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Stirling engine application in the production of freshwater

Nine of the fourteen nations in the Near and Middle East confront today a shortage of water resources, further made worse because of the frequent dry seasons, thus becoming in the world the regions more involved in the problem. In the rest of the world 26 nations, for a total of about 232 millions of people, can be considered countries with low water resources, particularly in the African continent, in the northern China, in California and in the south Europe. This can explained thanks to two fundamental reasons:
  1. just 1% of the total amount of water present in the Earth is available for the humane aims, since the 97% stays in the seas and in the oceans and the remaining 2% is trapped in the glasiers;
  2. not even the water provided by the meteoric outflow is sufficient to supply the whole water demand because of different factors as the evaporation, the violent flows and above all the high irregularity of the rains distribution in the world.
In spite of this problem, the actual tendency leads to a water consumption growth: when it is not possible to solve the water shortage problem trough a corrected utilization of the water available, the unique solution is represented by the seawater desalination.
These desalination technologies was born more than an half of a century ago and they utilize thermal, electrical or chemical energy to separate water and salt present in the seawater thanks to chemical processes as evaporation or filtration. Today, this opportunity can be further improved by using the desalination systems as a bottomer of a Concentrating Solar Power (CSP) plant. This latter system is based on the concept of concentrating solar radiation to provide high-temperature heat for electricity generation within conventional power cycles using steam turbines, gas turbines or Stirling engines. For concentration, most systems use glass mirrors that continuously track the position of the sun; the sunlight is focused on a receiver that is especially designed to reduce heat losses. A fluid (air, water, oil or molten salt) works as heat transfer fluids and, flowing through the receiver, takes the heat away towards a thermal power cycle, where e.g. high pressure, high temperature steam is generated to drive a turbine.
The possibility to integrate a CSP plant with a desalination system allows to improve the efficiency of the whole plant, thanks to the utilization of the thermal energy, got by the steam going out from the turbine, used for feeding the desalination process instead to be dissipated in the environment. Furthermore, water can be produced with very low costs and in a totally safe-environment way, since it does not use fossil combustibles but solar energy.
This coupling, in addition to the advantages said, is optimal because the regions with low water resources are, the most of the times, the ones with high solar irradiation and then by applying this type of combined plant it is possible to use one of the highest solar energy to produce water in the place where it is more required, with evident advantages not only technological, but economical and social as well.
The CSP technologies destined to the high power range (>1 MW) makes use of steam or gas turbines: for smaller applications, the best option is presented by the Stirling Engine. In this case, the mirrors used are in the form of "Parabolic dishes", like the one shown in Figure 1, with the power block positioned in the focal point. The main characteristics of the Parabolic dish-Stirling technology are shown in Table 1.

Table 1: Performance data CSP-Parabolic Dish
Solar field cost (€/m2)
Typical size (MW)
Max operating T (°C)
Land requirement (m2/(MWh*y))
Peak solar efficiency
Annual solar efficiency
Thermal cycle efficiency
Capacity factor (solar)
29% (d)
16-18%(d), 18-23%(p)
30-40% Stirl, 20-30% GT
25% (p)
(d) = demonstrated, (p) = projected, Stirl = Stirling, GT = Gas Turbine.

For integrating a CSP plant with a desalination plant, among the different technologies, the best results to be the Multiple Effect Distillation (MED) because uses thermal energy for feeding the process, respect to processes like Mechanical Vapour Compression (MVC) and Reverse Osmosis (RO) which make use of mechanical energy, and it offers smaller consumptions inside the various thermal processes (the most important is the Multi-Stage Flash). Table 2 shows the most significant data for the MED process:

Table 2: Performance data MED
Heat consumption
Electricity consumption*
Plant cost**
Time of commissioning
Conversion Freshwater / Seawater
Tmax brine
Product water quality
< 10
* Does not include power losses induced by cogeneration due to increasing outlet temperature at the turbine;
** it increases with product water quality and energy efficiency.

The combination of the systems said, for the realization of a plant CSP - Stirling - MED, thus results a great opportunity to develop the Stirling's technology and therefore it is worth delving, also and above all, for the humane reasons described.

For more information

Manolo BONIFAZI (Eng)


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