technology selection Storage Excess electricity can be stored in a high-efficiency fly-wheel for short term load balancing (PowerTHRU offers a suitable model for the site that costs 85.000 USD). Longer term technologies for on-site electricity storage include: • Metal-air batteries • • Compressed air storage Hydrogen Metal-air battery technology is rapidly advancing and represents high efficiency and longevity gains over existing battery technologies. We are investigating around a dozen emerging metal-air battery technologies. Recent years have also seen many advances in compressed air storage for electricity. Adiabatic storage stores the heat generated during air compression for reuse during expansion, achieving a theoretical 100% efficiency (70% in practice). New pilot projects are underway, such as the German ADELE project (2013). SustainX has commercial isothermal CAES, and LightSail offers a 70% efficiency system that has proprietary storage tanks that can handle very high pressures and don’t need to be buried underground. If hydrogen is generated, it must also be stored. Storage in the form of hydrogen has, in particular, advantages in terms of space. Though it is less efficient than compressed air energy storage (40 - 50% efficient, as opposed to 75 - 90% efficient), it can be stored at much smaller capacities. It is therefore economically best suited to scenarios where the total amount of energy stored is a more important factor than the overall efficiency, which is usually the case when energy is stored for longer than 1 - 2 days. A 250-liter pressure vessel designed to store 10 - 20 kWh of compressed air could store enough H2 to provide 150 - 300 kWh of electricity, reducing the metals (e.g. iron, titanium, aluminum, and sodium) that release H2 upon moderate heating (less than 200 F). be stored cheaply without high pressure at a very low temperature (-453 F) liquid (LH2 cost of storage capacity by more than a factor of 10. H2 There are many emerging technologies for safe hydrogen storage. Some of the more promising ones involve storing hydrogen bound in a solid alloy, which increases its volumetric energy density and removes the risk of explosion. This or a similar approach would need to be worked out for a safe hydrogen cycle to exist on site. HEAT Generation There are many potential sources of heat capture and generation throughout the CTP site, mostly detailed in the other scenarios: • • • solar heat collection from rooſtop collectors and greenhouses heat exchangers (air-to-air, air-to-water) fuel combustion (biogas or biomass) • waste heat from household equipment A key design challenge will be to define a smart heat cascade through the neighborhood to use high to low quality heat in the area and store any large excesses. Storage Short-term heat storage can be performed in insulated water storage tanks or insulated concrete blocks. Water has one of the highest thermal capacities (4.2 J per cubic cm). Concrete has around may also ) or by absorption in powders of abundant a third of this thermal capacity. However, it can be heated to a much higher temperature (around 1200 C by electrical heating and thus has a much higher overall volumetric capacity. An insulated cube of about 2,8 meters would have sufficient heat storage for a household (calculate). The Wiggenhausen-Sud solar development has used this approach to great effect. An alternative for short-term heat energy storage is to use high density ceramic bricks heated to a high temperature with electricity (at peak hours) and well insulated to release heat over a number of hours. Converting electricity to heat is unlikely to be a recommended option for this site, however, since electricity will be in shorter supply than heat. For seasonal heat storage, Phase Change Materials are once again an interesting option. Molten salts retain a high temperature in thermal storage systems in conjunction with concentrated solar power. Options for molten salts include: potassium nitrate, calcium nitrate, sodium nitrate, lithium nitrate. These absorb heat and then re-release it into the water to transfer energy as needed. The salt must be mixed in a eutectic mixture. WATER Collection & storage Rainwater harvesting is a standard and straightforward technology: runoff surfaces are coupled with gutters that collect water into tanks for purification and use. One option to explore in this scenario is how to integrate the water storage into the architecture of the CTP site in such a way that it is hidden from view and makes maximum use of gravity flows. 138 / 146 Pagina 137
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