Solar power technology seeks to harness the vast power of the sun by using the energy in the sun’s rays as input and transforming them into electric energy. Modern research has shown that the energy emitted from the sun in a single hour supplies more than enough energy to fulfill the world’s energy needs for a year[1]. In light of this, it makes sense that the sun has been honored in cultures around the world since times immemorial as life-giving. Solar power is a sustainable, non-polluting technology that utilizes the power of the sun for clean electric power, reduced electricity costs, and reduced greenhouse gas emissions as well as reduced waste, which leads to reduced pollution and increased public health.

This article will describe the two main types of solar installation: solar photovoltaic system (SPV) and concentrated solar power (CSP), and discuss one of the latest advances in SPV, dual junction III-V/Si solar cell.

Solar Photovoltaic

Solar photovoltaic is the most widely applied solar technology at present, comprising approximately 96-97%[2] of the total solar installations worldwide. Solar photovoltaics have a wide range of applications, from a single portable solar panel, individual home[3] and office installations, to large commercial installations[4]. SPV technology continues to utilize new types of materials and combinations of materials to harness a greater range of the sun’s rays and increase overall efficiency[5].

SPV Process

Photovoltaics uses semiconducting materials to convert sunlight into an electric current using the photovoltaic effect. In this process photons from the sun’s light excite electrons, enabling them to generate an electric current. SPV panels are made of either monocrystalline silicon, polycrystalline silicon, or amorphous silicon as well as cadium telluride and copper indium gallium selenide/sulfide[6]. The solar panels are made of individual solar cells joined together in a single solar panel covered with glass. The solar cells can be multi-junction, meaning two or more different semiconducting materials are sandwiched together to improve efficiency[7]. A single solar panel can have as many as 72 individual solar cells[8].

SPV Capacity

The capacity of single SPV panels, comprised of individual solar cells, ranges from 30 watt and 60 watt in portable panels[9] to 320 watt panels comprised of 72 individual solar cells[10]. Most home and commercial installations include more than one individual solar panel connected together, called an array. The panels are connecting by copper wires. A typical SPV array ranges in capacity from about 5 kilowatts[11] for a rooftop residential installation on a sloped roof, to a megawatt-scale commercial system, usually installed on a low slope or even flat roofs.

SPV Efficiency

SPV typically ranges from 10% to 20%[12] efficiency in field conditions. However, it is important to note that the energy noted on the panel takes into account the efficiency, so two 300 watt panels with different efficiencies, say one 10% and one 14%, will still both produce 300 watts of electric power in ideal conditions. The only difference is that the panel with the greater efficiency will be smaller in size. The most efficient, and also the most expensive solar panels are the monocrystalline silicon panels, having efficiencies from 17.8% to 18%[13] and as high as 25% in laboratory conditions[14]. 

The amorphous crystalline silicon panels are the least efficient, with efficiency topping at 20% in laboratory conditions[15] and also the least expensive[16]. Factors that affect efficiency are dust on the panels and overheating of temperature, as will be discussed below. In lab conditions efficiency can reach substantially higher, as much as 43%[17], but these rates have not been achieved in outdoor installations.

Dual-Junction III-V/Si Solar Cell (SPV)

The dual-junction III-V/Si solar cell, a type of SPV cell, represents one of the latest advances in solar research and recently set the world record for converting non-concentrated sunlight into electricity[18]. The concept of a dual junction solar cell, in which two solar cells are grown or attached together to increase efficiency, is not new, but the materials used and the type of connection in the dual-junction III-V/Si solar cell are unique. 

The dual-junction III-V/Si solar cell combines the 1.8-ev GaInP cell made of gallium indium phosphide created by the National Renewable Energy Lab, which topped efficiency at 20.8%, as the top cell with a crystalline silicon (Si) hetero-junction technology cell, created by the Swiss Center for Electronics and Microtechnology as the bottom cell. 

The GaInP and Si cells are grown separately, stacked, and joined with an adhesive, which allows the cells to be joined even if there are irregularities in the surface or architecture of the two cells. The dual-junction III-V/Si solar cell operates at 29.8% efficiency[19] under a single sun, meaning it can achieve this efficiency under normal operating conditions.

SPV Dependability

SPV modules are generally considered reliable for 25-30 years[20]. Because SPV modules (unless they track the sun) do not have any moving parts, there is lower risk of failure. However the individual parts of the panel, including semiconductors, transparent conductors, glass, bypass diodes, copper wires, etc. have individual mean time to failure, individual components may have a greater possibility of failure.

Testing to determine mean time to failure and durability of SPV are constantly improving. Testing involves subjecting SPV panels to rigorous tests to mimic real world situations at much greater intensity to accurately estimate long term reliability[21]. To test the panels in real world situations, tests of 25 years or more are needed. Andy Black, a leading expert in solar financial analysis, has shown that solar panels only lose about half a percent efficiency per year[22]. He found that panels manufactured 40 years ago still generate power at approximately 80% of the original expected efficiency.

SPV Constraints

The primary constraints on SPV efficiency are temperature, dust, humidity, and air velocity. Naturally, the sun also needs to be shining for greatest solar panel output.

Different solar panels react differently to variations in temperature, but for all types, efficiency decreases as temperature increases over a certain temperature, usually 25 degrees Celsius[23]. SPV panels are rated with a temperature coefficient. For example, a certain panel has a temperature coefficient of -.48%. That means that for every degree over 25 degrees Celsius, the efficiency decreases by .48%. It also means that on a sunny but cold day, the panel can actually be more efficient[24]. Some typical temperature coefficients for different types of solar panels are seen in Table 1:

Table 1 – Solar Panel Efficiencies

A way to combat solar panel overheating is to ensure maximum natural ventilation in the area they are installed. As air velocity increases, the temperature of the solar cell drops and therefore efficiency improves[28]. Therefore low air velocity can also be a constraint on solar efficiency.

Regarding dust, a wide range of reduction in performance of solar cells has been reported due to dust. In the US the average reduction in efficiency due to dust is 1.5% with 4.7% maximum during a two month period, whereas in Saudi Arabia efficiency was reduced by 32-40% during a six to eight month period. In Egypt reductions in efficiency due to dust were as high as 65.8%[29]. Therefore environmental conditions, such as proximity to desert areas prone to dust storms, can impact the efficiency of solar cells.

Finally, humidity almost always causes degradation in solar cell efficiency. This happens both through the effect of the water vapor particles on the intensity of sunlight reaching the solar panels and also because the humidity can get into the solar cells (moisture ingression) and cause erosion, decreased performance, or failure of individual parts. Although exact percentages of decrease in efficiency due to humidity are not known, it is widely understood that humidity will degrade solar panel efficiency over time[30].

SPV Economics

Residential SPV installations vary greatly in price – from less than $3.00 per watt to just over $7.00 per watt. The average residential solar installation costs was about $3.57 per watt[31] in the beginning of 2016. A 3 to 8 kW system will cost between $15,000 and $40,000. SPV does exhibit economies of scale, as generally the larger the system the lower the cost per watt[32].

The prices of SPV energy have also decreased dramatically in the last decades. According to the International Renewable Energy Agency, SPV levelized energy costs are around $.08 per kWh on average. SPV prices were double that in 2008 and more than 100 times that in 1977[33] Table 2 shows the price decrease and economy of scale from 2012 to 2013.

Table 2 — Solar Photovoltaic Prices Demonstrate Economy of Scale[34]

In the state of California, which has the greatest amount of solar installations in the US, there are 580,692 solar projects with a total installed capacity of 4537 MW. The average cost for installations smaller than 10 kW is $5.25 per watt and for installations larger than 10 kW of $4.36 per watt. Again, this demonstrates the economy of capacity[35].

The cost of the solar panels range in price from $0.85 to $1.25 per watt, with the remaining cost coming from inverters, solar batteries, connectors, additional equipment and installation costs[36].

Concentrated Solar Power

Introduction

The second type of solar installation is concentrated solar power (CSP). Concentrated solar plants make up about 3-4% of the total global solar installations[37]. Of interest, CSP has the lowest cost for large scale solar power generation[38], which will be discussed later in this report. The other advantage of CSP technology is that thermal energy can be stored for later conversion to electric energy. In fact, CSP plants can continue to produce energy when clouds block the sun or after the sun has set due to this ability to store energy[39]. The three main types of CSP installations with a range of output and efficiencies will be described briefly below.

CSP Process and Efficiency

Concentrated solar power (CSP) uses lenses and mirrors to focus a large area of sunlight into a small beam that is sent to a thermal receiver. The concentrated light is then absorbed and converted to heat by the receiver[40], and then it is transported to a heat engine connected to an electrical power generator[41]. This is also called Solar Thermal Electric, STE. A concentrated solar plant (CSP) requires high levels of sun exposure, access to water (as is the case with a coal-fired electric power plant), and large-scale deployments.

The three main types of CSP are the trough system, which uses mirrored parabolic troughs; the dish system which uses dish shaped parabolic mirrors; and the central receiver system which uses central receivers or towers with usually thousands of sun tracking mirrors called heliostats[42]. Each of these systems channels the light to generate heat and electricity to be stored or used immediately.

Efficiency for the parabolic trough system can reach 20% conversion of sunlight to electricity, but total conversion efficiency is usually around 17%.[43] However, efficiency among the systems varies greatly based on conditions, as will be discussed under constraints.

Environmental Effects

CSP requires about 700–900 gal/MWh[44] of water for cooling, and can need as much as 3,500 liters of water per MWh of energy generated. This is more than a coal plant (2000 liters/MWh) or natural gas power plants (1000 liters/MWh)[45]. Additionally, CSP requires 10 acres of land per MW of energy produced, compared to only 8 acres per MW for large-scale SPV arrays[46].

CSP Capacity

A concentrated solar plant (CSP) deployment is typically more than 20 MW. The largest CSPs in the world range from 150-392 MW[47]. Small and medium scale CSP installations are now being employed, ranging from 10kW to 10MW[48] Large scale applications have primarily been for commercial use, while small and medium scale applications are being used in developing countries for villages or local areas that would otherwise be without power, as well as in Europe.

CSP makes up only a small percentage of the global solar power capacity, mainly due to the size of installations, and the land and water required. Because of these factors and especially due to the scalability of an installation, SPV is more widely used by investors and consumers. As of 2014, SPV installations had a capacity of about 140 GW globally, whereas CSP only had a global capacity of about 5 GW[49].

CSP Dependability

In current operational experience, CSP plants have a mean time to failure of more than 30 years[50]. Regarding reliability, CSP frequently does not reach expected capacity[51]. CSP in real world installations often produce 25 to 60% less electric energy than projected capacity[52]. This is usually due to unexpected weather patterns such as substantial lack of sunshine compared to the expected level[53].

CSP Constraints

The primary constraint of CSP is temperature. In addition, the CSP installation requires sunlight to function, which means that substantial changes in weather, as mentioned above, will decrease output.

In CSP, efficiency dramatically decreases when temperatures are above 100 degrees Fahrenheit[54]. For temperature, either a wet cooling or dry cooling system is used in CSP. In the wet cooling system, more water is used even than traditional power plants, as previously mentioned. For dry cooling, there needs to be a significant difference between the exhaust steam and the outside temperature in order to condense the steam.

CSP Design

Design considerations that affect performance of CSP installations are the size of the solar field, water requirements, and thermal storage capacity. However, these elements are managed in the design of the installation. In addition, the size of the solar field must be proportionate to the capacity of the power block[55] and the tower needs to be a certain height above the mirrors to avoid or reduce shade and other obstructions[56].

Of course, CSP depends on radiation from the sun, but most CSP plants have a means to store thermal energy for use when the sun is not shining. Therefore an additional constraint is the total thermal energy storage capacity. If the solar field can produce more thermal energy than the power block is able to store, that energy is wasted[57].

Finally, each step in the process of CSP has its own level of efficiency—the concentration of sunlight into steam, the efficiency of the steam engine, the conversion of this into stored energy as thermal energy, and the conversion of thermal energy into electric energy. Therefore the ability of these many parts to work together efficiently is another constraint[58].

CSP Economics

Concentrated Solar Power has an average installed cost of $2.50 to $4.00 per watt[59], making it the most economical form of solar power for very large scale installations. A 250 MW CSP station would cost between $600 million and $1 billion dollars. Estimated levelized energy costs per kWh range from as low as $0.06 per kWh to $0.18 per kWh[60]. It is interesting to note that SPV and CSP both demonstrate economies of scale, with larger scale installations being less expensive per watt or kWh, making large scale CSP installations the best choice for large commercial installations.

Conclusion

In conclusion, both SPV and CSP technologies have the ability to harness the sun’s power for a stable, reliable, non-polluting source of energy. SPV has a far greater current application worldwide, with approximately 97% of the total solar power, although both CSP and SPV are expanding. SPV and CSP have a similar range of efficiency, 10-20% in field conditions, with research on how to increase efficiency ongoing. The dual-junction III-V/Si solar cell is one example of this research in the area of SPV, topping efficiency at about 29.8%.

For capacity, CSP is generally much larger than SPV, with installations ranging from 20 to 392 MW. CSP installations are also the most economical per MW of energy produced. SPV has the advantage of small scale installations by individuals, such as a 3-5 kW solar array on an individual home. Both CSP and SPV are shown to have a good level of dependability for 30 years, however research in this area is ongoing. SPV and CSP both have constraints that limit efficiency, so consideration must be taken regarding which type of installation is best for the specific environment. With ongoing research, CSP and SPV solar technology have the ability to fulfill the world’s energy needs in a sustainable way, reducing long-term energy costs and environmental impact.

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