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May 29, 2007

Categorizing solutions

Category: Alternative Power – Dan 12:21 pm

Here are some ideas for general categories for smart green energy solutions:

  • Local solutions – Technologies like photovoltaic cells and ground source heat pumps are inherently local in nature. It makes the most sense to employ these right where the energy use will take place. Local sources don’t require (and don’t incur the costs) of energy transmission, and they are inherently more secure than centralized industrial-scale facilities.
  • Transportation solutions – Energy for transportation must consider that the energy use (primarily creating motion) means that the energy supply must be portable. Transportation solutions must combine high efficiency with high density fuel sources and/or infrastructure that facilitates using stationary sources in some fashion.
  • Solutions for stranded energy resources – Both wind and concentrating solar power have vast potential to supply electricity at an industrial scale. But these sources are generally far from the demand (hence the industry term “stranded”). Solutions here must combine these sources with technology for energy transmission and storage, to make the energy available when and where it is needed.
  • Efficiency – Regardless of the energy source, efficiency is now the least expensive source of power in many (most?) cases.

This categorization scheme will be used in the next few posts to look at workable solutions in each area.

May 12, 2007

Personal choices

Category: Efficiency,General – Dan 6:42 am

What can you do to save energy, and to move the world toward a more sustainable future? When I tell people about this site, many assume that the primary focus is on personal choices.

My initial thought here is that although there are myriad things that individuals can do to contribute to the solution, all these things are in the current context of where and how we live. And that inherently limits what we can do as individuals. For example, if you currently live in the suburbs, and your job is 30 miles away, and there is no public transit, your choices are limited. Unless you move or change jobs (both hard choices), you must drive to work, and your car choices are limited to what’s on the market.

So here are two lists, one of things that are relatively easy to do, and the other a list of things that will have a greater impact but will require a change in habits:

Easy to do
Big impact
  • Install energy efficient light bulbs – Just do it. These CF lights are now inexpensive and save money and carbon.
  • Drive less – Think about the trips you make, and combine them. Cut out unnecessary trips. For most people, cutting driving by 20% is relatively easy.
  • Unplug your extra freezer or refrigerator – If your extra fridge is only used occasionally, unplug it the rest of the time. You might save 10% on your overall electric bill.
  • Choose clean power – If you can specify power source when you buy electricity, spend the (usually small) extra money and specify you only want green power.
  • Choose Energy Star appliances – There is usually an incremental cost, but again it is relatively small and the cost savings and impact (especially with refrigerators) can be quite large.
  • Get a home energy audit – Of course you should then implement the suggestions. Some will be small and make a big difference, others will be an investment that will pay off over time.
  • Buy locally produced food and products – One of the big hidden energy costs is transportation for the things we eat and consume.
  • Use less water – What’s water got to do with energy you ask? For starters, water is heavy to move. If you live high on a hill, you may already be paying a surcharge, because pumping water up the hill costs more than delivering to people on the flatlands. It takes lots of energy to deliver water, and it also takes energy to treat the water.
  • Buy carbon offsets – Again, these are not too expensive. Effectively you are investing in projects to reduce carbon use, and carbon offsets allow that investment to be tied to your personal carbon use.
  • Install solar hot water heating – Solar hot water makes sense in so many places. The incremental cost is small, and the benefit is that your hot water heater hardly ever comes on, and you don’t have to feel guilty about long showers!
  • Get an efficient car – The car you drive is your most important personal climate decision.
  • Fly less – From a personal perspective, air travel is the #1 carbon producer. If you fly for business, look to how you can reduce trips and do more on the phone. When you do fly, make the trip count — stay longer and make one trip count for two.
  • Move downtown – Cities are energy efficient. You have transit options, you can walk to shops and businesses. A bicycle becomes a practical means of travel.
  • Downsize – Smaller homes consume less energy. And (as one article puts it), you’ll have less space so you’ll buy less junk.
  • Become a vegetarian – This is hard, but meat production is energy intensive, so eating less meat has a big impact on carbon use. One list also recommends loosing weight (every 50 pounds you loose will give you a 1% improvement in gas mileage).

In general, I’m not in favor of preaching sacrifice — it doesn’t work for the population at large and won’t solve the problem. But from a personal perspective, you can make a difference. And there is an aspect of changing the culture of energy consumption. In the Nov/Dec issues of Stanford Magazine, there was an article “A Crude Awakening”, documenting a debate of energy experts. One quote stands out here:

This has to become a public policy issue. It’s not right now. Think about the way the market for cigarettes worked in this country 50 years ago, and think of how it is structured now. We have not just taxes but regulation – they can’t be advertised on television – and a national campaign trying to educate people about the health concerns. We need a similar effort on this issue.

Cigarettes have almost become socially unacceptable in many parts of the country. At least it’s a start if you develop energy saving habits and lifestyles, and show others the way forward.

May 7, 2007

Energy Storage 101

Category: Transmission/Storage – Dan 8:29 am


One of the biggest challenges with new alternative energy solutions such as wind and solar is that these sources are em>intermitent; power is only generated when the wind blows or when the sun shines. The good news is that there is vast potential, either wind or solar could in theory provide the entire US with our power needs. But the bad news is there is no obvious solution to provide storage of the energy from these intermittent sources. Fossil fuels are a form of high-density stored chemical energy — that’s why they are so popular. The stored energy can be transported to where it’s needed, and released (by burning) when it’s needed.

There are several ways to store energy on an industrial scale; let’s look at a few:

  • Compressed air – In compressed air energy storage, electricity is used to pump air into a sealed underground cavern to a high pressure. The pressurised air is then kept until needed, at which point it drives turbines as it is released. Note that the compressed air can be mixed with natural gas and they are burnt together, as in a conventional turbine plant, which improves efficiency as the compressed air will lose less energy (one problem with these systems is that the air heats when compressed, and this heat must be eliminated — then when it expands, it cools and this reduces efficiency in the turbine). [1]
  • Pumped water storage – If you have a hill (over 100 meters) nearby with a suitable reservoir, and a supply of water, you can create what amounts to a reversable hydroelectric facility. Excess power isused to pump water up to the reservoir, and when energy is needed the water runs back down through turbines. Hydropower is 80% efficient, but this affects you in both directions so the result is about 64% conversion efficiency. [2]
  • Hot fluid – Generally, heat isn’t an efficient means to store energy. The exception is when heat is what you started with, which is clearly the case with concentrating solar power (CSP). Trough-based CSP systems use long parabolic mirrors to heat a working fluid. This hot fluid is then used to generate steam to drive a generator. Some CSP plants have a reservoir of fluid to store heat generated during the day for use at night
  • Batteries – Batteries store electricity as chemical energy. There are battery configurations of several megawatts, making them suitable for various utility applications, and of course smaller configurations are common and widespread.
  • Hydrogen storage – Water can be electrolized into hydrogen and oxygen, and the hydrogen stored for later conversion to electricity when needed using a fuel cell. Note this solution can be combined with a pipeline so that hydrogen becomes the energy tramismission medium, rather than using electrical lines.

There are other options as well, such as flywheels, capacitors, and superconducting magnets, but the above four technologies are the likely best near-term energy storage alternatives for industrial scale energy storage.

Storage pros and cons

None of these storage solutions are perfect. Because energy is being converted to a different form, all storage solutions waste some of the original energy. But all of the above storage solutions have different pros and cons, and these lead to some logical conclusions. Before looking at some smart solutions, let’s consider the pros and cons of each technology:

Compressed air

The investment in a compressed air storage system is in compressors and turbines. These components affect the rate of energy storage/generation. The storage medium – the cavern within which air is kept – is site dependent but is usually vast (in terms of potential mWh storage) compared to the rate (in terms of mW) of the generators. This makes compressed air ideal for stranded wind generators which themselves are limited by the maximum generation rate, but might experience generation fluctuations of days, weeks, or even seasons.

Pumped storage

Pumped storage is limited by lack of suitable sites; you need appropriate geography, plus plenty of water. Pumped storage facilities are also a less desirable option environmentally, because the upper reservoir isn’t much use for anything else — it fills and empties like a bathtub, often on a daily basis. Beyond these limitations, pumped storage has many of the characteristics of compressed air — the volume of storage is quite large compared with the rate of storage or recovery. Pumped storage is also ideal for stranded energy generation.

Hot fluid

As previously stated, storing energy as a hot fluid makes sense only when hot fluids is what you start with. Within the context of CSP facilities, investing in storage makes sense in the context of shifting peak power demand. In other words, peak energy generation at CSP will be during the middle of the day when the sun is highest; peak energy demand is likely to be later in the day and into the early evening. The quantity of fluid stored need only be enough to carry the load over into the evening.


Batteries have characteristics almost opposite those of compressed air or pumped storage. The cost of expanding storage volume is linear; double the number of batteries and you’ll double the storage. Batteries are also relatively expensive (driven in part because the storage medium is a manufactured product vs. a cavern or a reservoir). The result of these characteristics is that batteries make the most sense to even out intermittent power sources near the point of consumption, where the value of the energy is highest. An ideal use for batteries, for example, is to extend the time that energy is available from residential photovoltaic installations.

Hydrogen storage

Hydrogen is relatively easy to generate given a supply of water which can be separated using electrolysis. And while hydrogen is bulky to store, there are no major technical obstacles. Hydrogen falls somewhere between batteries and compressed air storage in characteristics; there’s a higher investment required in the equipment to electrolyze water and with fuel cells to convert hydrogen to electricity. Hydrogen can be stored in suitable underground caverns as with compressed air. In addition, there are several proposals to build pressurized hydrogen pipelines from stranded energy sources. The pipeline would not only serve as the energy transmission technology, but would contain a significant quantity of hydrogen and thus be a storage medium as well. I’ve included more information on this in the early article “What About Hydrogen?” Arguably, this pipeline approach is the best use of hydrogen storage, because it is then well integrated into the overall source / transmission / storage infrastructure.