Friday, 12 November
AGEC 2010 Paper on New Options for Dry Cooling Towers
The QGECE colleagues Drs Guan and Hooman will present two papers at the Australian Geothermal Energy Conference in Adelaide next week on what improvements can be expected in dry cooling area. This is a topic of crucial importance to the Australian geothermal industry and one of the four research programs in our Centre. I will try to explain why it is important in today's blog. Please note that I am not going to tell you about the contents about the papers. You have to come to the Conference for that. The following is just some background information.
A thermal power plant is basically a heat engine operating between a high temperature supply of heat and a low temperature sink. The heat engine converts some of the high temperature heat to work and the remaining heat needs to be dumped. No heat engine can produce work if it is connected to only a hot source. It always needs to have a cold sink to dump its waste heat. The relationship between the heat supply, heat dump and the power is provided by thermal efficiency:
The second law requires that the power cannot be equal to the heat input and there should always be an amount of heat dumped to a low-temperature heat sink. If the cycle is efficient, a higher amount of the geothermal heat is converted to electricity and the waste heat, the amount of heat that needs to be disposed, is relatively small. More heat needs to be disposed of for less efficient cycles. This is shown in the following graph.
The above chart illustrates the point that as the efficiency is lowered, the amount of heat that need to be disposed of for every unit of generated power increases very rapidly. For example, a representative efficiency for coal-fired power plants built in the last century is about 33%. These plants would dump 2 MW of heat for every MW of electricity they generate. A geothermal binary plant at half that efficiency would dump heat at the rate of 5 MWthermal for each MW of electricity generated or two-and-a-half times higher; and it would get increasingly worse at lower geothermal cycle efficiencies as shown in the chart.
Seawater cooling
For a power plants located on the shore, the preferred option is to use the sea as the heat dump. The cold water is pumped from the sea, passed through the condenser, and the warm water is returned back to the sea. This is also referred to as direct cooling or once-through cooling. Such systems usually target a temperature difference of 5-15oC between the ambient seawater temperature and the temperature of the returned water. This results in a water suction rate of about 25-50 kg/s per MW of electricity generation. All of the water is returned back to the sea (or the lake as the case may be). Unless the plant is in a sensitive area, open cycle seawater cooling is an environmentally acceptable practice although environmental monitoring needs to be in place to minimise risk. Depending on the size of the power plant, lakes or even rivers can be used as the heat sink using this method. Historically once-through cooling was the preferred method of cooling for a significant number of Australian power stations.
Wet Cooling Towers
If the dissipative capacity of the water reservoir is limited, then the atmosphere can be used as the heat sink. The condenser heat is dumped into a cooling water loop running in a closed cycle as shown in the following figure. The warm water from the condenser is sprayed into a cooling tower. The water droplets trickle down over a packing medium and during this process some of the water evaporates into the air stream through the packing. This evaporation cools the remaining water stream down to the wet bulb temperature of the incoming air stream. The cooled water collects at the bottom of the cooling water and is pumped back to the condenser.
Initially, these towers were built as tall concrete structures with the air flow being powered by natural draft. Recent designs use lower heights with fans pushing the air through the packing and out to the air. The following is a photograph of the wet cooling towers at the Germencik geothermal power plant in Western Turkey.
Since most of the cooling occurs through evaporative cooling, the thermal pollution of the water reservoir is reduced compared to the open cycle cooling as in the previous section. On the other hand, the relatively high water consumption rate can be prohibitive in areas where fresh water is scarce. The water consumption in a wet cooling tower can be estimated by using the latent heat of evaporation:
At a temperature of T=30oC, the latent heat of evaporation is about hfg=2400 kJ/kg. Based on this number, the evaporation rate for a 1-MW of electricity generation at 33% efficiency would be about 0.8 kg/s or almost 3 tonnes per hour. In a wet cooling tower, there are additional losses grouped under the general term of leakage, generally limited to below 8% of the evaporative losses.
In general, it is better to represent the cooling tower water requirements in terms of the heat dump rather than the power generated. This makes it easier to calculate the water requirements for thermal power plants at different cycle efficiencies. On that basis, the water consumption is 0.4 kg/s per MW of heat being dumped. A 25-MW geothermal power plant running at 15% efficiency will need to dispose of 142 MW of heat. The wet cooling tower water consumption for this plant would be 56.8 kg/s or 1.8 million tonnes per year or 1800 megalitres/year. If the water is not freely available, it would have to be purchased. The current bulk water prices in South-East Queensland are about $1000/ML. At a price of $1000/ML, supply of water to the wet cooling tower would cost the power plant $1.8m per year.
Dry Cooling Towers
If the thermal power plant is not located on the coast or near a large lake and if the water is either unavailable or too expensive to use a wet cooling tower, then the only cooling option is using a dry cooling tower. In a dry cooling tower, the air absorbs the heat from the heat exchanger bundles placed in the path of the air stream. As in the wet cooling towers, the air stream is either driven by fans or by the buoyancy of the air (natural-draft). The fan-driven systems can be built quickly and at relatively low cost but their operating costs are higher due to their higher maintenance requirements and the parasitic losses associated with running the fans. The latter can be as high as 15% of the gross power generation for geothermal plants and higher on hotter days when the efficiency drops.
Natural draft dry cooling towers also suffer similar efficiency reduction on hot days but the net plant output reduction would not be as high because there are no parasitic losses. They have low maintenance requirements and low operating costs but cost more to build.
Exploration of options to increase the viability of natural draft dry cooling towers for Australian geothermal applications is a major research program for the Queensland Geothermal Energy Centre of Excellence. This is being pursued along a number of avenues, including the following:
- Advanced heat exchangers with more favourable pressure drop to heat transfer ratios (very interesting things ar ehappening in this area and they will be adressed in Dr Hooman's presentation at the AGEC 2010 next Friday)
- Optimised coling tower design using new materials and new construction methods
- Hybrid systems including
- Solar enhancement (this is an interesting concept in the cooling tower context and you will hear about it in Dr Guan's presentation at AGEC 2010)
- Using the shallow aquifers as additional heat dumps (suggested by Bob Collins in AGEC2009 and the topic for a QGECE Master student)
The Conference starts next Wednesday and be there to learn more on the above and other aspects of geothermal energy utilisation.
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