March 23rd, 2012

Ann Vole

Energy Storage

I have concentrated on storage since I was a little kid wanting power tree forts and go-karts. As I got interested in independent living (my dad thinking of building on a lot on the north side of a hill with tall trees all around off the property... no solar, no grid, no wind) energy storage became far more important and large scale (both in volume and in length of storage cycles). As everyone seemed to think solar and wind would solve all our energy needs, I knew they were missing the most important piece of the puzzle...matching energy production cycles to energy use cycles. Now as energy experts are actually being asked to design a better grid (rather then just politicians and tree huggers looking for a greener world), the governments are finally realizing they need storage and am starting to put development moneys into that stuff. Here are some of the categorizes involved so I can point out the glaring omission most people make when discussing this topic.

The grid itself has three restrictions to worry about: 1) the average use is restricted to a certain amperage based on the temperature of the air because the wire gets hot and expands and then sags closer to the ground. 2) Electricity flowing through a wire has inertia (called inductance) that increases as the wire gets longer. This means sudden changes in the load (use) or source of the electricity will change the voltage in the part of the grid where the changes occur. These voltage spikes such as when a power line is shut off when struck my lightning can ripple through the system causing other lines to react as if they have been hit by lightning too and shut off themselves making their own voltage spikes and brownouts (drop in voltage). 3) AC power (changes direction constantly but is needed to make transformers work that change the voltage) has different peaks for the voltage as for the amperage based on the types of equipment adding or removing energy from the grid. As these two get further out of phase with each other, the less energy is able to move through the grid and the less efficient that energy transfer is. To clean up this out-of-phase power, they can increase the voltage or just add more clean energy sources (such as fuel-powered energy).

The restriction #1 means the grid must be over-sized to handle air-conditioner load on the hottest days of the summer during the supper-time peak load. To keep this over-sizing to a minimum, energy needs to be stored close to the users of the electricity or increasingly the demand is filled by fuel-powered generating stations built in the middle of cities. Electronic meters are helping by making peak load power more expensive so big buildings and industrial power users incorporate private measures to reduce their use at peak times. One method is to run cooling equipment at night to freeze a very large tank of water with cheap electricity then cool the building through the day with this stored coldness (termed "coolth"). If you plot the energy use cycle with the solar production cycle, you get an area where they do not match. That area is proportional to the amount of energy that needs to be stored for a 100% solar system (from an electrical grid standpoint). To minimize the size of the grid between the industrial-sized solar energy production facilities and the people/industry using the power, storage is best done close to the users in the proportion of that shift mentioned above. For the base amount of energy, the smallest amount used during the daily cycles, it is best stored at the source of the energy to level out the times when clouds pass over the solar production or at night. When combined with the charging of the other storage near the users, it becomes the average energy use over the night time which needs to be sized for the longest nights in the coldest part of the year. I have not mentioned wind because it is not able to be predicted like with solar. To fix wind's unpredictability, storage needs to be close to the windmills and used to cover that average use profile on the source side of the grid.

The types of storage have different purposes. Batteries can react quickly to fix voltage spikes and brown-outs but are not fast enough for some of the energy fluctuations you might see with large scale wind power. Capacitors are like batteries but can react faster. The problems with both batteries and capacitors is they rely on expensive electronics to make AC power as they are DC (direct current... does not change direction) devices. Batteries and capacitors are rather expensive for the amount of storage and can only be charged and discharged a limited number of times before needing replacement. Flywheels can store energy directly from the AC and thus fix phase problems and instantly react to spikes and brown-outs. The problem with flywheels is they lose energy to friction so require new methods of friction-free pivots and extreme vacuum environment. Flywheels are also big, heavy and thus limited on volume of stored energy. For volume, two storage forms are usually only listed: hydroelectric and pneumatic. Not only can dams be used to provide peak power but excess energy can be used to pump water to an elevated location to be used later. This has low cost for volume but generally cannot be located near cities so is restricted to the size of the grid between the storage and the use. Matching such storage with wind or solar facilities might work. The biggest restriction is environmentalists concerns for wildlife and people's use of bodies of water for real estate and recreation. Pneumatic storage is using underground caverns to store high pressure air. This pressure can be piped a bit to put the electricity generation closer to the users and the sources of energy further from these underground caverns. Pneumatic storage is probably the cheapest per volume but is quite restricted by the locations of suitable caverns (usually depleted natural gas and oil deposits and retired mines). New concerns are also arising from water contamination from leaks moving oil and natural gas around often many miles from the cavern location. Some scientists suspect the stresses on the caverns has been the trigger of earthquakes. The often overlooked storage option for cheap-per-volume storage is thermal. Most large scale solar power facilities use oil or liquid salts to gather the solar heat then use that hot liquid to create steam and power a turbine with that (exactly the same as if they are burning a fuel to make the steam). It becomes obvious then to store that heat to both account for clouds during the day and to continue producing electricity into the supper-time peak only a few hours later then the solar peak. Several solar facilities are doing exactly this but they are still not doing it the way I think should be done. They usually have two tanks to hold the hot liquid and the "used" liquid (cooled while making steam). Some use a single tank and stratify the tank meaning they have different temperatures as you move up and down in the tank. Tanks are expensive and prone to damage and require insulation around the whole tank. Dirt and clay are used to make the few materials used to make high temperature kilns so can handle way more heat then is needed to make dry steam (steam without water droplets as is needed for efficient turbines). If you are making a large heat storage area of soil, you just have to keep water out of it and only have to insulate the top surface... if that. Dirt also acts as an insulator and if you already have a fenced-off area for hot parabolic solar collection mirrors and pipes, a sterile and hot ground surface is acceptable. I have only found one facility that is doing something like this and they encased the pipes with concrete for some reason (no explanation provided in the detailed engineering notes found on the internet). I also want to explore the idea of wind power used to make heat directly and store it for steam production for a steady power source no matter how unreliable the wind speed. Note that a few wind projects are now using DC generators and batteries within the windmill tower then using grid-tie electronics designed for solar systems.

The other option for fixing the grid is to use it less. Grid-tied solar panels on homes means the grid only needs to shift the peak source to the peak use and this is best accomplished with energy storage close to the users. While we are still using lots of natural gas for electricity production, the idea of generating both heat and electricity within the home using natural gas makes a lot of sense. For the life span of such equipment (say about 50 years), we are unlikely to stop natural gas use completely before then. People already find battery-operated equipment and tools to be convenient so it is just a matter of using timers and electronic metering to move that energy use for battery charging to off-peak times. People prefer radiant heat and some new electric heaters are now designed to heat up a heat storage during off-peak times then radiate that stored heat through the rest of the day. Grid companies can encourage the use of such devices. New phase-change materials are being developed to add to the drywall material to store heat and cool in the walls. Heat pumps are good at maximizing the heat or cool produced from the same electrical energy but there are few systems who are designed to store that heat and cool to be able to run the heat pump in off-peak times. Of course good insulation, air-tightness, heat exchangers for fresh air, and better windows and doors (including "low-E" surfaces) can reduce or eliminate the need for heating and cooling or allow the programmable thermostat to not use expensive peak electricity. I am going to explore using a low voltage battery system at the location of LED lights and electronic controls for motion-sensing. The battery will then be trickle-charged for very low amperage use of the 120VAC wiring (and in my case only charged when the sun is shining on my solar panels). Electrically Commuted Motors (ECM) use only enough energy to get the job done so are much more efficient and can be used for continuous flow fans. With continuous flow, heating and cooling systems can be made smaller and thus more efficient. With some foresight, the system can be designed to be running only in off-peek times without getting noticeable before the peak time is over. This last paragraph can only be influenced with electronic metering and high peak-use rates but is also the only thing us consumers have control to do.

originally posted here: http://www.envirolink.org/forum/viewtopic.php?f=3&t=17435&p=179720#p179720

edit: I did forget an important topic that is getting a lot of attention: production of fuels or charged electrolyte from electricity or made directly from the solar energy (including via biological agents like algae used to make methanol). I always forget this aspect because it will be needed so badly for converting our transportation sector that requires light-weight energy storage found in chemical energy storage. This might also be used for grid storage and production but only after the transportation sector takes what they can as consumers adapt to the new technologies.