Hybrid Natural Draft Dry Cooling Towers - An Enabling Technology for
Remote Area Thermal Power Generation in Australia
Stephen Gwynn-Jones1, Aleks Atrens1, Zhiqiang Guan1, Yuanshen Lu1, Hugh Russell1, Hal Gurgenci1, Kamel
Hooman1
1
Queensland Geothermal Energy Centre of Excellence (QGECE), Mechanical Engineering, The University of Queensland,
Abstract
The University of Queensland’s (UQ) Geothermal Centre (QGECE) has developed an innovative new
cooling tower technology to reduce water consumption and the cost of generating electricity in regional
Australia. Remote Australian communities need cost effective small scale power generation options [1,
2]. Small scale thermal power plants (1-10 MW) using renewable sources (geothermal, biomass and
solar thermal) could meet this need [2], and these technologies require cooling towers that work
efficiently at small scale without consuming excessive amounts of water. The QGECE has developed a
polymer-steel cooling tower that has a flexible design allowing operation across the range of dry, wet,
and hybrid cooling modes. This tower has a modular construction that is easily deployable to remote
sites and dramatically cheaper than concrete cooling towers, particularly at small scales. The
demonstration unit, built at the UQ Gatton Campus is large enough to contribute to the efficient supply
of power for up to 1000 people. The QGECE hybrid cooling tower at Gatton is a world first research
facility with profound implications for power generation.
Background
All thermal power plants (including geothermal, solar thermal, biomass, coal-fired and even nuclear)
produce waste heat as a byproduct. This waste heat must be continuously dissipated by a cooling tower
to allow the plants operate efficiently. The cooling tower is an integral part of a thermal power plant as
indicated in Fig. 1, and the performance of the cooling tower is crucial to the power plant efficiency.
TURBINE + GENERATOR
HEAT SOURCE E.G.
‐GEOTHERMAL
‐SOLAR THERMAL
‐BIOMASS
‐GAS
‐COAL
‐NUCLEAR
PUMP
COOLING TOWER
Fig. 1 – Cooling towers are an integral part of thermal power plants for all heat source types
Queensland Geothermal Energy
Centre of Excellence
The University of Queensland
Brisbane QLD 4072 Australia
E [email protected]
W http://www.geothermal.uq.edu.au/
Depending on the primary cooling method used, all power plant cooling towers are classified as either
wet or dry. In a wet cooling tower, which is the type most often used in coal-fired power plants, cooling
is achieved primarily by evaporation of water into the air flowing through the tower. Dry cooling towers
simply transfer heat from the power plant directly to the air.
Hybrid Natural Draft Dry Cooling Tower
Fig. 2 – QGECE's hybrid natural draft dry cooling tower at the UQ Gatton campus
The QGECE hybrid cooling tower (Fig. 2) is a novel combination of the following features:
1.
2.
3.
4.
Flexible cooling modes
Natural draft
Small scale
Modular construction
These features are described in more detail in the following sections. A patent application has been
submitted for several of the tower features (patent application no. 2015903610).
As indicated in Table 1, the hybrid cooling tower is capable of being connected to a thermal power
plant of sufficient scale to demonstrate power technologies suitable for remote communities of up to
1000 people.
Table 1– Key parameters of hybrid cooling tower
Parameter
Value
Tower height
20
Tower inlet height
5
Tower inlet diameter
12.5
Design heat rejection
1.5
Suitable for solar thermal plant (net) sizes 0.5 – 1.5
Suitable for geothermal plant (net) sizes
0.1 – 0.3
Units
m
m
m
MWthermal
MWelectric
MWelectric
Flexible Cooling Modes
Water is a precious resource in Australia, and it not feasible to use wet cooling for thermal power plants
in many arid areas.
Wet cooling towers are one of the largest consumers of water in power generation from a variety of
mechanisms including evaporation loss, blow down water loss, and drift water loss. Williams and Rasul
[3] report the water evaporation rate for a coal power plant of 350 MW capacity in Queensland is around
1.8 litres of water per kWh of power generated. This is about 630 thousand litres per hour or 5.5 billion
litres per year for a 350 MWe coal-fired power plant. Geothermal power plants would consume more
water due to their lower thermal efficiencies. Gurgenci [4] predicted that the total water evaporation
rate for a geothermal power plant is about 0.4 kg/s per MW of heat dissipated by the cooling tower.
This implies water evaporation of 8.2 litres per kWh of power generated for a plant running at 15%
thermal efficiency. A 25 MWe geothermal plant would consequently use 1.8 billion litres per year.
Dry cooling can conserve this precious water resource and may be the only viable option for thermal
power plants located in many remote areas of Australia. Dry cooling towers keep the working fluid
separated from the cooling air, consuming no water. While dry cooling systems can save significant
amounts of water, there is a trade-off in the form of more heat exchangers (higher capital costs), and
reduced plant efficiencies at high ambient temperatures. The higher capital costs are balanced against
the savings from not consuming water, and hybrid cooling be introduced to solve the intermittent high
temperature efficiency issue.
The QGECE hybrid tower has been designed such that each individual construction can be built for a
different cooling mode:



In areas where water is scarce, the tower can be built to operate as a dry cooling tower and
conserve precious water;
In areas where water is plentiful, it can be built as a wet cooling tower, and achieve optimum
performance; and
In arid areas where some water is nonetheless available, it can be built to operate in hybrid
cooling mode, using limited water to achieve high performance.
Hybrid Cooling
Dry cooling towers experience lower efficiency when the ambient air temperature is high (typically
during the middle of the day). Hybrid cooling is a method of pre-cooling the ambient air using small
amounts of water when the ambient temperatures are high. Hybrid cooling towers can be selectively
operated in dry mode depending on ambient temperatures and water availability, and in wet mode to
improve plant efficiency and increase power output. Hybrid cooling towers consume significantly less
water than traditional wet cooling towers. The QGECE has studied various methods of providing the
pre-cooling [5-10] and will design, build and demonstrate the technology using the tower at the UQ
Gatton campus. A small amount of water will be used to enhance the cooling performance of the
NDDCT during periods of high ambient temperatures.
In this hybrid cooling design, water is introduced into the inlet air stream of a dry cooling tower. The
water evaporates and reduces the air temperature toward the ‘wet bulb temperature’, the minimum
temperature that can be reached by evaporative cooling. The cooler air then extracts more heat as it
passes through the heat exchangers, increasing heat transfer efficiency. Numerical modelling indicates
that a power plant using this system can increase net power output during periods of high ambient
temperature by up to 20%. The addition of a hybrid system increases the range of ambient temperatures
in which the tower can operate. This allows the tower to be built smaller (and at lower cost) and
increases the total plant power output over the life of the tower. A summary of the performance
Hybrid Tower Heat Transfer Increase (%) improvements of running a natural draft dry cooling tower in hybrid mode is shown in Error!
Reference source not found..
100%
80%
60%
40%
20% Humidity
20%
0%
0
10
20
30
40
50
60
Ambient Temperature (˚C)
Fig. 3 – Heat transfer increase for hybrid over dry cooling tower for 20% relative humidity, adapted
from He et al. [8]
The other exciting benefit of hybrid cooling explored by the QGECE is that it can utilise brackish or
briny water without water pre-treatment. Traditional wet cooling towers cannot use this water without
expensive treatment processes. Preliminary tests suggest that using brackish water for hybrid cooling
may even improve heat transfer compared to clean water.
Natural Draft
In a natural draft dry cooling tower, no fans are required to drive the air flow. The air is driven through
the bundles of heat exchangers and the tower occurs by buoyancy effects. Buoyancy is due to a
difference in air density between the inside and outside of the tower; the tower is filled by heated air,
which, like the air in a hot air balloon, is lighter than the surroundings. The greater the temperature
difference and the height of the tower structure, the greater the buoyancy force. Therefore the volume
flow rate of air across the heat exchanger bundle is directly proportional to tower height and the
temperature difference between inside and outside. Large natural draft cooling towers up to 200 m tall
have been built for thermal power plants.
Using natural draft cooling towers in small thermal power plants removes the costs, power consumption,
and maintenance needs associated with fans. However, natural draft towers are normally built over
100 m tall, and the capital cost is traditionally higher than fan forced. The QGECE’s innovative new
design addresses these issues as described below.
Small Scale
Until recently, there has not been much attention given to the possibility of natural draft being applied
to small scale thermal power plants due to fixed costs and performance benefits of large towers.
Research on natural draft cooling towers in recent decades has focused on large towers with tower
heights of more than 100 m for large thermal power plants. With the increased desire to build small
scale thermal plants for remote areas, plant designers looking to maximise power output and efficiency
are looking for alternatives to fan forced cooling. Small, high performance, natural draft dry cooling
towers are consequently an enabling technology to improve the efficiency and power output of small
thermal plants.
A major design issue for small natural draft cooling towers is the negative effect of crosswinds on
cooling performance, which reduces overall plant efficiency. The performance degradation from
crosswinds is larger for small towers than for tall towers. In order to overcome this issue the QGECE
has studied the mechanics of crosswind interaction with cooling towers, and developed a solution,
windbreak walls, that will be demonstrated in the tower at Gatton at a scale applicable to industry.
The QGECE has designed windbreak walls to improve performance in cross wind conditions based on
numerical modelling and laboratory scale testing [11-13]. Three simple but effective windbreak walls
will be installed (after sufficient performance data is collected without them) to enhance the
performance of the demonstration hybrid tower under crosswind. These walls are used to divert
crosswind flow through the heat exchangers to increase the heat transferred, improving plant efficiency.
When there is no crosswind, the cooling air enters into the tower freely without any obstruction from
the walls. On a windy day the walls stop the crosswind flowing past the tower, redirecting it through
heat exchangers to improve performance. 3D simulation results show that the performance of the tower
with windbreak walls will be increased by nearly 40% at crosswind speeds of 10 m/s. A comparison of
the tower performance with and without windbreak walls has been made in Error! Reference source
not found..
Fig. 4 – CFD Simulation of airflow during crosswind conditions for a small natural draft dry cooling
tower without windbreak walls (left), and with windbreak walls (middle). The graph shows that
without wind break walls the tower performance is reduced to 62% with 5 m/s wind, whereas in the
case with the wind break walls the tower performance is increased to 115% at the same wind speed.
Adapted from [11]
Modular Design
An additional benefit of building small scale cooling towers is that they may be constructed out of
materials other than reinforced concrete.
To overcome the typical expenses associated with natural draft cooling towers, and improve economics
for small-scale towers, the QGECE has pioneered a modular construction from a steel frame and PVC
polymer membrane. These materials dramatically reduce costs relative to concrete while still providing
the same structural integrity and resilience to wind loading. The modular design also substantially
reduces construction time (by an order of magnitude or more), and thereby reduces construction costs.
Finally the modular design enables rapid deployment to remote sites: construction requires only a
concrete foundation and crane; workers can rapidly bolt together structural components in a few days.
Future Innovation
While the UQ Gatton demonstration hybrid tower already provides substantial technological advances
that will enable widespread use of small-scale natural draft cooling, the QGECE has plans for further
research and development. The QGECE will explore the use of solar enhancement to improve cooling
tower performance. In a solar hybrid natural draft dry cooling tower, solar collectors are added and
arranged radically at the base of the tower, and the heat exchangers are placed vertically at the outside
edge of the solar collectors. From this arrangement, extra heat from the solar collectors increases the
buoyancy of the air inside the tower and drives more air through the heat exchangers, improving the
cooling rate. The system exploits Australia’s abundant solar energy, which is most plentiful during the
hottest periods, and during which natural draft dry cooling has reduced performance. Modelling showed
that more than 10% net power can be achieved in a hot day.
Fig. 5 – Schematic of the solar enhanced natural draft dry cooling tower configuration (left), and
estimated increase in plant net power output with solar enhanced tower (right) [14]
Summary
The QGECE has developed a hybrid cooling tower that will be an enabling technology for small-scale
thermal power plants in regional Australia. This technology is possible due to the QGECE’s innovations:



Flexible cooling modes, including hybrid cooling, allowing the tower design to be tailored to
site-specific water availability;
Windbreak walls, allowing the hybrid tower to achieve consistent performance at any scale
even in the presence of crosswinds; and
Modular steel and polymer design, reducing construction costs & time, and improving
capability for isolated deployments, thereby enabling small-scale natural draft designs.
These key enabling technologies will be demonstrated at a scale applicable to industry in the tower
deployed at the UQ Gatton Campus. Ultimately the innovations provide the following key benefits:






Reduced or eliminated water consumption;
No fan power consumption;
Low maintenance costs;
Simple tower structure and low construction cost;
High efficiency heat exchangers (minimal pressure losses) providing excellent cooling
performance; and
Provides an enabling technology for remote area thermal power generation.
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