Technology Roadmap

The key technology options for heating and cooling in buildings have been narrowed down to those with the greatest long-term potential for reducing CO2 emissions. This roadmap covers the following technologies for space and water heating, heat storage, cooling and dehumidification:

• Active solar thermal
• Combined heat and power (CHP)
• Heat pumps for space heating and cooling, and for hot water
• Thermal storage

Active Solar Thermal (AST) systems have several advantages. They can be applied almost anywhere and do not require any energy infrastructure. They are either carbon-free or have very low emissions, associated with their electricity use for pumping and controls. Owners and operators of AST systems do not have to consider the risks of changing energy prices and potentially, carbon prices.

AST systems collect the incoming radiation from the sun by heating a fluid (generally a liquid, but occasionally air). The heated fluid in these collectors is used either directly (e.g. to heat swimming pools) or indirectly with a heat exchanger transferring the heat to its final destination (e.g. space heating). The amount of heat energy provided per square metre of collector surface area varies with design and location but typically ranges from 300 kWh/m2/yr to 900 kWh/ m2/yr.

The target for high-density energy storage for solar systems is that investment costs (after deployment) will be twice that of today’s sensible energy storage systems.

Current solar water-heating systems for single-family dwellings are relatively small, with collector areas of 4 m2 to 6 m2, and meet 20% to 70% of average domestic hot water needs with a 150 litre to 300 litre storage tank. Solar combi-systems for single-family dwellings, which provide space and water heating, are larger, with current systems typically having a collector area of around 12 m2 to 15 m2 associated with a 1 000 litre to 3 000 litre storage tank. Combi-systems can meet 20% to 60% of the space heating and water heating needs of a single-family house. In both cases, an auxillary heating system is currently required to meet the balance of demand. However, as low-cost compact thermal storage becomes available it will remove the need for these auxillary systems in many applications. Where district heating systems exist, solar thermal energy can be produced on a large scale with low specific costs, even at high latitudes, as successful examples at the MW scale in Sweden and Denmark show.

An emerging application for AST systems is solar thermal air conditioning. Two main technologies can use solar thermal collectors for air conditioning in buildings:

• Thermally driven chillers are used to produce chilled water in closed cycles, which may then be used with any space conditioning equipment.
• Open cycles are also referred to as desiccant evaporative cooling systems (DEC) and are used for direct treatment of air in a ventilation system.

Coupling solar thermal collectors with thermally driven chillers would enable systems to meet space heating and cooling, as well as hot water demands. The dominant technology of thermally-driven chillers is based on sorption. The basic physical process consists of at least two chemical components, one of them serving as the refrigerant and the other as the sorbent. Many sorption chillers are available commercially at different capacities, but few at 100 kWth or less.

Solar cooling is attractive because solar radiation usually coincides closely with cooling loads, while many service-sector buildings also have simultaneous heating and cooling requirements.

SEIDO Technology

Heat pipes act like a low-resistance thermal conductor. Due to their thermal-physical properties, their heat transfer rate is thousand's of times greater than that of the best solid heat conductor of the same dimensions. Sunda's SEIDO heat pipe is a closed system comprised of two meters of copper tubing, an evaporator section, a capillary wick structure, a condenser section and a small amount of vaporizable fluid. The heat pipe employs an evaporating-condensing cycle. The evaporator section is tightly bonded to the absorber plate, where it captures the heat from the absorber and evaporates the liquid to steam, which moves up to the condenser section. The condenser protrudes out from the evacuated tube and is inserted into the heat exchanger manifold. There this steam will be condensed by water flowing through the manifold. Latent heat energy will be released to the process water through this phase change of vapor to liquid. In vacuum tube solar collectors, the condensation zone is at a higher level than the evaporation zone. The transport medium condenses and returns to the evaporation zone under the influence of gravity. This process is repeated continuously thereby heating the water in the solar loop.

Advantages of the Technology

• High heat transmission rate
• Fast Start-up
• One-way heat conduction
• Homogeneous heat distributing on the surface of the condenser

The absorber is treated with an aluminum-nitride selective coating to achieve highest efficiency of the heat transfer. The coating is applied using a magnetic sputtering technique. This special optical coating transforms more than 92% of the incoming solar irradiation into heat and reduces less than 8% heat loss.

• High absorbance to guaranty the effective heat from solar irradiation
• Low emittance against the heat loss by heat emission

Heat pipes are inserted into the aluminium absorbers forming assemblies, which in turn are inserted into the glass tubes. The tubes are made of borosilicate glass which is strong and has a high transmittance for solar irradiation. In order to reduce the convection heat loss, glass tubes are evacuated to vacuum pressure of <10-5 mbar. Sunda uses a patented technique employing high heat and pressure to insure stable glass-to-metal vacuum seals. In order to keep the stability of the vacuum for a long time, a barium "getter" is used. Through evacuating air out of the glass tube the absorber material and the selective coating are protected from corrosion and other environment influences. This ensures a lifetime of at least 15 years without loss of efficiency.

• Effective hermeticity
• Resistance in harsh environments, e.g. against corrosive materials, vibrations, serious temperature fluctuations
• Long-term, reliable protection of the packaged component

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