A relative simple improvement to the Rankine cycle can boost plant performance and lower operating and maintenance costs, explains Francisco Javier Rubio of Spanish engineering company IMASA.

One of the major concerns of nations worldwide is how to deal with the continuing rising cost of traditional electric energy sources.

In a market economy, the industry has worked hard to provide innovative solutions to supply constraints. This has led the industry to invest heavily in order to increase efficiency and seek new sources of energy.

The future energy needs of the world population can be estimated relatively accurately, but it is more difficult to qualify and quantify the future sources of this energy. According to best estimates, a large portion will come from the transformation of solar energy into electricity and the use of other renewable sources. However, these sources are still dependent on timing and seasonality during which they can lie idle, so the industry has turned to the solution of uniting old technologies with new to overcome these serious deficiencies.

Among the older technologies commonly used in conventional thermal and nuclear plants is the Rankine cycle or steam cycle. The performance of this cycle has been improved considerably over the past 100 years and now it is set for a further substantial improvement, not only in efficiency but on other issues of interest to the energy industry.

A new Rankine cycle technology has been developed by IMASA, a Spanish multinational engineering and projects company. IMASA exclusively holds the rigths to this technology, and is planning multiple applications in biomass-fuelled plants, as well as in the solar thermal field.

The Rankine cycle is a thermodynamic cycle which transforms heat into work.Although its effectiveness is limited by the thermodynamic efficiency of a Carnot cycle operating between two heat reservoirs, the main advantage of the Rankine cycle is its industrial maturity, benefiting from a long and continuous development, and its high degree of applicability in different processes.

Improvements have been achieved mainly by increasing the temperature difference between the cold and hot sinks, resulting from the evolution of materials that are able to sustain increasingly restrictive conditions – up to supercritical water conditions.

Also, thanks to the improvement of thermal and mechanical designs, engineers have been able to design condensers with very low pressure at the exit of the turbine, down to or below 0.1 bar. This has helped increase the electrical efficiency of the cycle.

Other improvements have included a more integrated heat recovery in the boiler, using it to superheat or reheat steam, or using a bleed stream of the turbine to preheat the feedwater to the boiler (regeneration). In some cases, binary cycles have been introduced, where two cycles in series operate at different temperatures, as well as the Organic Rankine cycle, which, uses an organic fluid rather than water. The latest technologies have used modern CFD tools to develop turbines and more sophisticated equipment with a higher performance and lower maintenance.

At present, water availability and use of cooling towers are the Achilles’ heel of Rankine cycles. Water as a commodity is becoming ever more scarce and therefore expensive. Take, for example, the water limitations in the areas suitable for solar thermal power plants with high levels of annual sunshine. These conditions are detrimental to optimising the condensation pressure at the exit of the turbine and therefore lowers the power outputs. These factors heavily penalise the performance of the cycle and will compromise the viability and success of such a project.

Hygroscopic cycle
The configuration of the novel Hygroscopic cycle enables an absorber to be used as a condensor, which has a cost benefit

Furthermore, even when water is available and affordable, the use of cooling towers leads to increasing operating costs, adding water treatment (use of chemical additives) and environmental hazards, such as the growing problem of legionella. This results in a situation where it is difficult or impractical to install these cycles using wet cooling.

There is always the air-cooled condenser (condensation of water vapour by indirect contact with ambient air) that could potentially solve part of the problem, but both its price and the required space and power consumption makes this option both unprofitable and unattractive in most cases.

Finding a solution to all these problems is where there is room for improvement. A Spanish team of researchers, led by myself, has come up with a simple solution that we believe has multiple advantages in terms of space, operating and maintenance costs, performance, and most interestingly, a lower investment cost. The solution is the Hygroscopic cycle. This evolution of the Rankine cycle works essentially with hygroscopic compounds that improve the conditions for condensing vapour at the outlet of the turbine.

The Hygroscopic cycle uses the physical and chemical principles of the absorption cycle machines to provide higher performance and better cooling in an efficient Rankine cycle system. Knowledge and experience with hygroscopic compounds has been the motivation behind this development and international experts from leading institutes have given their backing to this invention.

Hygroscopic compounds are compounds which have a high avidity for the water in vapour form. These are generally salts (e.g. LiBr, NaCl, Na2SO4 to name a few) that are sometimes already present in low concentrations in regular water supplies. These compounds are not volatile, toxic or flammable – rather the opposite: they are stable, abundant and cheap.

Depending on their nature and concentration, they all share – to a greater or lesser extent – their dissolution abilities to water, with the benefit of increasing the condensation temperatures. The simplest example is found in the increase in the boiling point of water when mixed with small amounts of salt. Through absorption with hygroscopic compounds, lower pressures can be achieved at the outlet of the turbine by cooling this steam with water at higher temperatures than in a conventional Hygroscopic cycle. This immediately leads to lower cooling duties for the condensing water used in the condenser. In fact, a careful selection of compounds with high hygroscopic endothermic heat of dilution (e.g. KNO3 and NaNO3) substantially decreases part of the condensation energy in the absorber.

To exploit this effect to the fullest and reduce even further the cooling water consumption, air coolers could be installed, in order to dissipate the remaining energy. The Hygroscopic cycle benefits from a clever configuration, such that it allows the traditional condenser, or the air condenser, to be replaced by an absorber where the hygroscopic salts come into contact with the steam turbine output.

The configuration of the cycle using the absorber as a condenser has numerous advantages over the traditional condenser. The first is the cost, as it can be up to four times cheaper than traditional condensers. It occupies much less space (reduced cost of civil works), the pressure drop is negligible (less need to sub-cool the steam) and operating costs are also significantly lower.

Depending on the concentration of the hygroscopic compound chosen, greater or lesser efficiency is obtained, since it can work with lower condensing pressure without the ambient conditions limiting the cold sink. This optimisation results in an increase in electricity production of between 1 per cent and 5 per cent when compared with a current Rankine cycle with identical ambient conditions conditions (cold sink).

It is therefore a significant step in the evolution and implementation of Rankine cycles. The Hygroscopic cycle has enormous potential for development and innovation as future hygroscopic compounds are being developed, enriching the already large family they represent.

The Hygroscopic cycle has many similarities to the Rankine cycle, but especially noteworthy is the fact that you can apply the same improvements in the conventional cycle already introduced above and achieve even more from the latter, with the advantage of having better cooling conditions.

Often in engineering, efficiency gains are linked to the higher cost of equipment, largely due to the refined materials and manufacturing technology used. However, compared to current Rankine cycles, it can be stated that for low concentrations of salt or hygroscopic compounds, an immediate increase of up to 1 per cent more electricity is obtained.

Most importantly, a global reduction of over 90 per cent of the plant water consumption is achieved, together with lower operating costs and with an initial investment similar or lower than a conventional Rankine cycle. A thorough study of the plant has concluded that for these concentrations, conventional and commercial equipment could be used, keeping the costs down, and the manufacturers guarantee.

Certainly a higher salt concentration implies a higher cost of equipment, but the increase in cycle efficiency largely compensates for this increase, thus reducing pay-back This point is key when marketing the cycle since for similar investments the Hygroscopic cycle is more competitive, profitable and with the same reliability and warranty as traditional Rankine cycles.

This technology is immediately applicable to combined-cycle, solar thermal, nuclear and biomass combustion plants, as well as coal-fired plants. Furthermore, by increasing the condensation temperature, it enables it to be used in cogeneration plants for heat recovery, further expanding its scope and future potential.

To demonstrate the real potential of this cycle, we highlight a real-life example of a recent investment, in order to compare its performance and competitive investment. The benchmark is a newly-installed biomass-fired plant, with a power output of 15 MW. This plant has an availability of 86 per cent or an equivalent of 7500 hours/year, with a net electrical efficiency of 27 per cent using a traditional Rankine cycle with the latest optimisations.

One of the problems that this plant faces is water scarcity and continuous health inspections in order to avoid the problem of legionella in the cooling towers. Therefore, in the absence of water, the condensation pressure of the steam turbine output is limited, and thus the cycle efficiency limited.

In this particular case, the equivalent and competing Hygroscopic cycle has the potential to release the cold sink limit of the cycle and increase its performance at a lower cost. To start with, the Hygroscopic cycle does not require cooling towers or air condensers. It would instead be enough to install an air cooler that will dissipate the heat of condensation of the steam produced in the absorber.

Rankine
The cost of the Hygroscopic cycle has been estmated to be 5 per cent less than a conventional Rankine cycle

It is conservatively estimated that the total investment cost of this cycle is around 5 per cent less than the current Rankine cycle. It can even ensure a net increase of 1 per cent in yield, which corresponds in this case to an additional 150 kW net, since auto-consumption does not increase with this cycle. As for annual savings of water and additives for this plant, it is estimated at 400,000 m3 of water consumption in the cooling towers, and more than 10 m3 of chemical additives (NaClO, biocide and anti-fouling) per year. In comparison, the annual water consumption of the existing plant is the equivalent to 160 Olympic swimming pools and could supply more than 7000 people a year for a normal daily consumption of 150 litres of water per person.

In addition, over time the cost of industrial water is expected to increase, and this year it is estimated to reach an average cost of €2/m3 ($2.7/m3). Applying these figures will achieve a saving of €800,000 per year in make-up water. Cooling towers are not only expensive in terms of water consumption, but they also have chemical additives, operating and maintenance costs, and have to undergo constant inspections for legionella.

As for the net increase in economic benefits, the cycle has a net electrical output of 1125 MWh/year, representing an annual income of €135,000.

So we can conclude that for this biomass plant, the use of the Hygroscopic cycle instead of a traditional Rankine cycle would result in a €1 million net profit over a year. We should also remember that we are starting from a lower investment. Thus the investment will have a much higher rate of return, and a much lower environmental impact, keeping the same safety and reliability standards as in a conventional biomass plant. It is worth noting that in thermal plants, these profits could be slightly higher, since the best locations for the plant often corresponds to marginal/desert areas where the absence of water is a problem.

These values reflect the competitiveness of the investment associated with the Hygroscopic cycle, and the team of experts responsible for this new technology are eager to introduce this cycle to the industry.

Francisco Javier Rubio is the chief engineer of IMASA’s Energy Division. For more information, visit www.imasa.com.

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