower system was completed. The Jwaneng site was recommended and the installation of solar radiation monitoring installations at Jwaneng and Letlhankane was suggested.
• A Request for Expression of Interest (EOI) to construct, maintain, and ultimately decommission a scalable solar plant near either Jwaneng, the copper mines in the North West Region, or other areas in Botswana was issued on June 7, 2015. The EOI required interested parties to include proposals for IPP license agreements, power purchase agreements, and the location of specific sites. Although closing of these bids was due on August 19, 2015, the outcomes have not yet been made public.

This recent EOI request is rather open-ended, not specific regarding the capacity of the proposed operation, and is open to both CSP and PV options. Earlier government comments suggest that there is interest in 50 MW of generating capacity in the North West Region near the Discovery Metals Boseto and proposed Khoemacau copper mines and 50 MW near Jwaneng (a diamond-mining area). Interestingly, the previous feasibility study advocating a 100 MW CSP option at Jwaneng was not  published nor shared as part of the tender process. If I was bidding, I would have wanted to see that report. Owing to lack of information, many companies would have had to individually prepare their own assessments from the ground up and probably needed to spend a considerable amount of time on site in Botswana. It is likely that this complicated the process and raised the cost of bid preparation. Perhaps there are good reasons for not sharing the earlier report, but they are not obvious. Nevertheless, based on recent reports, a great deal of interest has been shown in this process and 118 EOIs have been received.

Due to the high cost of CSP systems, I anticipate that most of the received bids will be for straightforward PV plants without storage. Even so, I believe CSP is an attractive renewable energy option for Botswana and the inclusion of a thermal-storage component would also enable the generation of electricity until about midnight each evening. On reflection, the South African REIPPP has much to recommend it: it has specific windows for bidding and mandates for specific technologies, such as PV, CSP, and wind. This approach provides opportunities for different IPPs, it promotes renewable energy diversification (cheapest is not always better), and it has resulted in significant decreases in the costs of electricity from renewable sources in just a few years.

Costs of electricity from CSP operations are presently a limiting factor, but indications are that these will decrease. The US Department of Energy has projected that the cost of electricity from CSP operations with storage will continue to drop and has forecast a 3.5-fold decrease in electricity cost by the end of this decade (see the figure below).

Source: US DOE

I am certainly looking forward to more information on the Botswanan EOI and it will be interesting to review the nature of the proposals that were received. Given the open-ended nature of the request, ranking the proposals will be challenging, but I am hopeful that that the Botswana government will act on some of the proposals and that that we will see the installation of large grid-scale PV, and even some CSP projects, in the near future. 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)

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