Hybrid dry/wet cooling systems

minwatercsp

Figure 1: Hybrid heat exchanger (left) and a conventional forced draft air-cooled heat exchanger (right)

The scarcity of water in the arid locations that CSP plants are often erected in is one of the main drivers towards the use of dry-cooling (cooling with air) for power plant cooling purposes instead of a more conventional wet-cooling (cooling with water) approach.  However, even though dry-cooling methods allow an order of magnitude reduction in cooling water consumption, the overall power cycle efficiency is generally lower compared to cases where wet-cooling is implemented.

In order to provide an improvement in the performance of a dry-cooling system for CSP applications the MinWaterCSP project focusses on the development of a novel hybrid cooling system (indicated in figure 1) that can be operated in a dry and wet mode, depending on the atmospheric conditions.

The hybrid system relies on the development of three sub-technologies:

  • Higher efficiency low noise axial flow fans
  • Wire heat transfer surface finned tubes
  • Software tools for design and investigation

Axial flow fans

minwatercsp

Large diameter axial flow fans find application in forced or induced draft air-cooled condensers. These fans are typically constructed by either using extruded aluminium sections or composite material lay-up techniques. These construction methods sometimes limit the performance of the final manufactured product.  Additionally it only allows for a fixed number of fan configurations to be at the disposal of the cooling system designer. The following possibilities for improvement in large diameter axial flowfan design and manufacturing exist:

  • The aerodynamic design of a fan rotor to simultaneously render high total to static efficiencies and low noise levels. The aerodynamic design will consider the manufacturing technique to be used for the fan blade.
  • A blade manufacturing technique that would produce sufficiently stiff and light weight fan blades with a high level of accuracy (see figure 2). The manufacturing process will be adaptable in such a way that the manufactured fan can be tailored to the requirements of the cooling system designer.
  • The integration of a high efficiency electrical fan motor and drive system that will specifically cater for the requirements of large diameter cooling fans.

Figure 2: Scaled axial flow fan, manufactured using high accuracy techniques.

Wire structure heat transfer surfaces

minwatercsp

Present state of the art heat transfer surfaces in large air cooled condensers mainly consist of galvanized steel tubes with aluminium fins in various configurations.  Existing wire structure heat transfer surfaces can be adapted and developed for such applications. Main benefits of wire structures as heat transfer surface enhancement can be reduced material usage, higher heat transfer surface area as well as higher heat transfer coefficients. Therefore experimental and simulation based studies will be done on a variety of designs by Fraunhofer ISE, Freiburg.  If successful, samples for laboratory testing will be manufactured and tested to determine heat transfer and air-side pressure drop characteristics. Should the outcome show benefits compared to existing state of the art technology, a cooling system design based on the new heat transfer and air-side pressure drop characteristics will be carried out and overall gains will be quantified.

Software tools

minwatercsp

Cooling system design tools play an important role in the design and development of new cooling systems.  Kelvion Thermal Solutions is responsible for the development of performance prediction software for existing large scale cooling systems as well as a newly designed hybrid cooling system. The software is largely based on existing thermo-fluid dynamic theory and will be capable of predicting, among other things, heat transfer rates, steam-side flow distributions, water consumption and fan power consumption.  The results obtained from the newly developed software will be used as input to existing CSP simulation software (ColSim), developed by Fraunhofer ISE that is used for the life cycle analyses of CSP plants.