Interesting article about a new type of heat engine that could double the efficiency of photovoltaic power generation and make it competitive with coal. Apart from the exciting technical possibilities, it’s a wonderful rags-to-riches story of a poor kid from Mobile, AL.
Builders covet LEED certification — it stands for Leadership in Energy and Environmental Design — as a way to gain tax credits, attract tenants, charge premium rents and project an image of environmental responsibility. But the gap between design and construction, which LEED certifies, and how some buildings actually perform led the program last week to announce that it would begin collecting information about energy use from all the buildings it certifies.
I attended a seminar on LEED certification several years ago when it was first introduced. It’s basically a scoring system: buildings are awarded a certain number of points for each “green” feature, and based on the total number of points achieved, the building is certified at one of four levels. LEED Certified — the lowest level — requires between 26 and 32 points. LEED Silver requires between 33 and 38 points, LEED Gold requires between 39 and 51points, and LEED Platinum requires between 52 and 69 points.
But energy efficiency is not the only feature LEED rewards. For example, a building receives points for being constructed on a brownfield site. Bicycle racks and charging stations for electric vehicles earn additional points. These things are all well and good, but they don’t affect the amount of energy used by the building.
As any student of calculus knows, the integral of a function over a given range is closely related to the average value of the function over that range. One very useful average in studies of energy use is the daily average temperature. Building energy use is often a strong function of outdoor air temperature because about half of the energy required is for heating and air conditioning. Consequently, daily energy use in a building usually correlates well with daily average temperature.
Weather stations report temperature at regular intervals, and the best way to find the daily average is of course to take the average of all readings over a 24-hour period (assuming there are no gaps in the data). In historical studies however, sometimes the only information available is the daily high and low. But believe it or not, averaging the daily high and low temperatures gives a pretty good estimate of the daily average temperature.
Students in calculus classes are taught various methods of approximating an integral. The simplest is the rectangular rule, which in the case of daily temperature is equivalent to taking the average of all of the readings over the day (the integral would then be equal to the average multiplied by the total length of the interval, which is 24 hours).
Averaging the maximum and minimum of the function over the range gives about the same answer as the rectangular rule, but to my knowledge this method of approximating an integral is never taught. It would be interesting to know how “smooth” a curve has to be for this hold. It’s trivially true for a straight line, for example. What about second order curves?
Another interesting method of finding daily average temperature is to measure the temperature at 5:04:18 AM and 6:55:42 PM, and average the two readings. What is special about those two times? The answer is left as an exercise for the reader. Hint: it’s closely related to another method of approximating an integral, one that is occasionally still taught in classes on numerical analysis.
According to data provided on this page, a Tesla Roadster requires about 177 Watt-hours to travel one mile. Assuming that on average, the efficiency of electricity generation in the US is 30%, the Tesla requires 2.1 MJ of primary energy to travel one mile (surely there is a dependence on velocity, but that information is not provided).
My Honda Element gets 24 miles to the gallon, so it requires 0.0417 gallons of gasoline per mile. One gallon of gasoline contains 131.9 MJ, so my car consumes 5.5 MJ of primary energy to travel one mile.
A conventional vehicle would have to get 62 miles per gallon to consume the same amount of primary energy per mile as the Tesla Roadster.
I drive about 17,000 miles per year. At 24 miles per gallon and fuel costs of $1.80 per gallon, I spend about $1,275 per year for gasoline.
An electric vehicle that requires 177 Wh per mile would use 3009 kWh to travel 17,000 miles. At 7.5 cents per kWh, that amount of electricity would cost $226 — about 17% of the cost of a gasoline-powered vehicle. I could save over $1000 per year.
However, a new Tesla costs $98,000 — which means the simple payback would be just shy of the century mark.
The $787 billion Stimulus Bill signed by President Obama includes an $11 billion investment in smart grid technology. How many people have any idea what the smart grid is all about? Here is some good background. From the article:
The grid took more than a century to grow into the unwieldy beast it is now. Given the urgency of climate change, energy independence, and economic demands, we have only a fraction of that time to fix it. But the solution won’t spring forth fully formed. This, the greatest engineering challenge of our era, must be solved the same way it was created—piece by piece, with utilities and consumers acting in their own interests. For too long, those interests have been misaligned. It’s time for a reset.
Hoping to reduce energy consumption and environmental impacts, a number of states and federal agencies in the US have pledged to build new buildings in accordance with the Leadership in Energy & Environmental Design (LEED) program developed by the United States Green Building Council (USGBC). Consequently, one would expect LEED-certified buildings to use less energy than conventional buildings with the same function. But this is not the case.
In an article in Yale Environment 360 (a publication of the Yale School of Forestry & Environmental Studies), Harvey Sachs of the American Council for an Energy Efficient Economy (ACEEE) sums up the situation this way: “A green label building is not necessarily an energy efficient building.”
Dr. Sachs’ statement is supported by research commissioned by the USGBC itself. One way to assess the energy use of LEED-certified buildings is to compare them with Energy Star buildings. Energy Star is an EPA rating system that rates a building’s energy use against that of other buildings with similar functions: buildings consuming 25% less energy per square foot than their peers are awarded the Energy Star label.
So how do LEED-certified buildings stack up? It turns out that fully half of them would not qualify for the Energy Star! But to me the most amazing result from USGBC’s report is the following:
one quarter of these [LEED-certified] buildings had [Energy Star] ratings below 50, meaning they used more energy than average for comparable existing building stock.
A curious article in Wednesday’s New York Times about how US cities are going to use the flood of stimulus money earmarked for energy efficiency. The story’s focuses on Knoxville, TN and a group of energy auditors hired by the city to root out inefficiency in public buildings.
What the Times doesn’t say is that these audits are being done in advance of an Energy Savings Performance Contract (ESPC) that the city may enter into with Ameresco. You can read about that in this article from our local paper. Unless I’m mistaken, the potential ESPC has little to do with the stimulus funding, though perhaps some of the funds could be applied to the ESPC.
The great thing about an ESPC is that it’s revenue neutral. As an example, suppose the city of Knoxville is paying $1,000,000 per year in utility bills. An Energy Services company (ESCO) performs an audit and identifies $1,000,000 in energy efficiency upgrades that the city needs (new chillers, boilers, more efficient streetlights, etc.). These upgrades promise to reduce the city’s utility bills by $200,000 per year.
Under an ESPC, the ESCO secures financing and installs the new equipment at no charge to the city. The city agrees to pay the ESCO $200,000 a year until the loan is paid off.
So before the ESPC, the city was paying $1,000,000 in annual utility bills. After the ESPC, the city pays $800,000 per year to the utility and $200,000 to the ESCO. The ESCO makes a profit and keeps people employed. The banks make a profit on the loan. The taxpayers win because the city’s costs remain the same and they do not have to raise my property taxes.