The role of pumps in renewable energy

Renewable energy is increasingly displacing fossil fuels in the power sector, due to its lower carbon emissions and reduced pollution. flow takes a look at concentrated solar power – one renewable power generation solution where pump systems still have an important role to play.

MicroCSP collectors on the Big Island of Hawaii. Image courtesy of Xklaim, licensed under Creative Commons

There are two major classifications of solar power generation – solar photovoltaic (PV) and concentrated solar power (CSP). While in PV systems the sun’s energy is directly converted to an electric current, CSP involves the sun’s heat energy being concentrated and transferred to a fluid, which facilitates the use of a steam cycle for power generation. An advantage of a CSP over a PV system is that there is inherently some stored thermal capacity in the system, which helps maintain generation capacity when sunlight is not available. The stored thermal capacity of the system is often further increased through the use of a heat transfer agent. So, CSP is not dissimilar to more traditional combined cycle gas turbine plants, and consequently the use of reliable pumping systems are still a vital element of successful CSP projects. Typically, CSP’s combine three major systems to produce electricity by collecting and concentrating sunlight with mirrors and lenses in a heat transfer fluid (HTF) such as synthetic oil or molten salt. Through a heat exchanger system, pumps move the HTF and heat water to generate steam. The power block then produces electricity using a steam turbine and a generator. Pump technology is a vital feature for many elements of a CSP facility – boiler feed pumps, heat transfer pumps, tank pumps, vacuum pumps, condensate extraction pumps, and cooling water pumps, for example.

VARIABLE OPERATION The start-stop cycle requirements inherent in CSP plants are said to be among the most demanding in power generation. According to Ruhrpumpen, CSP plants are often subject to highly variable operation with daily stopping and starting, large temperature differentials and corrosive environments and this requires heavy-duty pumps that can manage

temperatures as high as 600°C. With special materials and design considerations, its HVN and J Line pumps are said to offer a solution to handling heat transfer fluids. At the same time, its VLT model can pump molten salt in heat transfer and thermal storage systems.

HANDLING HIGH TEMPERATURES Flowserve boasts a range of pump solutions that is said to include pumps for every CSP application including those with the ability to handle hightemperature HTF, superheated feedwater, condensate and cooling water. Its Molten Salt VTP, for example, is a vertical pump designed for molten salt circulation. A mixture of sodium nitrate and potassium nitrate molten salt possesses high thermal conductivity, allowing temperatures to reach 600°C which can lead to heat distortion problems on vertical pumps. Flowserve successfully overcame this issue through the use of thermal mapping software which ensures heat dissipation is adequate and distortion is mitigated. Flowserve provided a variety of critical application pumps for the world’s first commercial CSP plant, which was built in 2011, near Seville in Spain. The 19.9MW plant employs molten salt as the HTF which allows for the continuous generation of electricity 24 hours a day for many months throughout the year. ClydeUnion Pumps (a brand of Celeros Flow Technology) provides boiler feed solutions for solar power projects from its range “CSP is not dissimilar of multi-stage dual volute and double case to more traditional diffuser pumps. These are designed with a high head per stage capability which means combined cycle gas turbine plants.” that fewer stages are needed, resulting in improved shaft deflection characteristics of the pump. Cooling water pumps are needed to circulate cooling water from a source to condense steam in the condensers and to provide auxiliary water in many CSP applications. ClydeUnion Pumps can provide large capacity vertical turbine pumps or horizontal volute pumps, which are said to ensure optimum operational performance and longer times between overhauls which are an important consideration for CSP plants in remote locations. chevron-circle-right

Energy-efficient pumping solutions for solar district heating

DESMI’S ENERGY-EFFICIENT, IN-LINE, vertical NSL centrifugal pumps were specified for use in one of the world’s largest solar heating plants – The Dronninglund Solar Thermal (District Heating) Plant in Denmark. This solar district heating plant has a water thermal storage reservoir of 60,000m 3 – 2,982 solar panels with a total surface area of 37,275m 2 . DESMI pumps are used for circulation applications in primary as well as secondary areas, and it also provided shunt pumps. Typically, these types of pumps have energy efficiency levels well above 80%. With proper use and maintenance, it is not unusual that they can perform over the entire lifespan of such a solar plant (25 years). Furthermore, its vertical pumps are said to save around 50% floor space compared to horizontal pump installations, which means that the plant can be more compact – which again helps to save costs. District heating in Denmark involves the distribution of hot water below 100ºC. For such applications DESMI developed the ‘Spacer Design’ which allows the motor to stay on the pump during service and the pump

can remain installed in the pipework, while it is being serviced, which helps simplify maintenance operations.