METHODOLOGY

This section will evaluate the efficiency of a single-phase inverter in a solar power system. These inverter has been widely evaluated in terms of performance but little has been done with regards to its performance at low power. The design will provide the structure and easement framework in MATLAB an PSICE.

Solar Power Converter

Various converter technologies are used in different applications with the aim of reducing CO2 emissions from the power sector. High voltage converters of at least 15 kWh rating are required in power system to ensure seamless operation of the utilizes (25). EV converters should have a higher energy density than those of plug-in hybrid power system s since the support longer driving ranges. Although full EV and PHEVs converters are different, the European standards categorize them in the same group, class 3, since they must satisfy the high-energy demand to propel cars. The common feature of these converters is high energy densities (23)(25). Also, the converters have a fast charging capability to reduce the recharging duration and increase the range with a quick recharge. The converters have a low internal resistance and this increases their lifespan although manufacturers aim at making them last the car’s lifetime.

There are various requirements for EV converters. The voltage range for all full electric power systems is 250-500V and up to 800V for electric buses (25). The converters should have high energy content for extended driving. The electric energy requirement largely depends on the power system. Assuming a power system weighs 1 metric ton, the minimum converter storage capability required is 14kWh for a range of 100km. A similar hybrid power system requires 10kWh converter capacity for a distance of 60km. In relation to discharge power, a mid-size EV or PHEV requires 100 kW of power performance. The recharge power of a full electric power system can reach up to 50 kW depending the car’s ability to absorb energy from the braking system during deceleration.  EVs converters have high turnover capacity since they are involved in driving the electric drive system. The table below outlines performance profiles of common EV converters.

Lead- Based
NiMH
Lithium-ion
NaniCl2

Power density

at 25oC, 10 sec

300 W/kg

1.000 W/I

150 W/kg

440 W/I

300 W/kg

750 W/I

180 W/kg

270 W/I

Energy density at 25oC, 5h
30-35 Wh/kg

70-110 Wh/I

66 Wh/kg

180 Wh/I

110 Wh/kg

275 Wh/I

120 Wh/kg

185 Wh/I

Self-discharge rate at 20oC

~3% monthly
~15-20% monthly
~5% monthly
No self discharge although slightly dependent on the converter application

Optimal ambient T/oC
0 to +40oC
-10 to +45oC
-10 to +25oC
-40oC to +60oC at converter level

Operating ambient T/oC
-30 to +75oC
-10 to +45oC
-25 to +50oC
270oC to 350oC at cell level

Operational lifetime
3-8 years without active cooling
8-10 years for HEV application with active cooling
European automakers qualify industrial applications for 10 years

Cost EV
100-250$/kWh

10-25$/kW

400-500$/kWh

910-1140$/kW

300-450 $/kWh

100-200 $/kW

500-700$/kWh

750-1060$/kW

Cost PHEV

800-1200$/kWh

30-75$/kW

Improving Inverter Efficiency at Low Power Using a Reduced Switching Frequency

Operation of solar converters

EVs converters have positive electrodes that have high potential and negative electrodes that have low potential. The electrolyte between the electrodes allows the flow of ions but limits the flow of electricity. When the converter is connected to a power source, the positive electrode becomes the anode at reduction reactions take place at this terminal while the negative electrode is the cathode and oxidation reactions take place at this terminal (3)(6). During discharging, the process is reversed and the positive electrode becomes cathode while the negative electrode becomes anode. In converters with sealed cells, a separator holds the liquid electrolyte to prevent a short circuit. The separator also contains extra electrolyte and has space to facilitate expansion of the electrolyte (27). In NiMH batteries, the separator serves as an ammonia trap and in Li-ion batteries, it prevents formation of Li-dendrite. A schematic of NiMH converter is illustrated below.

Concept of operation of Solar Converters

Solar cells are made up of semiconductor materials such as silicon. Silicon is naturally abundant and makes the PV module environment-friendly. The photovoltaic (PV) cells contain at least two semiconductor layers, one positive and the other negatively charged. The two semiconductors are doped with the desired element to make them either p-type (positive) or n-type (negative). They are joined together to produce a p-n junction. When an external load is applied across the p and n layers, an electrical current flows across the load because of the flow of electrons. Sunlight consists of tiny energy capsules called photons, whose number depends on the intensity of solar radiation and their energy content in the wavelength band (30).

When the sunlight strikes the PV surface, three possibilities can occur: it can get reflected, can permeate the cell, or can be absorbed by the solar cell. When the negative layer of the p-n junction absorbs enough photons, the n-type material frees electrons. The required number of photons should be to such intensity that their energy is greater than the Fermi level of the material so as to dislodge the electrons for the photovoltaic effect to take place. The freed electrons from the n-layer migrate to the positive layer creating a voltage differential like that of a battery. This fete is achieved through the electron-hole combination in the p-type layer.

Connecting the two layers to an external load such as an electric lamp completes the circuit, causing the electrons to flow through the circuit generating electricity. When there is no illumination, the current that flows from the p-type layer to the n-type layer (in the reverse direction to the flow of electrons), is known as the dark current. The solar cell has the capacity to produce between 1-2 watts of electricity from solar energy. The intention of using solar energy is to provide enough electricity to run a given load, and this can be achieved by combining several solar cells into a module, and the modules can then be connected in series or parallel into a solar array. The combination of energy from each of the solar cells in the array yields the required amount of electrical power.

Efficiency in the solar cell and the converter -Improving Inverter Efficiency at Low Power Using a Reduced Switching Frequency Essay

The conversion efficiency of a solar photovoltaic cell can be defined as the percentage of solar energy that is converted into useable electrical energy when it shines on a PV device. The conversion efficiency depends on four factors namely wavelength of light, recombination of charge carriers, temperature, and reflection (32). Photons have a range of wavelengths of operation corresponding to different energy levels. The solar spectrum that reaches the earth’s surface has the regions known as ultraviolet, visible light, and infrared region. Different wavelengths of the solar energy are reflected at different angles. Therefore, when the solar light strikes the surface, some photons are reflected, some pass through the material while the rest are absorbed by the PV module. The energy of some of the absorbed photons of solar energy are converted to heat and are thus wasted, while the other option has enough energy to dislodge the electrons from the n-type material and produce electric current. From this explanation, we can see how that most of the solar energy is not converted into usable electrical energy.

Electrical energy can only flow when the charge carriers, primarily electrons and holes, are able to flow across the material. Holes denote the absence of electrons and represent positive charge carriers. When an electron meets a hole, they can recombine and cancel the production of electric current. In direct combination, the electrons and holes generated by light can meet and recombine to emit a photon, which is the opposite of how electrical energy is produced by a solar cell. This phenomenon greatly limits the efficiency of a solar cell. Indirect recombination involves the electrons meeting an obstacle such as an impurity, which makes them recombine with holes to release heat energy. The ideal condition is that the electrons generated from the impact of solar energy flow through the external circuit in the form of electrical current.

Temperatures also contribute to the efficiency of solar cells, which normally work best at low temperatures. When the temperatures are high, there is a shift in the properties of the semiconductor materials, which causes an increase in the amount of current produced, with a corresponding decrease in the voltage, which reduces the power output. Extremely high temperatures may damage the modules making their lifespan shorter. Hence, a solar array needs to be equipped with cooling systems to limit the effect of heat on the material and on the efficiency.

Reflection reduces the efficiency of the solar cell since an increase in the amount of light reflected limits the number of photons that can be absorbed to dislodge the electrons to produce electric current. Untreated silicon has been found to reflect over 30 percent of the light incident on its surface. Therefore, one of the ways to improve the conversion efficiency of the solar cells is to use an anti-reflective coating, which is normally dark blue or black in color.

From a scientific perspective, the efficiency of the solar cells can be obtained by examining the current-voltage curves plotted under certain temperature and illumination conditions. From the curve, the values of the short circuit current (Isc) and the open circuit voltage (Voc) are obtained (no load conditions). The efficiency of the solar cell can be obtained as a ratio of the product between the current and voltage produced when a load is connected across the solar cell to the product of the incident solar flux (Is) and the area of the solar cell (Ac) [30]. We can practically obtain the efficiency by taking the values when the cell is exposed to a constant and standard level of light while maintaining a constant temperature.

Single-Phase Standalone Converters