DESAL LOOKS TO FUEL CELLS

How can the trusty fuel cell help in the fight to overcome the future global water crisis?

Between two and 7 billion people will face water shortages by
the year 2050, says the UN, and it is estimated that the amount of water per person will shrink by a third during the next two decades. Desalination is seen as a key technology for addressing such supply problems in many regions. The downside? It can be an expensive and energy-hungry business. But Rajindar Singh of Siemens Water Technologies explains that fuel cells can play their part in bringing costs down.

Inadequate supplies of good-quality water coupled with higher water demand due to rapid population growth and industrialisation in developing countries, are among the major reasons for the worsening water situation. Current shortages of potable water around the world and looming water scarcity
– especially in the developing countries – is the driving force behind the implementation of membrane technologies for seawater and brackish water desalination.

Desalination – thermal and membrane– supplies potable water on the order of only 0.1% of overall fresh water usage. However, the rapid expansion of population in arid and semi-arid areas has provided an impetus for the construction of medium to very large (0.3 Mm3/day) seawater reverse osmosis (SWRO) desalination plants in the last five years.

SWRO desalination is becoming viable mainly for two reasons:

- A substantial reduction in the cost of desalinated product water from US$1.75/m3 in 1990 to US$0.95/m3 in 2006;

- Substantial reduction in specific energy usage from 8 kWh/m3 to 4 kWh/m3 in the last decade.

Comparable figures for brackish water RO (BWRO) desalination are US$0.25/m3 and 0.8 kWh/m3.

Efforts are underway to reduce the energy consumption to less than 2 kWh/m3 in order to make SWRO desalination competitive worldwide.

One technique for increasing the efficiency and reducing the cost of producing desalinated water is to use integrated systems i.e. a thermal desalination system coupled with a single-stage SWRO system. Co-generation using dual-purpose plants that use both electricity and waste heat to produce potable water by membrane desalination is another attractive option.

Because of the scarcity of water in remote areas, high energy consumption and environmental pollutant emissions from conventional power plants that use fossil fuels, renewable energy based desalination systems such as solar distillation have been built in the last 30 years. Renewable energy sources such as wind power and solar energy have been successfully integrated with RO plants of low to medium capacity since 2000.

Wave-powered desalination and fuel cellpowered integrated desalination are among the newest alternate energy sources under investigation.

The rationale for developing integrated energy-water systems is to reduce capital cost, energy consumption and the cost
of desalinating seawater by RO, as 50% of the operating costs are due to energy consumption.

Furthermore, desalination systems powered by conventional energy sources can have significant operating costs resulting
from a large variability in energy costs reflecting fuel price volatility. The efficacy of an integrated fuel cell-RO membrane
desalination system for reducing energy consumption, and thereby reducing the cost of producing potable water from saline water (especially in remote locations) is the focus of this article.

Fuel cell power

Fuel cells are electrochemical engines that convert the available chemical-free energy in a fuel – usually hydrogen and oxygen– to electrical energy directly, without going through the heat-exchange process.

Gaseous fuel is fed continuously to the anode where it gets oxidised and the oxidant – air or oxygen – is fed continuously
to the cathode where it gets reduced.

Electrochemical reactions take place at the electrodes to produce an electric current in the external circuit. The electrolyte separating the anode from the cathode conducts
ions between the electrodes completing the electric circuit.

Since the chemical energy is directly transformed into electricity, the theoretical efficiency is not limited by the Carnot cycle as it is in the case of conventional power plants. In theory, it is possible to construct a fuel cell of 80%-90% efficiency. In practice, because of irreversible losses (over-potentials), the efficiency of a fuel cell system is 40%-45%, based on the lower heating value of the fuel.

However, efficiencies of 80% have been achieved for fuel cell power plants with cogeneration i.e. combined heat and power
systems – and hybrid fuel cell/reheat gas turbine cycles – have efficiencies approaching 70%. Importantly, since fuel cells operate at nearly constant efficiency independent of size, small plants operate nearly as efficiently as large ones.

Fuel cells power plants are also environmentally benign; emissions of sulphate (SO2) and nitrogen oxide (NOx) are nearly 1,000 times lower than those of fossil-fuel power plants, and the carbon dioxide (CO2) emissions profile is better based on overall plant efficiency.

In fact, the overall emissions are so low that fuel cell plants are exempt from air permitting in the South Coast and Bay Area Air Quality Management Districts in California – which have some of the most stringent limits in the United States.

Similarly, a 200 kW fuel cell system running on anaerobic digester gas in New York City has negligible emissions; <0.5 ppm of CO, <0.5 ppm volatile organic compounds, <1 ppm SO2 and <0.4 ppm NOx.

There are several types of fuel cells depending on the electrolyte and operating temperature:

• Alkaline fuel cell (AFC at 50-90 oc);
• Direct methanol fuel cell (DMFC at 50-120 oc);
• Polymer electrolyte fuel cell (PEMFC at 80-120 oc);
• Phosphoric acid fuel cell (PAFC at 180-210 oc);
• Molten carbonate fuel cell (MCFC at 600-650 oc);
• Solid oxide fuel cell (SOFC at 800-900 oc).

Of these, the PAFC has been commercialised for stationary applications, and PEMFC is the most attractive for vehicular systems.

Both MCFC and SOFC are best suited for dual heat and power applications.

A recent promising development is the successful demonstration of intermediate temperature or IT-SOFC operating at 500-600 oc using cerium gallium oxide electrolyte
on stainless steel substrate – instead of conventional yttrium-stabilised zirconium on ceramic based high temperature SOFCs.

The single cell potential for most fuel cells is 0.6 to 0.8 volts (d.c.) i.e. it is 0.65 volts at a current density of 160 mA/cm2
at 200 oc for a PAFC, and 0.75 volts at a current density of 430 mA/cm2 at 85 oc for a PEMFC.

A typical fuel cell power plant consists of a fuel processor that reforms the fuel i.e. natural gas or methanol to hydrogen; a multi-cell fuel cell stack (5 to 500 kW); a power conditioner that converts fuel cell d.c. power output to a.c. power; and a heat exchanger (for heating during start-up and for removing heat due to irreversible losses during operation). The electrochemical cell stack is thus a small but vital component of a fuel cell power plant, as shown in figure 1.

Integrated fuel cell RO desalination processes

The integration of fuel cells with desalination units has been investigated in the last few years. Specifically, different configurations have been evaluated, in which a fuel cell stack
provides electricity to a RO unit, while stack waste heat is recovered via a heat exchanger for preheating feed to a multi-stage flash thermal desalination unit and/or the RO unit.

These studies have shown that regardless of the type of desalination process (membrane or thermal), when fuel cells are integrated with desalination units, waste heat and power generated by the fuel cell is efficiently utilised by the desalting process.

Since the demand for electricity and water is never constant in a 24-hour period – while fuel cells work more efficiently at constant loads – two different scenarios are envisaged for fuel cell integrated RO plants:

• Off peak operation;
• Peak operation such that the hybrid system alternates between water production and electricity supply to the grid.

During off peak hours when electricity demand is low, fuel cell power would supply more electricity to the RO unit to produce
water continuously, whereas during peak hours when electricity demand is high, it would be more economical to generate more electricity and sell it to the grid with reduced production of RO purified water. Typical electrical consumption in SWRO plants operating at 40%-45% product water recovery and with energy recovery from the high pressure reject stream currently is about 4-5 kWh/m3.

The near-term goal of the industry is to reduce electrical consumption by half using a combination of energy efficient
RO pumps, advanced energy recovery turbines, high performance low energy RO membranes, and by using alternate energy powered RO systems, thereby, also reducing
the desalinated product water cost by more than 33% to less than US$0.80/m3.

A schematic diagram of a fuel cell-RO hybrid system is shown in figure 2, where the fuel cell stack waste heat is used for preheating saline feed water. According to one analysis, energy consumption for desalination is reduced by 8%-10%, due to lower feed pressure when the feed water temperature
is increased from 20 oc to 28 oc i.e. for a 0.1 Mm3/day product water desalination plant operating at 39% product water recovery (PWR) at 20 oc and 43% PWR at 28 oc.

Higher feed water temperature also results in higher productivity; a rule of thumb is that membrane flow rate increases 3% per oc rise in temperature as a result of reduced viscosity. Higher productivity, in turn, means fewer membrane elements because lower membrane surface area is required for the same flux (L/m2.hr), resulting in reduced capital and operating costs. There is, however, a small penalty in terms of higher dissolved ions concentration in product water; product water quality decreases with rise in temperature due to higher osmotic pressure, and because solute (ions) flow through the
membrane has a higher activation energy than water flow.

One intangible but significant benefit of a fuel cell powered RO desalination plant is the limited water required for fuel cell
operation compared to massive quantities of water required for conventional power plants (CPP) for system cooling, e.g. a 20-MW fuel cell plant requires only 7-10 m3/hr water for the fuel processor (see figure 1).

Fuel cell plants also reduce or eliminate adverse environmental impact on aquatic ecosystems; for example in the case of CPP
integrated SWRO systems, power plant thermal discharge to the oceans is a serious drawback.

Integrated fuel cell – RO desalination application

The studies so far have been based on high temperature fuel cells – MCFC and SOFC. However, since neither of these fuel cells is economically viable yet, it is more prudent to investigate commercialised fuel cell technologies, specifically PAFC systems. Several hundred 200-kW PAFC systems operating on various fuels – natural gas, propane, butane, methane, pure hydrogen, and methane from anaerobic digesters at wastewater treatment plants or landfills – have been installed around the world during the last decade. These on-site distributed units generate power for hospitals, hotels, schools, military installations, manufacturing plants, municipal facilities and wastewater treatment plants.

A typical 200-kW PAFC unit generates enough power to supply electricity to nearly 150 homes, and 176,500 kcal/hr of usable
heat at 60 oc. As independent stand-alone units, therefore, they are ideally suited for providing power to remote communities as well as power and waste heat to membrane
desalination units.

Based on preliminary calculations, the above waste heat figure of 176,500 kcal/hr is adequate to heat 45 m3/hr of RO plant
feed water from 20 oc to 28 oc. The electric power required by the RO high pressure pump to deliver 45 m3/hr feed water to the RO membrane unit at 33 bar g (total delivery head) is 62 kW based on a pump efficiency of 75% and a motor efficiency of 90%.

Assuming a PWR of 50%, the RO plant will produce 22.5 m3/hr of desalinated potable water, i.e. 150 L/hr (1 US gallon = .785
litres) per family for a rural community of 150 homes. In addition, the PAFC system can supply at least 0.6 kW of electric power to each family in the community after supplying
power to the RO plant.

To conclude, even a modest fuel cell-RO membrane hybrid plant is a very promising technology for meeting the critical needs of energy and potable water around the world.