By Béla Lipták, PE, Columnist
Solar technology began in 212 BCE when Archimedes concentrated the sun's rays to ignite Roman ships. Now our space stations are powered by solar panels and lifted into space by hydrogen peroxide engines. It was in 1861 when Auguste Mouchout created the first steam engine powered by solar energy, and in 1883 Charles Fritz first turned the sun's rays into electricity. It took almost a century until, in 1954, Calvin Fuller and his team at Bell Laboratories constructed the first silicon solar panels, which had an efficiency of 6% and a cost of $300 per watt of electricity produced.
Today, solar cell efficiency can exceed 30%, and the installed costs have dropped below $10/Wp. Wp is the peak electricity produced under "standard conditions"—equivalent to bright sun in the tropics (25 °C and light intensity of 1 kW/m2). On average, each Wp produces about 5 kWh/day. The key limitation of PV cells is that any given semiconductor only responds to a certain wavelength of radiation and can therefore convert only a portion of it to electricity. This limitation is gradually being overcome.
The cost of conventionally generated electricity in the United States averages about 10¢/kWh. In contrast, solar electricity costs around 15¢ to 17 ¢ per kWh. Once installed, the fuel (solar energy) is practically free, and the useful life of the cells range between 25 to 35 years. The energy payback period (the number of years it takes for the cells to generate the energy that was needed to manufacture them) ranges between one and four years.
As of 2010, the total global electricity generating capacity is 4800 gW, the installed solar generating capacity in only around 25 gW or 0.5%. On the other hand, solar is the fastest growing power-generation technology in the world, increasing at an annual rate of 60%. Last year the First Solar Co. ranked Number 7 on Fortune's list of the fastest growing companies, and thin-film and nano-solar technologies might even exceed that rate. Today's market share of thin-film solar cells (copper indium gallium selenide (CIGS) is approximately 15%. The commercially available thin-film solar cells have an efficiency of 11%.
The total use of renewable energy in the United States (including hydraulic and biomass) is around 10%, and the total use of solar energy is about 0.2%, while that of Portugal, for example, is nearly 50% (Figure 1). On the other hand, the trend suggests that we will be catching up in the coming years. For example, the Westland Solar project in the San Joaquin Valley will be the world's largest solar power plant. It will generate 5 gW on 30,000 acres of land. Increased use of solar energy is also motivated by factors such as the high transportation expenses of gasoline. In Afghanistan, for example, the cost of delivering gasoline is hundreds of dollars per gallon.
Solar Energy Storage and Transportation
In the coming articles, I will concentrate on the software needs for control and optimization of solar energy storage and transportation systems, giving particular emphasis to my reversible fuel cell design (http://belaliptakpe.com/solar-hydrogen-power-plant/). Some of these software packages might seem overly complex, but at a time of unmanned drones and Google's unmanned cars, they should be feasible. I will start the discussion with small units serving individual homes and smaller industrial plants.
The optimization software needed for PV systems with both storage and grid connections has to 1) provide total automation, including record-keeping, 2) provide capability to reconfigure the system to automatically maximize profitability and 3) provide self-diagnostics.
In developing the software package, the primary goal must be simplicity, so its operation does not require more understanding from the homeowner than what's needed for operating a thermostat. To provide this requires total automation, so that the system will safely operate on its own, without the need for the homeowner to take any action at all. On the other hand, the software must allow the homeowner (or preferably the electric company) to update any data or setpoint, such as changes in the price of electricity (during normal, night or peak periods, if they differ). If the home has electric car(s) charging during the night, the software package must be provided with the target amount of electricity needed to be in battery storage in the morning.
Grid Connected PV System with Net-Billing Meter
The main components of a simple installation are the solar collectors and the intelligent, bi-directional electric meter, which is furnished by the electric utility company. Such a system has no electricity storage capability. Therefore, when solar electricity is insufficient or unavailable (at night or on cloudy days), it obtains some or all of the electricity needed from the grid (red direction of electricity flow in Figure 2). Inversely, when excess solar energy is available (beyond the needs of the household) it is sent to the grid (green direction in Figure 2).
Billing is done on the basis of net electricity used (or generated) during the month. This net amount is a function of the size of the solar collectors, the insolation in the area, the weather conditions and other factors (dirt build-up or collector maintenance, etc.)