Solar Energy Efficiency

The subject of solar energy efficiency is frequently discussed and comparisons are made with conventional electrical power generators efficiency. This page discusses issues that are related to the solar energy efficiency of photovoltaic solar panels and how are these issues related to residential solar power systems cost

What is energy efficiency?

Any electric power generator convert some sort of energy into electrical power. The ratio between the output electric power energy to the invested energy (input energy) is the energy efficiency of the generator; it is usually multiplied by 100 and expressed as percentage. The input power for a fossil burning power plant is expressed in BTU and the output is expressed in kWh (kilowatt-hour). Energy of 1 kWh is equal to 3412.3 BTU. Theoretically, if the input to a power plant is 3412.3 BTU and its output is 1KWh, the efficiency is 100%. However, the second law of thermodynamics says it is impossible to convert heat completely into useful work and the upper limit was set by the French scientist Carnot.
Similarly, the input to the solar panel is Sun radiation and the output is electrical power. Photovoltaic solar panels are tested under the Standard Sun Conditions. Standard Test Condition STC is under irradiation (solar radiation density) of 1000Watt/square meter (93 watts/square feet) at ambient temperature of 25 degrees C (77 degrees Fahrenheit). There is an upper theoretical limit on the achievable efficiency of photovoltaic solar cells, governed by the properties and structure of the semiconductor that is used. One will never get 1000 Watt from 1 square meter panel; he will get much less, well below 30%. If the solar energy efficiency is 15% (typical efficiency for most commercially available solar electric panels) , the 1 square meter panel can produce 150 Watts.

Energy efficiency of conventional power plants

Natural gas power plants exhibit Heat Rate of between 6,300 BTU/kWh to 8,000 BTU/kWh. 6,300 BTU/kWh translates to 3412.3/6300x100=54%, 8,000 BTU/kWh translates to 3412.3/8000x100=42.6%. Coal power plants are less efficient than natural gas power plants; their Heat Rate is between 7,700 BTU/kWh to 10,400 BTU/kWh (energy efficiency between 44.3% and 32.8%).

Where else energy is lost?

Power plants consume a tremendous amount of natural gas, coal and other fossils. These burning materials are dug, drilled transported and stored. Digging, drilling, transporting and storing are energy consuming processes.

On the output side, electrical power is transported and distributed, a lot of electrical power is wasted on the high-voltage lines, transformers and lower-voltage lines.

Environmental issues related to power plants

Not only energy is lost, power plants pollute the air and increase the carbon dioxide in the atmosphere. The environmental damage of power plants is not calculated into the price of electricity, the public health burden and the catastrophes caused by climate changes are imposed mostly on society, government budgets and insurance companies.

The capital cost of a power plant

The capital cost to build a new power plant, not including transportation infrastructure and power distribution infrastructure ranges around $0.50 per watt production for a natural gas power plant to $1.4 per watt production for a coal burning power plant. Usually, as the power production capability increases the grid (high voltage power lines, distribution line and other infrastructure elements) must expand as well.

Energy Efficiency of Solar Panels for Homes

Run of the mill solar panels for homes exhibit solar energy efficiency in the range between 12% and 18%. There is a race to develop newer technologies that will reduce the price to the user and will exhibit better efficiencies. For the homeowner what really counts is the price in terms of $ per watt production. Lower solar energy efficiency translates to a bigger size panels but most roofs can accommodate the bigger size panels.

Advantages of Solar Panels for Homes

From the perspective of an homeowner, a residential solar power has many advantages. Here are some of the advantages:

Solar panels use an otherwise unused real estate on the roof of the building.

Photovoltaic electricity production is local. The old paradigm of centralized electricity production facilities, super high voltage power lines and power distribution systems is vulnerable and somehow not efficient. This was proven again during the latest events in Japan.

Photovoltaic electricity production is clean, it does not release carbon dioxide and it is silent.

With the federal and states energy tax credit the pay on the investment is rapid. The operational cost of solar power systems are very low, once the capital investment is covered, electricity is practically free of charge

Disadvantages and Limitations of Solar Panels for Homes

Solar power systems produce electricity only when the Sun is up in the sky, the efficient hours are only 6 hours out of 24. Solar power systems are still expensive; the price however continues to drop consistently.

$ per Watt and the price of 1 kWh

The investment in utility scale power plants is smaller in terms of $ per watt production, a utility power plant can produce 24x7, the price per 1 kWh reflect the return on the investment, the cost of maintenance and operation and the ever increasing price of fossil fuels. A hidden cost lies in the fact that some of the higher operational cost power plants are operated not 24x7 but only during peak consumption hours.
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