Archives October 2007

After reviewing some of the details of Honda’s experimental solar-power hydrogen refueling station in Torrance, CA and its fuel cell vehicle several questions concerning efficiency and practicality come to mind. It most be noted that solar and hydrogen don’t emit harmful byproducts such as carbon dioxide or carbon monoxide so both technologies are important to our energy security. First let’s look at the efficiency of hydrogen and second the efficiency of generating hydrogen from solar.

As we learned from science class, hydrogen is the most abundant element in the universe. Hydrogen has approximately 3 times the energy per unit mass as gasoline and requires about 4 times the storage volume for a given amount of energy according to a Hydrogen Energy report from Stanford University. In further review of additional information on hydrogen we are also making some adjustments to our fuel-ranking table.

We are revising Table 1 that was used in our post of October 3, 2007 for data on the energy density for hydrogen from 2.5 kilowatt-hours (KWH) per gallon to 10.1 KWH/gal and is reflected in the revised Table 1 below. The discrepancy lies in measuring the weight of hydrogen in liquid volume. We are calculating the energy density of hydrogen using the high heat values of hydrogen of 61,000 British Thermal Units (BTUs) and a weight 0.57 pounds per gallon from the Stanford Hydrogen Report.

As a reference: 1 KWH = 3,600 kilojoules = 3,412 BTUs

Revised Table 1 Specific Energy, Energy Density & CO2

Hydrogen offers tremendous energy potential, but as we see from Table 1, hydrogen has a low energy density meaning it requires a large storage container to make it practical for use in a motor vehicle. Several car manufacturers including GM and Toyota have developed hydrogen vehicles. Hydrogen can be used in internal combustion engines replacing gasoline or in fuel cells to generate electric to power the vehicle. However, there are some limitations to the current technology that may limit the economic viability hydrogen powered vehicles in the near term. But there are no detrimental emissions with hydrogen as apposed to hydrocarbon fuels thus providing tremendous benefits as vehicle efficiency improves.

Honda’s solar-powered hydrogen fueling station takes nearly a week in sun to produce enough hydrogen to power Honda’s FCX concept hydrogen fuel cell vehicle. Honda employs a Proton Exchange Membrane Fuel Cell (PEMFC) that converts hydrogen to electric that in turn, powers the vehicle. The Honda FCX fuel cell vehicle has two fuel tanks that can be filled with up to 156.6 liters of hydrogen or about 43 gallons that offers 430km (267 miles) driving range. The hydrogen fuel cell vehicle provides a reasonable driving range, but with a fuel efficiency of 6.5 miles per gallon (MPG), suggests more research is needed.

BMW’s Hydrogen 7 can travel 125 miles on hydrogen and 300 on gasoline before refueling. In tests the BMW 745h liquid-hydrogen test vehicle has 75 kg tank has a Hydrogen Fuel Efficiency of 10 km/liter or about 25.2 MPG and cruising speed of 110 MPH. Not too bad for an internal combustion engine that is able to run on gasoline or hydrogen.

Figure 1 Specific Energy

Given the changes to hydrogen’s energy density we are also adjusting hydrogen density (Figure 2) to reflect liquid hydrogen and high-energy value as noted by Hydrogen Properties College of the Desert.

Revised Figure 2 Energy Density: KWH per Gallon

We still have more questions given hydrogen’s very high specific energy, (3 times that of gasoline) and low energy density (4 times the volume of gasoline). Hydrogen is more efficient then petroleum fuel, yet when used as a fuel cell in a vehicle Honda’s MPG of 6.5 MPG is quite low. The fuel efficiency of BMW’s Hydrogen 7 of 25.2 MPG is only at parity with gasoline.

The efficiency of using solar energy to generate hydrogen may not be the most efficient method. One report from Walt Pyle, Jim Healy, and Reynaldo Cortez Solar Hydrogen Production by Electrolysis indicated that a 1-kilowatt solar photovoltaic device could generate 1 cubic meter of hydrogen in 5.9 hours. Essentially, 5.9 KWHs from a 1KW solar cell produces 1 cubic meter of hydrogen. We know that a pound of hydrogen in liquid state equals approximately 61,000 BTUs (51,500 BTUs at low heat value) or 17.9 KWH.

Research at Caltech, suggests that photoelectrochemistry The Lewis Group may offer a more efficient means of generating hydrogen. We will continue to explore solar efficiency and hydrogen fuel cells to evaluate the economics of alternative energy.

The bottom line is that our dependence on foreign oil is the biggest threat to national security and without cultivation of alternative energies we continue to endure an untenable situation. Further research into solar and hydrogen fuel cells could significantly ameliorate our dependence on oil.

Despite efforts that have enabled the U.S. to limit its demand for oil, world oil demand is up significantly. Advances in technology such as solar energy and vehicle fuel cell could help the world reduce its dependence on oil.

Figure 1 Oil and Gold Prices

The U.S. Department of Energy (DOE) and the U.S. Environmental Protection Agency (EPA) today released the Fuel Economy Guide for 2008 model year vehicles Fuel Economy Leaders: 2008 Model Year Coming in first place is the Toyota Prius (hybrid-electric) with city/highway miles per gallon (MPG) of 48/45. With higher fuel costs more people are factoring in fuel efficiency into their purchase decision. However, it is the purchase of pickup trucks and SUV that account for most of the vehicle purchases in the U.S. and these vehicles are dramatically less fuel-efficient than hybrids and small four-cylinder automobiles.

Despite the trend towards larger vehicles, the U.S is not experiencing a rapid rise in oil demand. Yet oil prices continue to climb. While geopolitical risk may account for the bulk of the recent price increase, latest information from the U.S. Energy Information Administration (EIA) Total Petroleum Consumption shows increasing oil demand from China.

Figure 2 Oil Demand: U.S. and China

Figure 2 illustrates that while oil demand in the U.S. has grown only modestly since 2000, the growth in China’s oil demand is rising rapidly. The recent data from the EIA shows oil demand through Q2/07. The demand for oil in the U.S. is up 5% from 2000 while in China oil demand is up 59% over the same period.

Improving vehicle fuel efficiency may abate rapidly rising oil demand in the U.S., but more emphasis on diesel and hybrids could take us a lot further. For example, Toyota has been slow to introduce its diesel line of pickup trucks in the U.S. while it offers a broad line of more fuel-efficient vehicle outside the U.S. Toyota offers several cars and trucks in Europe with impressively high fuel efficiencies that are not available in the U.S. Infact, the Toyota Hilux two-wheel drive pickup truck offers a four-cylinder diesel engine with an MPG of 44.8 on the highway and 29.1 in the city.

We are also seeing progress on fuel cell vehicles that could ultimately ameliorate are demand for oil, if not eliminate it entirely, all with no carbon dioxide or other emissions. We see most major automakers developing hydrogen powered fuel cell vehicles. Honda for one has the right concept in employing solar energy to make hydrogen.

Honda’s experimental hydrogen refueling station in Torrance, CA increases the solar incre3ases the efficiency of hydrogen fuel by using solar energy to produce hydrogen. The hydrogen is then used to power Honda’s Honda’s FCX concept hydrogen fuel cell vehicle with the only emission being pure water vapor. These fuel cell vehicles may not be ready for prime time, they provide a clear reality to what is achievable.

The bottom line is that supply and demand dictate price and the availability of cheap oil is on the decline. Further research into solar and hydrogen fuel cells could significantly change our dependence on oil.

With the Solar Decathlon is taking place in Washington DC today promoting commercially available solar energy technology and oil prices pushing to an all time high of $84/barrel, where are we in terms of cost parity with hydrocarbon fuels. Advances in solar energy technology, lower production costs, and rapidly rising oil costs could bring solar energy into parity with hydrocarbon fuels within five years.

The cost of solar energy remains prohibitively high at about $0.45 per kilowatt-hour (KWH) for an installed system. (a kilowatt-hour equals a 100-watt light in operations for 10 hours and equals 3,412.14 British Thermal Units – BTU) As a reminder, there is significant variance in cost per KWH ($0.23-to-$0.68) because a solar energy system is dependent upon daily available sunlight, which varies from about two hours a day in Alaska to almost six hours in Arizona. Solar energy costs are based on data from solar photovoltaic (PV) supplies SunPower (SPWR) and Sharp Solar.

In comparison to hydrocarbon fuels, one gallon of gasoline has an energy value equal to approximately 125,000 BTUs and cost around $3.00/gallon. The energy value of a gallon of gasoline equates to 36.6 KWH, therefore its price is about $0.08 per KWH. So there is significant disparity between gasoline at $0.08/KWH and solar energy at $0.45.

Fortunately, there are a number of initiatives seeking to improve the economics of solar energy. When we measure the cost of solar energy in dollars per watt we find a complete solar energy systems cost around $8.90/watt. (If we amortize the $8.90/watt over a twenty year expected life of a solar energy system, it amounts to $0.45/KWH) To improve the economic value of solar energy we can lower production costs and/or improve solar efficiency in terms of raising the KWH of electric produced per solar panel.

From the Lewis Group. at Caltech, we learned about several technological approaches to improving solar energy efficiency. Among some of the leading solar technologies are multi-junction semiconductor cells and solar concentrators. In a multi-junction cell, individual cells are made of layers, where each layer captures part of the sunlight passing through the cell. One of the key limits to solar efficiency are the semiconductor band gaps where excess solar energy is lost to heat instead of being converted to electric. Solar concentrators amplify available sunlight and increase the watts per square meter per solar panel.

Last year the Department of Energy reported that solar energy achieved a new record in efficiency performance. The news release, New World Record Achieved in Solar Cell Technology. suggesting that solar concentrators and multi-junction PV cells could surpass the Shockley-Queisser single-junction solar efficiency limit of approximately 31% for single layer solar cell. The multi-junction, concentrator solar cell produced by Boeing-Spectrolab (a Boeing company BA) achieved a world-record conversion efficiency of 40.7% in lab experiments while commercial products operate at 26.5%-to-28.3% efficiency range.

The DOE believes breakthroughs in solar efficiency such as these may lead to systems with an installation cost of only $3 per watt, which translates into a cost of $0.15/KWH. In addition, solar PV cell suppliers are diligently working to reduce production costs. SunPower is targeting a 23% solar efficiency as a goal to reduce its solar energy system cost by 50% by 2012. Semiconductor solar cells benefit from economies of scale, which could reduce cost by 20% for each doubling of production. These solar initiatives should act to reduce solar energy costs.

Meanwhile, as energy costs rise with oil at $84/barrel, the cost of gasoline is expected to increase. If gasoline at the local filling station rises to $6.00/gallon, its cost in KWH jumps to $0.16/KWH. At that price gasoline would be above those achievable through solar energy advancements of $0.15/KWH.

Figure 1 Photovoltaic Device: Efficiency/Cost

With pricing on PV cells falling below $3.00/watt (or $0.15/ kilowatt-hour), solar energy will be close to parity with conventional hydrocarbons. If the cost of gasoline rises to $6.00 a gallon, it would cost $0.16/KWH while current cost of electric is about $0.11/KWH because of the low cost of coal. The bottom line is innovative research initiatives have the potential to deliver disruptive technologies that significantly changes the economics of delivering an energy solutions to all nations.

Further review:
Please see research at California Institute of Technology’s division of Chemical and Chemical Engineering at The Lewis Group.
Imperial College, Jenny Nelson’s Third generation Solar Cells.
DOE’s National Renewable Energy Lab (NREL) Third Generation Solar Photon Conversion (Exceeding the 32% Shockley(Exceeding the 32% Shockley—Queisser Limit)
Solar Energy Market Data Solarbuzz

My September electric bill arrived the other day and I was interested in comparing my energy savings after swapping 60 and 100-watt light bulbs for Compact Fluorescent Light bulbs (CFL), as recommended by the U.S. Department of Energy (DOE). Our progress in migrating to solar and wind energy is moving slower than expected. The CFL bulbs were a cheap investment so last year 12 standard light bulbs (two 100-watt and ten 60-watt) for ten 60-watt and two 100-watt CFL bulbs.

The results are impressive with improving energy reductions and money savings. Energy usage as measured by kilowatt-hours (KWH) is down an average of 30% from last and attributable to CFL, outdoor solar lighting as well as electric conservation efforts. However, the savings attributable to the CFL bulbs, of nearly $8 per month equate to an impressive return on investment of over 190% in one year.

While our initial calculations suggested energy savings (for lighting) called for reductions of over 70% when switching to CFL bulbs, the electric bill reduction was not that dramatic because large appliance usage account for a larger portion of electric power bill. However, when measuring the return on investment for a fast, cheap, and easy step to lower your electric bill, the CFL produces real savings,

The CFL bulbs cost around $4.00 for either a 100-watt or 60-watt equivalent light bulb. GE’s compact fluorescent lights were installed in August 2006 at a total cost of $48.00 (12 times $4.00 a light). According to GE the 60-watt CFL used 15-watts of power and the 100-watt CFL used approximately 26-to-29 watts of power. So theoretically, energy use, assuming lights were in operation for 4 hours per day, would save about 71 KWH a month. Our electric rates are currently at $0.108 per KWH which is at par with the U.S. average rate of Electricity Prices for Households $0.104 per KWH in 2006. Therefore, the CFL bulbs are saving about $7.71 per month from our electric bill amounting to $92.50 in savings over a year. That yields an investment return of 193% on a $48 investment in CFL bulbs.

Figure 1 CFL Energy Savings

Now of course, power usage varies by household, including the diligent habits of our children, so savings will vary. The bottom line is little steps sometimes produce big results – CFL bulbs do help reduce your electric bill with a small investment and also help the environment as each 1.3 KWH reduction in power use reduces carbon dioxide (CO2) emissions by 1 pound. Coal generates about half the electric power in the U.S. and produces roughly ¾ of a pound of CO2 for every KWH of electric. That means for every 1.3 KWH of electricity used (a 100-watt light used for 13.3 hours) produces one pound of CO2. CFL help reduce CO2 emissions by approximately 1.4 pounds per bulb based on light usage of just 1-hour/day a month.

The U.S. Department of Energy (DOE) is quite correct in suggesting that if every household in the U.S. substituted a 100-watt standard light bulb for a Compact Fluorescent Light bulb (CFL), it would eliminate an amount of carbon dioxide (CO2) equivalent to one million automobiles. However, it is the bigger picture that matters, – motor vehicles contribute the most to CO2 emissions. We must not forget that by focusing on CO2 emissions, they are admitting that CO2 is a real issue that potentially leads to global warming and climate change.

Let’s look at some facts about our carbon footprint. A 100-watt light in operation for 13.3 hours produces approximately one pound of CO2 when the electricity is generated by coal. Coal has significantly higher carbon emissions per kilowatt-hour (KWH) than oil or gas. Please see Carbon content of fossil fuels . Coal generates about half the electric power in the U.S. and produces roughly ¾ of a pound of CO2 for every KWH of electric. That means for every 1.3 KWH of electricity used (a 100-watt light used for 13.3 hours) we produce 1 pound of CO2. And remember it’s the oxygen in the air that contributes nearly 73% to the weight of CO2. This is why more CO2 is created than the actual weight of the fuel.

Using the same fuel emissions data, a motor vehicle with an average fuel efficiency of 22 miles per gallon (MPG), produces approximately 90 pounds of CO2 for every 100 miles driven. A gallon of gasoline produces nearly 20 pounds of CO2. That equates to one pound of CO2 for every mile driven by an SUV with a fuel efficiency of 19 MPG. (19.9 pounds/gallon times 1 mile divided by 19 MPG)

While it makes sense to address the issue of CO2 emissions, particularly as coal accounts for half of electric power generation and has higher CO2 emissions per KWH than oil, the real issue is an energy plan that givers us energy independence. Energy independence should equate to national security.

With the rapid growth in vehicle use around the world, it would be nice to know what are the most efficiency, economic, and least carbon emitting fuels. The number of motor vehicles on the road is increasing rapidly. The number of cars and trucks in China is up over 3,600 percent in the last thirty years. Data from the U.S. Department of Energy (DOE) and Ward’s Communications, Ward’s World Motor Vehicle Data provide an interesting view of the growth in motor vehicle use.

Figure 1 China Truck and Car Registration

While the U.S. still accounts for the largest motor vehicle market, the rest of the world is quickly accelerating towards more vehicles on the road. Figure 2 shows the number of vehicle registrations over the last thirty years for China, the U.S. and the rest of the world (ROW). Vehicle registration growth in the U.S. has been growing at a 2% per year rate from 1975 to 2005. The largest growth in vehicle registration is in China and India where growth in the last ten years is up 195% and 99%, respectively.

Figure 2 World Vehicle Registration

With an explosion in motor vehicle use, what fuel should we be using to better performance and reduce emissions? Let’s go back to two basic concepts of energy: Specific Energy and Energy Density. For a quick review, (Molecular Weight Calculator) the specific energy of a fuel relates the inherent energy of the fuel relative to its weight. Specific energy is often measured in kilo-joules per gram (kj/g). One kilo-joule equals one kilowatt-second meaning one kilowatt-hour (KWH) equals to 3,600 kilo-joules. Also one British Thermal Unit (BTU) equals 1,055.05585 joules. A reference to the specific energy and energy values of most fuels can be found at Hydrogen Properties

Figure 3 Specific Energy

By specific energy hydrogen is the clear leader. However, vehicles must inherently carry their fuel supply, so to determine which fuel is best for motor vehicles, energy density of the fuel is the next measurement. While vehicle fuel efficiency is dependent upon a number of factors such as engine type and performance, make and model of vehicle, road conditions and fuel, we are focusing on fuel energy.

Figure 4 Energy Density: KWH per Gallon

Figure 4 illustrates how fuels compare with respect to energy density, that is, energy relative the container size. We again are using KWH to measure energy value. Hydrogen, because it is so light, requires 15.9 times the container volume to provide the same energy as diesel. Biodiesel provides more power per gallon than Ethanol, which requires 1.6x, the container size for the same amount of energy as diesel. Biodiesel and diesel are relatively similar with respect to energy density. While both Ethanol and Biodiesel are both form of renewable energy, Biodiesel offers more bang per gallon. Before we are able invest more into hydrogen and solar energy to bring alternative energy into parity with conventional hydrocarbon fuels, diesel and biodiesel offer better energy efficiency among hydrocarbon fuels.

Table 1 Specific Energy, Energy Density & CO2

As a final assessment of hydrocarbon fuels, let’s compare carbon dioxide (CO2) emissions among our list of fuels. CO2 emission is a function of carbon concentration and combustion process of the fuel. Fuel energy research at the Department of Environmental Protection (EPA) and DOE indicate 99% to nearly 100% combustion of with fuels used in vehicles. That means almost all of the atoms in the fuel are converted to either heat or byproducts such as CO2.

Figure 5 illustrates how much CO2 is produced per gallon of fuel. Remember the molecular weight of CO2 is about 44 with oxygen contributor nearly 73% of the weight and is taken from our atmosphere during combustion. This is why more CO2 is created than the actual weight of the fuel. A second factor needs to be considered when evaluating CO2 emission and that is how much CO2 is produced per energy value. In comparing CO2 emissions per KWH of energy, Ethanol produces about 7% less CO2 than diesel or Biodiesel and 5% less than gasoline. Neither of these estimates considers the emissions from the processing to produce Ethanol or Bioiesel.

Figure 5 CO2 per Gallon

The bottom line is Ethanol and Biodiesel provide marginal relief to our energy crisis with biodiesel offering better efficiency and Ethanol marginally less CO2 missions. The only real solution to our imminent energy crisis is alternative energies such as solar, hydrogen fuel cells, and wind.

From our post last week we visited some research at California Institute of Technology’s division of Chemical and Chemical Engineering The Lewis Group and its focus on energy conversion and trade-offs between costs and efficiencies for photovoltaic (PV) devices.

We are providing a brief and very oversimplified analysis of the physics behind solar energy, and to that end, an overview of semiconductor band gaps and new technological approaches to optimize solar energy conversion efficiency. Our prior discussions on energy conversion process, dealt with of the properties of various materials used in silicon and thin-film solar cells. (see Research at Caltech may provide clues to improving solar cell efficiency Energy bands and band gaps provide a way of understanding some of the limitation of solar energy efficiency.

Semiconductor Band Gaps
In semiconductors, the band gap refers to the separation between the valence band, where the electrons are shared among the atoms in the material, and the conduction band where electrons can travel. When the material absorbs light, the electrons, when excited to the band gap threshold level, are able to jump to the conduction band and in the process, create electrons and positively charged holes, that in turn generate electric current.

The energy bands refer to the large number of tightly spaced energy levels associated with crystalline materials. Because electrons behave in wave like movements, their movement interferes with neighboring atoms. The physical properties of semiconductor material are much then those of metals. The minimum energy required to free an electron in metal, ( photo-electric effect) is significantly less than that of semiconductors. For metals the energy bands are more or less continuous, the valence band and the conductive band overlap.

In semiconductor materials such as crystalline, energy bands are grouped into bands, hence energy bands. In crystalline material, the energy bands are split between an upper band called the conductive band and a lower band called the valance band. The energy band gaps in photovoltaic semiconductors are different depending upon materials used. The separations of energy bands in crystalline materials results in gaps between the energy bands and are referred to as the energy band gaps. There are discrete and direct band gap materials. Direct band gap refers to having the conductive band minimum energy value and the maximum valance band energy at the same value such as metals. Please see Bart Van Zeghbroeck’s Principles of Semiconductor Devices chapter 2 for a review of energy bands and semiconductor band gaps.

Figure 1 Semiconductor Band Gaps

With a host of physics equations, models, functions, and laws, one can calculate the energy band gaps for semiconductor materials. In addition, one can alter the band gap of the material through, heat, pressure, or introducing impurities into the material (doping the material) to change its physical properties. In general, the more direct the band gap, the more efficient the band gap material. However, research is working processes to improve efficiencies through doping the material and introducing new materials. The processes seek to narrow the band gap of the semiconductor and thereby improve their efficiencies.

One must remember the cost efficiency tradeoff in evaluating which semiconductor material is best in achieving efficiency and cost parameters. Again, the Shockley-Queisser limit of approximately 30%-to-32% for single junction solar cell and a thermodynamic limit of about 83% are also constraint to improving solar energy efficiency. Please see “Detailed Balance Limit of Efficiency of p-n Junction Solar Cells” (W Shockley and HJ Queisser, Journal of Applied Physics, 1961). Semiconductor materials have limited efficiency because excess photon energy generates heat and not electric current.

Research studies on improving solar energy efficiency are embracing numerous fronts including concentrators and multi-junction semiconductors to impact ionization and spectral conversion. For a quick review, please Imperial College, Jenny Nelson’s Third generation Solar Cells. Multi-junction semiconductors employ two or triple junction semiconductor material to capture excess photon energy. A good overview on solar energy is presented by the DOE’s National Renewable Energy Lab (NREL) Third Generation Solar Photon Conversion (Exceeding the 32% Shockley(Exceeding the 32% Shockley—Queisser Limit)

Figure 2 Approaches to PV Cell Efficiency

There should be considerable excitement given the potential that greater efficiency should be able to drive solar energy costs lower. With PV cells below $3.00/watt (or $0.15/ kilowatt-hour), solar energy with be close to parity with conventional electric rates which are about $0.11/KWH. The bottom line is innovative research has the potential to offer the disruptive technology that significantly changes the economics of delivering an energy solutions to all nations.