With water conservation becoming a major environmental issue for power plants, Francisco Javier Rubio Serrano offers a new technological solution
IMASA Engineering & Projects’ test plant in Gijón, Spain
The steam or Rankine cycles are intrinsically connected to our vital resource, water. This resource is unevenly distributed through the different areas of the planet, and its administration and procurement can be very expensive depending on the geographical point where it is found.
As the population is fast growing (nearly 8000 million people in 2020), our use of water, linked to the dynamic and industrial expansion, will increase, strengthening the already hard strains on this resource. Our regular sources of fresh water will worsen its quality due to higher levels of pollution, and exhaustion of wells.
According to calculations by economists, industrial water consumption will double by 2050, and in rapidly industrializing countries such as China this will be multiplied by five, and therefore so will its price.
Over 40 per cent of the global population will live in river basins with severe water shortages, mainly in northern and southern Africa and in Central and South Asia. While seawater desalination is being used to lighten this problem, the process is intensive in terms of energy demand and a large amount of unwanted brine by-product is obtained.
Therefore, efforts to optimize and reduce water consumption by power generation facilities is of utmost importance in the face of society, the environment and the profitability of a project of this type. As the United Nations has concluded, “addressing water scarcity is one of the greatest challenges of the century”.
The popular Rankine cycles are large consumers of water, used for the feeding of the cycle itself, for cleaning and for the cooling system. The latter is the largest consumer of the three, and this consumption depends on the type of technology used, the cooling system selected, climate and weather conditions around the plant and cycle performance.
Cooling is an essential part of the cycle, and a major optimization to increase the performance of the Rankine cycle is to reduce the temperature of the cold focus. This requires having a suitable cooling system allowing condensation of the turbine exhaust steam to the lowest possible thermal level.
Three systems are currently used for refrigeration: once-through (or open circuit), evaporative cooling (cooling towers), and air condensers.
Once-through cooling is the cheapest of the three, although given its extensive water withdrawal and environmental impact, it is the least used. As the water flow required is so high, pumping costs are high as is the size of the necessary pipes. For this reason, power plants should be near the water source. The environmental impact is greater than with other systems due to the thermal and chemical pollution (mainly biocides) of discharges.
The evaporative cooling (semi-open) circuit with cooling towers is the traditional technology used to remove the heat of condensation in a Rankine cycle. Several types of cooling towers are available (induced draw, forced draw, natural draw), and in all of them the hot water comes in contact with air producing air saturation (evaporative cooling), thus stealing heat from the water flow and cooling it by this process, with a small amount of evaporation taking place.
In addition, a small part of the water escapes into the environment due to drag by the air stream. Depending on the humidity of the area, the cold temperature attained is well below ambient temperature.
Among its advantages are the small space required per kW of cooling obtained and the low temperature reached, especially in the warmer months, compared to other technologies, thus allowing it to work with very low pressures in the condenser (below 0.07 bar (a)).
Currently its disadvantages are becoming more prevalent than its advantages. They require significant water consumption and need to control their discharge, since an amount of water is evaporated and another part is evacuated to prevent salt concentration. They also need proper management and treatment of feed water to this circuit (cleaning, disinfection and storage). Cleaning operations are tedious and subject to strict health regulations in many countries.
The UN states that water scarcity is one of the greatest challenges of the century
According to the European Environment Agency, approximately 45 per cent of water resource in Europe is used for cooling thermoelectric plants. Other sources indicate that 33 per cent of the water demand in Europe is used in the production of electricity, and for the US it is 41 per cent, since the weight of electricity in this country lies more with thermal power plants.
Francisco Javier Rubio Serrano
On the other hand, the average cost of industrial water is highly variable from one country to another, and within their own regions. In Spain the average price of industrial water in 2013 was €2/m3, ranging from €1.20/m3 in Castilla y León to €3.95/m3 in the Balearic Islands. These figures are still far away from the European average of €4/m3. With these results, and based on future prospects, both consumption and the average cost of water will increase significantly in the coming years, somehow penalizing the implementation of Rankine cycles using wet cooling.
The third technology of refrigeration, with air condensers, began to be implemented in areas where the availability, accessibility or cost of water negatively affected the profitability of power generation plants.
This technology is especially gaining importance in solar thermal power plants, which are generally located in high-insolation desert areas where water resources are very limited or just not available.
This requires, in some cases, to drill wells with very poor water qualities, which require intensive treatment, or to carry water from great distances. In the air condensers, the turbine exhaust steam is passed through the finned tubes which are in contact with the air stream driven by large fans.
The environmental impact of this technology is much lower than the mentioned. The main disadvantages are decreasing electric performance of the system (more than 1 per cent less of net electrical output), since the condensing temperatures of the steam that can be achieved are higher than with the previous systems (up to 20°C higher in warmer months), and the increase in the investment cost of the plant (the air condensers often have a cost of three to five times the surface condensers used in previous systems).
Sometimes, due to a lack of water in the area, power is generated using both cooling systems previously mentioned. These are called hybrid systems, where part of the cooling is done using cooling towers, and elsewhere through air condensers. These systems can reduce up to 50 per cent water consumption compared to wet cooling with little influence on the performance of the installation. The main drawbacks are a significant increase in the investment cost, as well as increased costs of operation and maintenance.
Therefore, the use of one or another technology depends on the location of the power generation facility, which will mark the availability and cost of water for cooling.
In the effort to find a technology that can solve the above disadvantages, a group of engineers has developed a practical, economical and efficient solution called hygroscopic cycle.
This innovative thermodynamic cycle is a Rankine cycle where most of the equipment is exactly the same. The hygroscopic cycle works with hygroscopic compounds dissolved in water which, in addition to treating the feedwater of the cycle, allow for an efficient and economic condensation at the outlet of the steam turbine.
The solution from IMASA developed to improve the steam cycle
Based on the concentration of hygroscopic compounds chosen, the condensation temperature of the steam turbine output is superior to existing Rankine cycles for the same condensing pressure, and can work with dry cooling technologies instead of wet without reducing the electrical performance of the plant or increasing investment costs.
In the hygroscopic cycle, the air-cooler condenser, or surface condenser, is replaced by a steam absorber or mixing condenser in which the steam turbine outlet is put in contact with the hygroscopic compounds. The heat of condensation can be released in a cooling tower, in a once-through (or open circuit), or with a dry cooler, the latter being one of the leading innovations of the technology.
Dry coolers cool the hygroscopic flow leaving the absorber by passing throughout finned tubes which are in contact with an air stream. This hygroscopic flow works as a reflux of the condenser and is responsible for dissipating the energy of condensation.
Unlike the air condensers, the size of these devices is lower since the specific volume of the stream to be cooled is much lower (in this case cooling is done over a liquid stream and not over a vapour at vacuum) and energy costs as well as the investment price are lower. In some locations this technology uses an adiabatic dry coole, the main feature of which is the use of water at times when temperatures are high. The saturation of air with water lowers the temperature of the air to avoid affecting the electrical performance of the installation. In such cases there are savings in annual cooling water of at least 85 per cent.
There are several advantages of using the hygroscopic cycle rather than the Rankine cycle.
The first is an increase in electric power generation of around 1 per cent (net electrical efficiency increases from 0.25 per cent to 1 per cent) due to the optimization of the condensation temperature and heat and chemical recovery through the boiler blowdown, which is used in the absorber. Therefore, emissions of CO2 and other gases (NOx, SOx…) per kWh are reduced, as is fuel consumption. Another feature is that the hygroscopic cycle’s internal power consumption is similar to, or even lower than, that of a traditional Rankine cycle.
The second advantage is that this technology allows power generation facilities to work with dry coolers, which guarantee saving in cooling water consumption of 100 per cent a year. In addition, it is possible to save 50 per cent of the demineralized water supply.
The third advantage of this technology is an estimated 25 per cent reduction in operation and maintenance costs. Most Rankine cycles use surface condensers with wet cooling or cooling towers. The cleaning, fillings change, waste treatment, chemical additives and other operations related to cooling towers could be eliminated using the hygroscopic cycle technology. Steam absorbers and dry coolers are characteristic of the aforementioned technology, and have a minimum need for maintenance.
Finally, the fourth advantage is a reduction of around 5 per cent in investment costs, given the equipment involved in the Hygroscopic Cycle with respect to a conventional Rankine cycle.
In short, these advantages allow lower costs of electricity production in power generation plants that use a steam or Rankine cycle.
The hygroscopic cycle is a development of the Rankine cycle, which reduces production costs and decreases the environmental impact. This technology helps to achieve the challenges set in the COP21 Paris Climate Conference.
This technology is applicable to any plant using a Rankine cycle for power generation – thermoelectric, biomass, concentrating solar power, combined cycles, nuclear, cogeneration, waste-to-energy etc), new and existing – and it is compatible with any of the improvements in the Rankine cycle.
It is an innovation that we can utilize to preserve our water – ‘blue gold’, a natural, essential, valuable and limited resource for the development and survival of humanity.
Francisco Javier Rubio Serrano is head of engineering of the energy division at IMASA Project Engineering Corp. He led the group of engineers that developed the Hygroscopic Cycle. IMASA has inaugurated its first test plant in Gijón, Spain. www.hygroscopiccycle.com