Tag Archives: PV electric

Solar PhotoVoltaic Primer

The cost of photovoltaic systems (panels and inverter) has dropped to about 2 to 3 dollars per watt. At this price systems have payback times in the 10 to 15 year range, regardless of size. This assumes a cost of about 10 cents a kilowatt hour (kW-hr) for electricity.

Here are a number of nuts and bolts issues for those interested in solar power. First and foremost you must have a location with southern exposure. Even a small amount of shade can seriously reduce energy production. For most this means a roof top location, but it needn’t be if you have the space to put the array on the ground. The simplest mounting puts the panels flat on the roof. The pitch of the roof is not all that important as long as it faces south.

sun's path

sun’s path

The amount of space needed for an array of course varies as to how much total power you want to produce. Different manufacturers make panels in different sizes (watts) but the total space needed is the same because all PV panels have the same efficiency, about 15 %. Five 100 watt panels will take up the same space as one 500 watt panel. One kW requires about 80 square feet of space.

A big decision is whether the array is isolated or connected to the electrical grid. Grid-tied systems here in Arkansas can take advantage of net metering. This means that the power produced by the panels can actually make a meter run backwards if they are producing more power than the home is consuming at any time. About the only disadvantage of a grid-tied system is that when the line goes down, so does the solar power production. This is necessary to protect power line workers.

PV Grid-tied system

PV Grid-tied system

The alternative to grid-tied is to go entirely off line by buffering production with batteries. This avoids the aforementioned problem, but greatly increases the cost and “hassle factor” of the system. This is only practical when connection to the grid is cost prohibitive, as in remote locations.

The total amount of energy produced by a system is obtained by the total wattage of a system. For example a 1 kilowatt system can produce a maximum of one kilowatt hour only when the sun angle is ideal. Averaged over a year, a simple rule of thumb is that you can get 4 hours of net production per day. Hence a 1 kW system can be expected to produce 4 kW-hrs per day, more some days, less others.

Let’s use an average consumption of 1000 kW-hrs per month (close to the average in Arkansas) to determined a system sized to replace 100 % of electric needs. 1000 kW-hrs per month means 33 kw-hrs per day. Divide that by 4 to get a a little over 8 kW system. To allow for some inefficiencies say we use a 9 kW system. At 2.5 dollars a watt, the total cost would be 22,500 $. The 30% federal tax rebate brings the final cost down to 15,750 $. Sales taxes and installation will add to the cost, but these numbers can be used to approximate a cost if you are interested in going solar.

Energy Storage

The success of transitioning to sustainable energy supplies in the United States relies to a large degree on our ability to store energy produced by intermittent energy sources such as solar, wind and biomass. We have plenty sunlight and wind to go around. Conversion of biomass to a liquid or gaseous fuel is a convenient method for storing energy, but photosynthesis is quite inefficient compared to other ways of capturing solar energy. Also any biomass to energy scheme will involve burning something which always has some negative health consequences.

The future could be powered by electricity from solar and wind exclusively but how will we store the electricity for use when the sun isn’t shining or the wind isn’t blowing? Batteries are an obvious way of storing energy but are impractical for storing energy on the scale of an electric utility.

grid scale batteries

grid scale batteries

Batteries for powering transportation are in use now and will expand greatly in the future.

Most electric cars today use Lithium ion batteries. They have the best energy to volume and energy to weight ratios referred to as energy density. The problem is that even the best batteries pale in comparison to the energy density of gasoline. Liquid fossil fuels like diesel and gasoline are very energy dense and can produce 50 times as much energy as a Lithium ion battery of equal weight or volume. With current technology the Nissan Leaf, an all electric vehicle, has a range of under one hundred miles. Batteries being developed now can increase the energy density by five to ten fold, giving electric cars a range of several hundred miles.

Storing energy for the electrical grid can accommodate a wider range of methods. One of the simplest ways of storing energy is to pump water up a hill.

Pumped storage

Pumped storage

All you need are two reservoirs, one higher than the other. When energy is available it is used to pump the water to the upper reservoir. When energy is needed, the water is released, causing the turbines to reverse direction and generate rather than consume energy. The only limitation is space and geographic relief.

Another utility scale energy storage method being examined is compressed air. Just as pumping water up a hill stores energy, so does compressing a gas. In the latter part of the nineteenth century, several European cities used compressed air for energy storage. Rather than convert the energy in the compressed air to electricity, it was piped and metered to do mechanical work. Everything from motors for heavy industry to sewing machines ran on the compressed air. The major limitation of compressed air storage is the necessity of a large underground reservoir to hold the compressed air. Wind turbines in the midwest will in the future store energy with compressed air.

compressed air

compressed air

Flywheels are another method to store energy. An electric motor spins up the flywheel, later when energy is needed the motion is used to power a generator. The big advantage of flywheel storage is that it can be done anywhere. No need for a big hole in the ground or pairs of reservoirs at different altitudes.

One of the best methods to pair production and storage of energy is solar thermal. Simply heat a fluid with sunlight. When electrical energy is needed use the heat to power a generator. Power towers have a collection of mirrors pointed at the top of a tower. A fluid is circulated through the heated zone, then sent to a storage site for later extraction of energy.

All these techniques involve converting one form of energy to another, but can ultimately be used to generate electricity even when the wind isn’t blowing and sun isn’t shining.

large solar array

Solar Steel

People often think that solar photovoltaic panels are OK to put on a roof to cut ones electric bill a little but really doesn’t go far to fill the needs of the nation when it comes to electricity. Or that it’s OK for light weight usages like lighting in parking lots but can’t provide for heavy industries like steel mills. I would like to disabuse those folks of the idea that solar can’t keep us going.

First some fundamentals. Electrical energy is measured in Watt-hours (Wh) or multiples there of. If your monthly electric bill is about a hundred dollars, close to the Arkansas average, you are using a MegaWatt-hour (MWh), which is a million times a Watt-hour. This amount of electricity is available year around from a space about twenty seven feet on a side. It easily fits on a south facing roof. A system like this will not just lower your bill but eliminate it.

Let’s talk about power for heavy industry, and it doesn’t get much heavier than steel mills. Nucor Corporation operates twenty-three steel mills

electric arc furnace

electric arc furnace

across the United States producing twenty-two million tons of steel annually employing electric arc furnaces. If we can figure out how to do this with solar panels we can do anything.

It takes about one and a half Mwh electric to produce a ton of steel. On average each plant produces a million tons of steel a year, so we need one and a half TeraWatt-hours;



a TeraWatt is a million times a MegaWatt. How much land do we need per plant? It works out to one thousand five hundred acres. This is equal to the land use of less than four average farms in Arkansas. That’s it. The land occupied by four farms in Arkansas will provide enough sunlight to power a steel mill. Cool, huh?

When you look at total electric use in the United States over a year the numbers get really big. The national annual electric use is four PetaWatt-hours; a PetaWatt is a billion times a MegaWatt. So how much land would it take to generate all the electric power we use in the United States? A surprisingly small nine thousand square miles. This is an area smaller than Rhode Island.

The numbers I cite are good for the amount of sunlight in Arkansas using flat plate collectors. If the national power grid originated in Nevada using tracking panels, the area needed is less than five thousand square miles. There are counties in Nevada much larger than that. There is no question that sunlight alone can provide all the electric power we need in this country.

The obvious fly in the ointment is the need for storage when the sun doesn’t shine, or transmission to where the sun doesn’t shine, but both those limitations are under study and are an achievable goal in the near future. And that’s just solar Photovoltaics as an energy source. That amount of energy is available from wind turbines and the potential for geothermal is greater still.


What about China?

There is a consensus among virtually all scientists that humans, by burning fossil fuels, are contributing to global warming and thus changing the climate. The climate has changed many times over the billions of years of earth’s existence, so what if we are changing it, why does it matter?

It matters because we are changing the climate on a timescale never seen before; hundreds, even thousands of times faster than any naturally occurring climate change. We are changing the climate at a rate which can cause massive extinctions as plants and animals fail to adapt.

The only viable solution is to stop burning fossil fuels. Coal, oil and natural gas represent carbon that was removed from the atmosphere over hundreds of millions of year. We are burning up these fuels at a prodigious rate, returning all that carbon to the atmosphere over a couple of hundred years. This has resulted in much, much higher concentrations of heat trapping gases and particulates in the atmosphere. Additionally we are making the oceans much more acidic.

We have to stop! We have to decarbonize as quickly as possible. We have started but only by baby steps. The fastest growing carbon free alternative for producing electricity in the US is wind power, which has increases by thirty per cent over the last five years. That’s the good news, the bad news is that that represents less than three percent of our total production.

Our solar electric production has increased by a phenomenal five hundred per cent, but has further to go with only a tiny fraction of one per cent of total electric production. We have a long, long way to go. And there are impediments. One argument against abandoning fossil fuels is that we will be at a competitive disadvantage with other countries that continue to rely on fossil fuels.

So what are our economic competitors doing? What about China? If the objective is to limit carbon release to the atmosphere but China isn’t why should we? And India, if India is still polluting, why do we have to stop? You know in some childish, schoolyard way I guess that makes sense. But we need to be adults about this. We need to provide the global leadership to show the world how it can and should be done.

The US consumes close to one quarter of the world’s resources, yet we constitute a bare five per cent of the global population. It shouldn’t be a matter of what others are doing, but what we need to do to get our house in order.

Actually the “what about China” question is an UH-OH. China is already the world leader in wind power; growing by leaps and bounds, twice as fast as the US over the past five years. How about wind generation as a fraction of total energy production?



Denmark beats us by an order of magnitude, with over twenty per cent of total electric production from wind.

We are similarly behind for solar electric. China is the world leader in the production of Photovoltaic panels, and Germany leads in per capita production, over twenty times the US ratio.German-Solar-Houses

We still have the largest economy in the world and if we were to invest in renewables we could be a world leader in preparing for a carbon-free future. Think American Exceptionalism.

Biofuel is Inefficient

The United States attained the position of a superpower to a very large degree by our ability to utilize fossil fuels. Our way of life requires burning massive amounts of those fossil fuels. The wastes released by burning these fuels is leading to global warming and ocean acidification. If we want to preserve any semblance of a natural environment on this planet we must stop.

To maintain our lifestyle we have to adopt energy production systems that are free from carbon pollution and have long term sustainability. Direct solar, wind, and biofuels derived from crops are three strategies being exploited on a small scale already.

These three energy sources all derive from the sun but are they of equal efficiency? The short answer is NO, in capital letters. Not only are biofuels very inefficient in terms of land use, but also compete with food crops for acreage, fertilizer, and water.

Although the direct tax credits for biofuels like Ethanol and Biodiesel have been discontinued, we continue to subsidize these energy sources by crop price supports and mandates for biofuel use. This is certainly good for agribusiness, but is it good for society?

Consider the productivity of Ethanol from corn. In the United States, we use about half the corn we grow for ethanol production, roughly 50 million acres per year. For this we get 3 billion gallons of gasoline equivalent from ethanol. The problem is that we use over 130 billion gallons of gasoline a year. If we put every arable acre of land in the country in corn (580 million acres), we still would only be able to produce less than half of the fuel we need.

And we would have nothing to eat! The problem with biofuel is that photosynthetic efficiency is very low. That’s why it took hundreds of millions of years to accumulate the fossil fuels were are now consuming.

Of course, there are alternatives to biofuel.

wind turbines

wind turbines

If that same land area is used for wind turbines, solar thermal or photovoltaic applications, much more energy can be harvested. The 60 gallons gasoline equivalent per acre from corn ethanol represents less than 2000 kilowatt-hours per acre per year. Dedicate that same land mass to wind turbines with “good” winds and you get 130,000 kilowatt-hours per acre per year. And the land beneath the wind farm is still available for crops or pasture.

Photovoltaic systems are even more productive.rooftop_PV Virtually anywhere in the US, 800,000 kilowatt-hours per acre per year is attainable with current technology, That is 400 times as efficient as corn ethanol. We don’t need cropland, we can do it on our roofs. We get to eat.

In summary, photosynthesis is a very poor choice when it comes to energy production because it is so inefficient and it competes with food crops for land and water. Solar energy production methods such as photovoltaics and wind with current technology can sustainably power our future, now.

5.4 kW grid-tied

Cost of PhotoVoltaic (PV) Systems

Because of the previous high cost of photovoltaic (PV) panels, their use has been limited to rather specialized and unique purposes. PV panels have and continue to be used to power satellites, and remote sensing facilities on earth where traditional energy sources such as electricity provided by power plants was unavailable.

In the 1970s those PV panels that were used to power satellites cost close to eighty dollars per watt. By the turn of the century the cost of panels was down to the vicinity 8 dollars a watt. And the price continues to drop.pvcost When I put in the panels to power my home in 2008 the cost was around five dollars a watt.

The current cost of PV panels is now approaching one dollar a watt for small home scale systems. This results in a system payback time of about ten years. Imagine, you pay one upfront cost for for a system, it pays itself back in about ten years, and you have free electricity for the rest of the life of the panels which is well over 25 years!

So lets talk about the nuts and bolts of a system. It consists of three components, the PV panels which produce direct current (DC) electricity, an inverter to converter the DC to Alternating Current (AC), the stuff that powers you home, and storage for when the sun isn’t shining.

If a home is already connected to the grid then storage isn’t an issue. A “grid-tie” can be used as the storage, eliminating the considerable cost of batteries. The net metering law in Arkansas allows producers to utilize a bi-directional meter. When the sun isn’t shining power is drawn from the grid, when the sun is shining the meter runs backwards, crediting the production. A system can be sized to produce as much energy as is consumed.

And it doesn’t matter how big the system is, it always pays off in ten years or less. A larger system will cost more, but produce more so the break even point is the same regardless of size.

The average home is Arkansas uses about 1000 kWhs per month and hence has an electric bill in the vicinity of 100 dollars a month. Here is an illustrative calculation for the cost of a system to totally cover the electric needs for that household. The panels for a PV system should cost about 10 grand. Add in about 2 grand for a system inverter, and another 3 grand for installation and you get a total cost of about 15,000 dollars. There is a 30 % federal tax credit, a credit not a deduction, which will then lower the cost of the system to about 10,500 dollars.

For this household the system will generate all the energy needed to offset the billed amount of electricity, saving the consumer, now a producer, 1200 dollars a year. The payback time is less than nine years. And the system will continue to produce at this rate for two or three times as long as has been paid for already.

Do you have access to the southern sky? Then your roof, or open space can be utilized to pay for your electricity, and the cost can be spread over the life of the system. In the future companies like Entergy will be mainly involved in distributing energy, rather than producing it. Production will be at home.

Power to the people.

Electric Highways and Byways

President Obama recently announced new fuel efficiency standards for cars and light trucks. By 2025 the Corporate Average Fuel Economy of these vehicles will be 55 miles per gallon of gasoline or its equivalent from other energy sources. It is unlikely that this standard can be met by sticking with the internal combustion engine. In fact the standard has encouragements built in which favor alternatives.

Foremost will be gas/electric hybrids, but plug-in hybrids and pure electric vehicles will contribute to the mix. Gas/electric hybrids such as the Prius gain efficiency and therefore higher mileage because they have batteries and electric motors to boost power when needed. The batteries are recharged from the gasoline engine when less power is required. Plug in hybrids and pure electric vehicles get some or all of their power from batteries charged from the electrical grid.

The greater simplicity and efficiency of electric motors mean that their mileage is much better than cars powered by internal combustion engines. Another advantage of electric cars is that the energy used to charge the batteries can be produced at locations remote from urban areas. This will have the effect of lowering pollution where people live and work. Overall less energy is needed to power an electric fleet of vehicles, regardless of how the energy is produced.

The significant advantage of gasoline powered cars is one of energy density and refuel time. A simple comparison is illustrious. A gasoline powered car can travel several hundred miles before refueling is necessary, and then the refueling time is only a matter of a few minutes. Currently available pure electric vehicles have a range of less that one hundred miles and recharging the batteries takes hours, not minutes. Research is ongoing to improve both the energy density and recharge times for batteries, but an alternative would be to charge the batteries on the fly.

This is the way a gas/hybrid electric works but it requires hauling a gasoline engine around with you. Ideally if you could charge the batteries as you go without a gasoline engine then the issue of battery life and recharge times becomes immaterial. We need a technological leap to get there but it should not be that difficult.

Enter microwave power transmission. Imagine a highway system with microwave broadcast antennas imbedded in the pavement. As electric cars drive over a segment of an antenna, computer controls on board would turn on the broadcast antenna, and a receiving antenna in the car would use the received power to charge batteries, essentially continuously. No power would be wasted as the broadcast antennas only function when a car is overhead and signaling to receive power. The system could begin in urban areas, extend to the interstate highways, and finally to the byways.

In all but the most remote areas, cars could still operate on batteries big enough to get them “off-grid” for reasonable distances, say a one hundred mile range. To minimize transmission losses, power could be provided from solar panels lining the highways. Or how about decking over the highways with panels? Both protect the highways and drivers from the weather and power the vehicles at the same time! I won’t go so far as to say the possibilities are endless, but there are a lot of novel ideas out there to be exploited.