Steam conditions at the turbine outlet are defined by the temperature at which the steam is condensed and the latent heat of vaporisation.

By CSP Today staff writer 

The efficiency of a Rankine cycle is defined, in large part, by the pressure and temperature of steam both entering and leaving the turbine.

For conventional fossil plants (coal and NG), higher steam cycle efficiency means lower fuel cost. In the case of a solar thermal plant, higher efficiency means less collector area is required for the same plant output, resulting in lower capital cost for the project.

Higher efficiency can be achieved by increasing inlet pressure temperature or lowering exhaust pressure and temperature.

The right ambience

The turbine exhaust conditions are dictated by site ambient conditions, says Babul Patel, senior consultant, Nexant.

According to Patel, if wet cooling is used, water temperature and ambient relative humidity or dew point will determine the minimum condenser pressure that can be achieved. Since there are limited options available for the turbine exhaust improvement, focus is normally on increasing steam inlet temperature.

With current trough design and oil as heat transfer fluid (HTF), the upper limit on steam temperature is 370ºC (700ºF). New HTF will have to be developed that can be heated to higher temperatures and still maintain low vapour pressure.

“Research is being carried out with molten salt that can be used in troughs that will allow raising HTF temperature and hence the steam temperature for the steam cycle to 500º–550ºC,” said Patel.

However, he added that the high freezing temperature of salt (220ºC) is a problem that will have to be addressed and taken into design before salt can be used in the troughs.

It should be noted that a central receiver with direct steam generation or indirect steam generation through molten salt as HTF can achieve higher temperatures in the order of 500º–550ºC, but design of the receiver and material of construction are a challenge, added Patel, who is scheduled to speak about cycle efficiency at the Concentrated Solar Power Summit US in San Francisco (June 30–July 1).

Reliability with commercial potential

A solar combisystem is described as a solar heating system that provides renewable heat, both space heating and cooling and hot water from a common array of solar thermal collectors, normally linked to an auxiliary non-solar heat source.

In the past, it has been mentioned that integration of a small thermally driven chiller in a solar combisystem is a suitable solution to manage the heat produced by solar collectors in a better way. The share of building loads met by solar energy can be increased, thereby reducing conventional energy consumption and giving this technology a better possibility of penetrating the market.

A hybrid system with solar heat and conventional fossil fuel heat source as back-up provides the reliability that most commercial systems require.

“This technology has good potential,” said Patel.

However, the added capital cost of the solar energy collection system has to be justified against the savings in fossil fuel costs, he added. Some incentives will have to be provided for the renewable portion of the system to justify the added investment in the hybrid system.

Wet is best

In the past, organisations like NREL have conducted studies to assess the preferred design conditions for a dry cooling tower, and the anticipated increase in the levelised cost of energy.

Experts point out that wet cooling tower technology has a much lower effective cost than dry cooling in large power plants.

Dry cooling systems have significantly higher capital cost and also suffer from efficiency losses when ambient temperature is above 20ºC (assuming wet cooling system is designed for 18º–20ºC wet bulb temperature). The efficiency loss and decrease in turbine output occur at high temperatures when ideally the plant should be operating at maximum output.

“However, in the areas where solar thermal plants will be developed, water is normally a scarce commodity and dry cooling may become a necessity, albeit at higher cost,” said Patel.

One alternative is to design a hybrid dry/wet cooling system where water consumption can be reduced to about 10% of the wet system; but the annual plant output is maintained at about 90%–95% of the wet system.

Preferred design conditions for dry system are less than 20ºC ambient temperature.

“In my opinion, dry cooling is manageable for a plant size of 50MW net electrical,” said Patel.

Over 100MW plant size, dry cooling becomes very massive and pressure losses across the system become significant.

Concentration is key

Wet cooling towers achieve cooling through evaporation. The water losses through evaporation, entrainment and blowdown will determine make-up water required and hence the operating cost of the wet cooling tower (assuming one pays for the cost of water).

Evaporative losses are a function of dry bulb and wet bulb temperature difference and overall heat transfer, and are normally site dependent.

The two areas, according to Patel, that should be taken into consideration are – minimise entrainment losses and maximise concentration cycles (or minimise blowdown).

“Concentration cycle of four to five was the norm in the past, but now concentration cycle of 10 or higher is seriously considered in the design of a wet cooling tower,” said Patel.

Condensation temperature 

Condensation temperature at the end of the steam cycle correlates to saturation pressure in the turbine exhaust. Lower the exhaust pressure, and more effort will be extracted from the steam. Hence, any lowering of the condensation temperature can be useful; however, this temperature is dependent on cooling water temperature or ambient temperature.

3rd Concentrated Solar Power Summit US

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