1.2 Efficiency of Conversion
Efficiency is a very important metric in energy conversion. It is most commonly used for evaluating and comparing various methods and devices in terms of technical performance, which is, in turn, related to cost of the technology. The efficiency concept is frequently used in cost estimates and commercial decision making. So, we should spend some time refreshing our basic understanding of the efficiency as a universal metric of conversion systems.
Please refer to this Efficiency of Energy Conversion book chapter, and refresh your basic knowledge of the efficiency definition and use. This text uses a number of simple efficiency calculation examples related to traditional fuel systems. I encourage you to learn from those, and then we will see how the same approach may apply to solar energy systems and devices.
Based on this reading, can you answer the following questions?
Check Your Understanding - Question 1
If an electric motor consumes 150 W of electrical power to produce 120 W of mechanical power, what is the efficiency of this device?
Check Your Understanding - Question 2
How would you determine the energy conversion efficiency of a power plant that consists of three conversion sub-systems with efficiencies η1, η2, and η3, respectively?
Check Your Understanding - Question 3
A light bulb converts electric energy to light and heat. Can you estimate efficiency of a 40 W light bulb emitting 950 lumens of light energy (assume 1 lumen equivalent to 0.001496 W of power)?
We see that efficiency of conversion, η, is a key metric of system performance. When applied to solar energy conversion systems, efficiency of solar energy conversion would be defined as the ratio of the useful output power (delivered by the conversion device) to the incident power (of the solar radiation):
We can answer the following questions from the efficiency analysis:
- What fraction of available energy is lost in the conversion?
- How one device is compared to another?
- What is the performance limit?
PV efficiency measurements
When the efficiency is compared for different types of photovoltaic (PV) cells, we need to make sure that conditions under which the cells are operating are standardized, so that any difference in cell performance is due to the properties of materials and design and not due to the variability of external factors. The nominal efficiency of PV devices is measured at standard conditions [ASTM G173 guide]:
- Air temperature 25°C
- Solar irradiance of 1000 W/m2 (clear sky)
- Air mass (AM) of 1.5G
- Cell (panel) oriented perpendicular to the light beam
When the external conditions are kept constant, measured efficiency is solely a device characteristic. To determine efficiency experimentally, we need to measure both the solar irradiance and the power of the cell.
For your notes: [Solar Conversion Efficiency Cheat Sheet].
Example of Efficiency Calculation
Generally, to estimate the efficiency of solar energy conversion, you would need:
- solar irradiance data, and
- performance data
Consider the example below, which shows estimation of the standard efficiency of a PV module.
Standard solar input (irradiance) at the module surface: S = 1000 W/m2
Identifying power input to the PV cell:
Pin = S =1000 W/m2
Identifying power output from the PV cell:
Pout = E × I / Area = 112.5 W/m2 (Note: from physics, power is equal voltage times current)
Then, for efficiency, we can write:
η = Pout / Pin = 112.5 / 1000 ×100% = 11.25%
Conclusion: only 11.25% of energy flowing to this panel is converted to electricity.
The reason that energy conversion systems have less than 100% efficiency is that there are losses. The origin of those losses can be a complex issue, which could be better understood based on the physics and design of a particular conversion device – PV cell, concentrator, or thermal collector. We will get back to those considerations when talking about specific conversion technologies in detail in respective lessons of this course.
Quality and quantity of solar conversion
There is an important distinction between the total power (measured in Watts) and power density or flux (measured in W/m2). When we talk about the performance of a particular solar energy conversion device (for example, a solar cell), power density characterizes the "quality" of the energy conversion - how much power is generated by each square foot or square meter of the PV cell area. That may depend on properties of the cell material, design, and physical principles behind the conversion process. In contrast, the total power reflects the overall output - the "quantity" of usable energy generated by the whole device per unit of time. In applications of solar energy (say, if we want to power a building), we often look at the total wattage of the system and ways to maximize that total "quantity" of energy supply.
For example, imagine a solar module. At a particular moment of operation, the output power of the device can be expressed as
Pout (total) = Pin(total) (η/100%) = S A (η/100%)
- η = efficiency (%)
- S = sunlight power density (irradiance) at the cell surface (W/m2)
- A = total cell area (m2)
Logically, to increase the total output from that module, we need to either increase the efficiency or increase the total input power.
The avenue of raising cell efficiency leads us to the physics of the conversion process, materials properties, and cell design. The main research and development question here is how to make a better working cell.
The avenue of increasing the total input power lead us to three issues: (i) concentration of light, (ii) sun tracking, and (iii) system scale-up. Concentrating the ambient incident light would indeed increase the amount of energy supplied to the module per unit of time via increasing the S parameter in the above equation. Tracking - i.e., the orientation of the solar panel perpendicular to the sunlight beam - is another way to maximize the amount of absorbable radiation and also contributes to increasing the S parameter. Finally, increasing the size of the module by adding more cells to the system, increasing cell area, or multiplying modules (scale-up) would increase the total active area of conversion (A).
The technology scale-up is the way to match the solar power to commercial applications and consumer's needs. The utility-scale solar power, which is the primary focus of this course, is discussed in the next section.