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Solar energy
Ahteshamul Haque Department of Electrical Engineering, Faculty of Engineering & Technology, Jamia Millia Islamia University, New Delhi, India
Abstract
The Sun is the biggest source of energy for our planet. This energy is known as solar energy. The lifecycle of many creatures on Earth is not possible without solar energy. From ancient times attempts have been made by human beings to trap and use this energy in applications like heating, etc. Due to the lack of technology and limited resources solar energy was not used much in the past. With the advancement of electrical engineering, it has now become possible to utilize solar energy as electrical energy. The demand for electrical energy is increasing exponentially. The conventional fossil fuels are used to meet this demand. These fossil fuels have limited reservoirs and emit greenhouse gases. This leads to serious concerns of energy crises and climate-related threats. Among all renewable energy sources, solar energy has received tremendous attention from researchers as it is available in abundance and free of cost all across the globe, and has limited safety concerns. The objective of this chapter is to describe the fundamentals of utilizing solar energy for use in a solar passive energy system and for electrical energy using solar photovoltaics.
Keywords
solar energy
passive
active solar energy system
photovoltaic
module
partial shading
integration
3.1. Introduction
Human beings have tried to harness solar energy in the past for their convenience. In the fifth century BC, passive solar systems were designed by the Greeks to utilize solar energy for heating their houses during the winter season. This invention was further improved with the advancement of mica and glass, which prevent the escape of solar heat during the daytime. Another invention was made in USA to use solar energy to heat water. The first commercial solar water heater was sold in the 1890s. In the nineteenth century scientists in Europe constructed the first solar powered steam engine [1].
In the 1950s scientists working at Bell Labs developed the first commercial photovoltaic (PV) cells. These PV cells were capable of converting sunlight into electrical energy to power electric equipment. These PV cells began to be used in space programs, that is, to power satellites, etc. Further advancement in the technology reduced the price of solar PV and it began to be used for household applications [2].
Currently, global demand for electricity is increasing [3]. The limited reservoirs of fossil fuels and emission of greenhouse gases have led to serious concerns regarding energy crises and climate threats. These concerns led researchers to look for alternative sources of energy, and solar energy is considered as the most acceptable source among all renewable energy sources. Solar energy is available in abundance and free of cost all across the globe. It is reported that Earth receives energy from the Sun, which is 10,000 times more than the total energy demand of the planet [4].
The conversion from solar energy to electrical energy is done by using solar PV. Solar PV has a nonlinear characteristic and its output varies with ambient conditions like solar irradiation, ambient temperatures, etc. [5].
In this chapter passive and active solar energy conversion is discussed. PV modeling, its operation, module, integration, and evaluation parameters are described. Finally, practical problems are given.
3.2. Passive solar energy system
The passive solar energy system (PSES) is used to utilize the Sun’s energy for heating and cooling of a living space. PSES can be used for reducing heating and cooling energy bills and provides increased comfort. In this system, either the complete living space (building) or parts of it take advantage of natural energy characteristics. PSES requires few moving parts, low maintenance, and eliminates the need for mechanical heating and cooling systems.
The design of PSES is not very complex but knowledge of solar geometry, climate, and window technology is an essential requirement. PSES can be integrated into any building if a suitable site is available.
The following are the categories of passive solar heating technique [1].
1. Direct gain: In this type, solar irradiation directly penetrates into the building and is stored.
2. Indirect gain: In this type, solar irradiation is collected, stored, and distributed using thermal storage materials like a Trombe wall.
3. Isolated gain: In this system, solar irradiation is collected in an area of the building that can be open or closed off selectively.
The basic aim of PSES is to maximize solar heat gain in winter and minimize heat gain in summer. The following specific techniques are used for PSES implementation:
• Orientation of the building is kept with the long axis running east to west.
• Glass is used and its size and orientation are chosen to maximize solar heat gain in winter and minimize heat gain in summer.
• Overhangs are sized south facing to shade windows in summer and allow gains in winter (Figure 3.1).
• Thermal mass is stored either in walls or floors for heat storage.
• Daylight should be used to provide lighting.
Figure 3.1 Orientation of a house for a solar passive energy system.
With effective shading, window selection, and insulation, the natural air can be used to cool the building. In many PSES designs when considering climate conditions, the windows can be opened at night to flush the air inside and closed during the daytime to significantly reduce the need for supplemental cooling.
The drawback of PSES is that its effectiveness only lasts for 16–18 h daily. The rest of the time (morning hours for heating) heating/cooling is dependent on a supplemental heating/cooling system. However, heating/cooling by using PSES saves significant amount of money.
3.3. Active solar energy system (photovoltaic)
PV cells are used to convert solar energy into electrical energy. This concept was discovered in 1839 by French scientist Edmund Becquerel and is known as the photovoltaic effect. The PV effect was first studied in solid like selenium in 1870. The converting efficiency of selenium solar was 1–2% and it was very costly, which prevents engineers using it in energy converters. With expansion of work in this area a method was developed for producing highly pure crystalline silicon in the 1950s. In 1954, Bell Labs developed silicon PV cells whose efficiency was 4%, which was further improved to 11%. At this time a new era of power-producing cells began. In 1958, a US space satellite used a small array of cells to power its radio. The currently available PV cells are made of silicon and are also known as solar cells [2].
3.3.1. Principle
Sunlight is composed of photons. These photons contain different amounts of energies corresponding to various wavelengths of light. When photons strike a PV cell, they may be absorbed, reflected, or pass through these cells. Absorption of photons in solar cell results in the generation of an electron hole pair. This generation of electron hole pairs result in the generation of a voltage, which can drive the current in an external circuit. Figure 3.2 shows the effect of light on a silicon PV cell. Figure 3.3 shows the connection of a PV cell to an external load/circuit.
Figure 3.2 Effect of light on a silicon PV cell.
Figure 3.3 Connection of a PV cell to an external load circuit.
3.3.2. Types of solar PV cells
Solar PV cells are made of silicon, which is available in abundance. Solar PV cells are categorized into the following manufacturing technologies:
• Monocrystalline
• Polycrystalline
• Bar crystalline silicon
• Thin film technology
The conversion efficiency of monocrystalline cells ranges from 13% to 17%, whereas for polycrystalline the range is 10–14%. The polycrystalline cells are economical as compared to monocrystalline. The expected life of polycrystalline cells is between 20 and 25 years and for monocrystalline it is between 25 and 30 years. The conversion efficiency of bar crystalline is around 11%. The production cost of thin film technology is reduced but the efficiency is very low and ranges between 5% and 13% with a lifespan of around 15–20 years.
In addition, the latest technology is organic PV cells.
3.4. Ideal PV model
The equivalent circuit of the ideal model of a PV cell is shown in Figure 3.4 [5]. The output current I is:
(3.1)
Figure 3.4 Ideal PV model.
where,
(3.2)Ipv,cell is the current generated by the sunlight, Id is the diode current given by Equation (3.2), Io is the reverse saturation current of the diode, q is the electron charge (1.60217646 × 10−19 C), k is the Boltzmann constant (1.3806503 × 10−23 J/K), T is the temperature of the diode (K), and a is the diode ideality constant.
3.5. Practical PV model
In practical applications, PV cells are connected in series and parallel combinations. PV cells are connected in series to increase the output voltage and the output current. This arrangement is known as an array. The basic equation (3.1) is the ideal equation of one PV cell. However, in practical applications PV arrays are used and therefore other parameters are to be considered. The equivalent circuit of a practical PV model is shown in Figure 3.6.
Figure 3.6 Practical PV model.
The equation for a practical PV array is:
(3.3)
(3.4)
(3.5)where, Ipv and Io are the PV and leakage current, respectively; a and b are ideality factors; KI is the short circuit current temperature coefficient at ISC; S is the solar irradiation (W/m2); ISC is the short circuit current at 25°C and 1000 W/m2; EGO is the bandgap energy for silicon; Tr is the reference temperature; Ior is the saturation current at temperature Tr; Vt = NskT/q is the thermal voltage of the array; and Ns is the number of PV cells connected in series.
If the array is composed of PV cells connected in parallel, then Ipv = Ipv,cellNp and Io = Io,cellNpRs is the equivalent series resistance of the array and Rp is equivalent parallel resistance of the array.
The plot of Equation (3.3) is shown in Figure 3.7 (for a particular solar PV array model) and is known as the I–V characteristic curve of solar PV.
Figure 3.7 I–V characteristic curve of a solar PV.
Manufacturers provide electrical parameters of PV rather than equations. These specifications are labeled and highlighted in Figure 3.7.
The open circuit voltage is the maximum voltage across the PV array when no external circuit is connected across it. The short circuit current is the current when the PV array terminal is shorted. The other parameters are VMPP and IMPP, that is, the PV array voltage and current at maximum power point.
3.6. Effect of irradiance and temperature on solar cells
The characteristic equations for solar PV are given by Equations (3.3–3.5). The I–V (PV array output current and voltage) and P–V (PV array power and voltage) characteristics are shown in Figures 3.8 and 3.9, respectively. The data for PV for the characteristic curves are taken from Kyocera model no. KC200GT, summarized in Table 3.1.
Figure 3.8 I–V characteristic curve in different ambient conditions.
Figure 3.9 P–V characteristic curve in different ambient conditions.
Table 3.1
Parameters of solar PV Kyocera model no. KC200GT @ 1000 W/m2, 25°C
| Parameters | Value |
| Voc | 32.9 V |
| Isc | 8.21 A |
| IMPP | 7.61 A |
| VMPP | 26.3 V |
| Pmax | 200 W |
| Ns | 54 |
| KI | 0.0032 A/K |
As seen in Figure 3.8, the plot between PV array output current and voltage is shown. It can be seen that with the increase in solar irradiation at constant ambient temperature the current for the same voltage increases. The same trend is seen in Figure 3.9 where the maximum power point is increasing with the increase in solar irradiation. The power of solar PV increases with the increase in solar irradiation and it varies nonlinearly.
The other case is when ambient temperature varies and solar irradiation is constant. Figure 3.10 is the curve showing this variation. It is seen that with the increase in ambient temperature both the maximum power and the open circuit voltage decrease.
Figure 3.10 P–V characteristic curve in different ambient temperatures.
The best climate condition for PV is high solar irradiation and low ambient temperature. However, the rating of PV should be decided based on the accurate solar irradiation levels.
3.7. PV module
The basic unit of solar PV is a solar cell. Solar cells can produce only a small amount of power. Power generated by a solar cell depends on its efficiency. Depending on the cell efficiency the power generated per unit area varies in the range 10–25 mW/cm2, which corresponds to 10–25% cell efficiency [6]. The typical area of a single solar cell is 225 cm2. With 10% cell efficiency the maximum power generated would be 2.25 Wp. To meet the requirement of high power, these cells are connected in series/parallel combinations to form modules. Currently solar PV modules are available whose power rating range is from 3 Wp to 200 Wp. These modules can be connected further to form arrays, as shown in Figure 3.11.
Figure 3.11 Solar PV cell, module, and array.
A solar PV array can provide power ranging from a few hundred watts to several megawatts.
3.7.1. Series and parallel connections of cells
Series and parallel connections of solar cells are made to generate high power. To increase the output voltage solar cells are connected in series and to increase the output current they are connected in parallel. It is assumed that all the parameters of solar cells are identical for making series and parallel connections.
Figure 3.12 is the I–V characteristic curve of solar PV connected in series. It can be seen that open circuit voltage is increased. In Figure 3.13 solar PV cells are connected in parallel for comparison. It can be seen that short circuit current is increased.
Figure 3.12 I–V characteristic curve PV cells connected in series.
Figure 3.13 I–V characteristic curve PV cells connected in parallel.
3.7.2. Mismatch in solar cell parameters
In solar PV modules, the solar cells are connected in series or parallel combination with the assumption that all the parameters of connected solar cells have identical parameters. However, in practice the mismatch may happen due to the following reasons:
• Cells or modules have the same ratings but are manufactured differently.
• Cells are processed differently.
• Outside conditions, that is, partial shading conditions, are different.
• Glass covers, etc., can be broken.
Mismatch in a connection can be caused by electrical parameters but the most common mismatch is seen in two parameters, that is, Voc and Isc. Among these two parameters, the short circuit current mismatch is a matter of concern particularly when solar cells are connected in series, which is common.
The mismatch in Voc is an issue when solar cells/modules are connected in parallel.
Figure 3.14 is the analysis of the effect of Isc mismatch. It shows the case when two solar cells are connected in series and their Isc is not the same. Cell 1 has the higher Isc compared to Cell 2. As per the convention the short circuit current flowing through the external circuit will be equal to the lower value of Isc, that is, Cell 2 in this case. If the combination operates in short circuit condition the sum of the voltage across both the cells is zero, that is, on the Y axis (where the dotted line is crossing). To meet this value of current the solar cell of lower Isc value is forced to operate in reverse bias condition. Due to this effect, there will be significant power loss.
Figure 3.14 I–V characteristic curve cell parameters are mismatched.
3.7.3. Hot spot due to partial shading
In the solar PV module there are many cells connected in series. It may happen in cloudy weather that one or more cells of the string receive no sunlight as shown in Figure 3.15. Under short circuit conditions the shaded cells of the string will become reverse biased and will be forced to work at Vbias to maintain the same current (Figure 3.16). This may lead to heavy power loss in the partially shaded cell and due to excess heat this cell may break down completely. Also, the negative voltage may bring the diode to reverse breakdown voltage and may lead to the same result, that is, breakdown. Due to this effect, the string will become open circuit and the solar power generating system could fail.
Due to the reverse bias operation the cell generates heat, which is treated as hotspot on PV array. These hotspots and failure can be avoided by using bypass diodes. These diodes are connected in parallel to the cells to limit the reverse voltage and hence the power loss in the shaded cells [7]. It is proposed by researchers that in a 36 series connected cell two bypass diodes can be connected across 18 series cells (Figure 3.17). The bypass diode restricts the voltage of the shaded cell to reach reverse breakdown voltage and also provides an alternate path for the current.
3.8. Daily power profile of PV array
As discussed in the previous section, the power output of the solar PV system depends mainly on solar irradiation. Also, the output power varies nonlinearly with ambient conditions. Based on the climate condition the output power profile of the PV array varies. The total solar irradiation in the plane of the solar array is known as incident solar irradiation. The power profile is measured in terms of performance ratio. It is defined as the ratio of the daily PV system electricity generation effectiveness to the rated array efficiency. It could be done on a daily, monthly, or annual basis. The performance ratio is a PV system metric that is normalized by both PV system capacity and incident solar radiation. An idealized performance ratio of 1.0 would imply that the PV system operated at standard test conditions over the reported period, without any losses [8]. Figure 3.18 shows the PV power production profile for the whole year on an hourly basis. It can be seen that power is delivered when Sun is available. Figure 3.19 shows daily performance ratio over a year.
3.9. Photovoltaic system integration
The photovoltaic system is broadly classified into three parts:
1. Standalone (off-grid) PV system
2. Grid connected (on-grid) PV system
3. Network connected solar power plants
3.9.1. Standalone (off-grid) PV system
These systems are used in areas where there is no electricity available from the national grid, that is, rural areas. In this type of system, the energy is stored in a battery generated by solar PV. The DC–DC converter is used to regulate the generated DC voltage and an inverter is connected to convert DC to AC for standard home appliances. The basic structure is shown in Figure 3.20.
Figure 3.20 Standalone PV system.
3.9.2. Grid connected (on-grid) PV system
In this type of PV system, the solar PV generates the power, which is regulated by a DC–DC converter followed by a DC–AC inverter to supply the AC power to standard home appliances. If the power generated by the solar PV is more than the load requirement of a house, then it is supplied to the grid measured by the smart meters and with proper protection. The typical structure is shown in Figure 3.21.
Figure 3.21 Grid connected PV system.
3.9.3. Network connected solar power plants
This system is used to generate large amounts of power and the capacity of power generation ranges from several hundred kilowatts to hundreds of megawatts. The photovoltaics are installed on a localized large area and are also connected to the network. These systems are installed on large industrial facilities or terminals. A photograph of one such installation is shown in Figure 3.22.
Figure 3.22 Large-scale grid connected PV system. Courtesy NREL website photos: http://www.nrel.gov./esi
3.10. Evaluation of PV systems
The following parameters are calculated to evaluate the PV systems.
3.10.1. Net PV system production
The net electricity produced by the PV system and delivered to facility loads or exported to the grid.
Reported as: monthly totals, annual totals, and average daily totals per month.
Units: kWh/month, kWh/year, kWh/day.
3.10.2. Equivalent daily hours of peak rated PV production
The daily PV production normalized by rated PV capacity.
Reported as: monthly average hours per day per rated PV capacity.
Units: h/day/kWp of rated capacity.
Calculated by: Net PV system production (kWh/day)/rated PV capacity (kWp)
3.10.3. Equivalent annual hours of peak rated PV production
The annual PV production normalized by rated PV capacity.
Reported as: annual thousands of hours per year per rated PV capacity.
Units: 1000 h/year/Wp of rated capacity.
Calculated by: Net PV system production (kWh/year)/rated PV capacity (Wp)
3.10.4. Facility electrical load offset by PV production
The net electricity produced by the PV system compared to the total facility electricity use.
Reported as: percentage of annual and monthly electrical facility load met by the PV system.
Units: nondimensional value expressed as a percentage × 100(kWh/month, kWh/year).
Calculated by: Net PV system production/total facility electricity use
3.10.5. Facility total energy load met by PV production
The net electricity produced by the PV system compared to the total facility energy use. A building that produces more energy than it uses would result in 100% or greater total facility energy load met by the PV system.
Reported as: percentage of annual and monthly total facility energy load met by the PV system.
Calculated by: Net PV system production/total facility energy use
3.10.6. Total electricity delivered to utility
When the PV system produces more AC electricity than is used at the facility, the excess production typically will be exported to the utility grid.
Reported as: monthly and annual total electricity that is exported to the utility grid.
Units: kWh/month, kWh/year.
3.10.7. Total incident solar radiation
The solar radiation in the plane of the solar array. It is calculated by summing the time-series solar radiation flux per unit area and multiplying by the PV array area.
Reported as: annual and monthly total incident solar radiation.
Units: kWh/year, kWh/month.
Calculated by: PV array area/total incident solar radiation
3.10.8. PV system AC electricity generation effectiveness
The time-series, monthly, and annual effectiveness of the PV system in converting incident solar resources to AC electricity used in the building or exported to the grid.
Calculated by: Net PV system production/total incident solar radiation
3.10.9. PV system performance ratio
The ratio of the daily, monthly, and annual PV system AC electricity generation effectiveness to the rated PV module efficiency. The performance ratio is a PV system metric that is normalized by both PV system capacity and incident solar radiation. The performance ratio can indicate the overall effect of losses on the rated PV capacity due to system inefficiencies such as cell temperature effects.
Reported as: daily, monthly, and annual average performance ratio.
Calculated by: Rated PV module efficiency (at standard test conditions)/PV system AC electricity generation effectiveness
3.10.10. Reduction of peak demand resulting from the PV system
The monthly peak demand reduction that resulted from the PV system supplying AC electricity to the facility. If a building electrical system does not feature demand-responsive controls, it is possible and straightforward to measure the demand reduction afforded by the PV system.
Reported as: monthly demand reduction resulting from PV system.
Units: kW or kVA.
Calculated by monthly values of: Peak demand of total facility electricity use without PV system (kW or kVA) − peak demand of net facility electricity use (kW or kVA)
3.10.11. Energy cost savings resulting from the PV system
The energy cost savings that are a result of the PV system supplying useful electricity to the building. The energy cost saving is the difference between the calculated energy costs without the PV system and the actual utility bills.
Reported as: monthly and annual facility electricity cost savings accruing from the PV system.
Units: $/month, $/year.
Calculated as: Facility electricity costs without PV system − facility electricity costs
3.11. Advantages of solar energy
Solar energy is seen as most reliable source of energy among all renewable energy sources. It has the following advantages.
3.11.1. Price saving and earning
The installation of a solar PV system saves on electricity and other energy bills and can generate money if supplied to the grid.
3.11.2. Energy independence
The consumer receives full energy independence by using rooftop solar PVs.
3.11.3. Jobs and economy
Due to the large integration of solar systems, new industries are setup, which creates new job and strengthening the economy of the country.
3.11.4. Security
The guarantee of energy security is very high, which is a major advantage.
3.12. Disadvantage
The disadvantage is that Sun is not available 24 h. So during times when there is no Sun, alternative arrangements need to be made.
3.13. Summary
This chapter provided the fundamentals of solar energy application in terms of passive and active (PV) solar energy systems. Details about the working principle of the PV system, its dependence on ambient conditions, power profile, and evaluation parameters were given and discussed. PV system integration was also described. The advantages and disadvantages of solar energy were deliberated.
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