24-07-2012, 04:55 PM
Design of Commercial Solar Updraft Tower Systems – Utilization of Solar Induced Convective Flows for Power Generation
[attachment=28775]
ABSTRACT
A solar updraft tower power plant – sometimes also called
'solar chimney' or just ‘solar tower’ – is a solar thermal power
plant utilizing a combination of solar air collector and central
updraft tube to generate a solar induced convective flow which
drives pressure staged turbines to generate electricity.
The paper presents theory, practical experience, and
economy of solar updraft towers: First a simplified theory of
the solar tower is described. Then results from designing,
building and operating a small scale prototype in Spain are
presented. Eventually technical issues and basic economic data
for future commercial solar tower systems like the one being
planned for Australia are discussed.
INTRODUCTION
Sensible technology for the wide use of renewable energy
must be simple and reliable, accessible to the technologically
less developed countries that are sunny and often have limited
raw materials resources. It should not need cooling water and it
should be based on environmentally sound production from
renewable or recyclable materials.
FUNCTIONAL PRINCIPLE
The solar tower’s principle is shown in figure 1: Air is
heated by solar radiation under a low circular transparent or
translucent roof open at the periphery; the roof and the natural
ground below it form a solar air collector. In the middle of the
roof is a vertical tower with large air inlets at its base. The joint
between the roof and the tower base is airtight. As hot air is
lighter than cold air it rises up the tower. Suction from the
tower then draws in more hot air from the collector, and cold
air comes in from the outer perimeter. Continuous 24 hoursoperation
can be achieved by placing tight water-filled tubes or
bags under the roof. The water heats up during day-time and
releases its heat at night. These tubes are filled only once, no
further water is needed. Thus solar radiation causes a constant
updraft in the tower. The energy contained in the updraft is
converted into mechanical energy by pressure-staged turbines
at the base of the tower, and into electrical energy by
conventional generators (Schlaich and Schiel, 2001).
COMMERCIAL SOLAR TOWER POWER PLANTS
Scale-Up
Detailed investigations, supported by extensive wind
tunnel experiments, show that thermodynamic calculations for
collector, tower and turbine are very reliable for large plants as
well (Schlaich et al. 1990). Despite considerable area and
volume differences between the Manzanares pilot plant and the
projected 200 MW facility, the key thermodynamic factors are
of similar size in both cases. Using the temperature rise and air
velocity in the collector as examples, the measured temperature
rise at Manzanares was up to 17 K, wind speed was up to 12
meters per second during turbine operation, while the
corresponding average figures from simulation runs for a 200
MW facility are 18 K and 11 meters per second, respectively.
Therefore measurements taken from the experimental plant
in Manzanares and solar tower thermodynamic behavior simulation
codes are used to design large plants with an output of up
to 200 MW. Results of such a simulation are shown in Fig. 10.
Shown are four-day-periods for summer and winter. This plant
with additional storage covering 25% of total collector area
operates 24h per day, at or close to nominal output in summer,
and at significantly reduced output in winter.
SUMMARY AND CONCLUSIONS
The updraft solar tower works on a simple proven
principle, its physics are well understood. As thermodynamic
efficiency of the plant increases with tower height, such plants
have to be large to become cost competitive. Large plants mean
high investment costs, which are mostly due to labor costs. This
in return creates jobs, and a high net domestic product for the
country with increased tax income and reduced social costs (=
human dignity, social harmony), and in addition no costly
consumption of fossil fuels. The latter reduces dependence on
imported oil and coal, which is especially beneficial for the
developing countries releasing means for their development.
[attachment=28775]
ABSTRACT
A solar updraft tower power plant – sometimes also called
'solar chimney' or just ‘solar tower’ – is a solar thermal power
plant utilizing a combination of solar air collector and central
updraft tube to generate a solar induced convective flow which
drives pressure staged turbines to generate electricity.
The paper presents theory, practical experience, and
economy of solar updraft towers: First a simplified theory of
the solar tower is described. Then results from designing,
building and operating a small scale prototype in Spain are
presented. Eventually technical issues and basic economic data
for future commercial solar tower systems like the one being
planned for Australia are discussed.
INTRODUCTION
Sensible technology for the wide use of renewable energy
must be simple and reliable, accessible to the technologically
less developed countries that are sunny and often have limited
raw materials resources. It should not need cooling water and it
should be based on environmentally sound production from
renewable or recyclable materials.
FUNCTIONAL PRINCIPLE
The solar tower’s principle is shown in figure 1: Air is
heated by solar radiation under a low circular transparent or
translucent roof open at the periphery; the roof and the natural
ground below it form a solar air collector. In the middle of the
roof is a vertical tower with large air inlets at its base. The joint
between the roof and the tower base is airtight. As hot air is
lighter than cold air it rises up the tower. Suction from the
tower then draws in more hot air from the collector, and cold
air comes in from the outer perimeter. Continuous 24 hoursoperation
can be achieved by placing tight water-filled tubes or
bags under the roof. The water heats up during day-time and
releases its heat at night. These tubes are filled only once, no
further water is needed. Thus solar radiation causes a constant
updraft in the tower. The energy contained in the updraft is
converted into mechanical energy by pressure-staged turbines
at the base of the tower, and into electrical energy by
conventional generators (Schlaich and Schiel, 2001).
COMMERCIAL SOLAR TOWER POWER PLANTS
Scale-Up
Detailed investigations, supported by extensive wind
tunnel experiments, show that thermodynamic calculations for
collector, tower and turbine are very reliable for large plants as
well (Schlaich et al. 1990). Despite considerable area and
volume differences between the Manzanares pilot plant and the
projected 200 MW facility, the key thermodynamic factors are
of similar size in both cases. Using the temperature rise and air
velocity in the collector as examples, the measured temperature
rise at Manzanares was up to 17 K, wind speed was up to 12
meters per second during turbine operation, while the
corresponding average figures from simulation runs for a 200
MW facility are 18 K and 11 meters per second, respectively.
Therefore measurements taken from the experimental plant
in Manzanares and solar tower thermodynamic behavior simulation
codes are used to design large plants with an output of up
to 200 MW. Results of such a simulation are shown in Fig. 10.
Shown are four-day-periods for summer and winter. This plant
with additional storage covering 25% of total collector area
operates 24h per day, at or close to nominal output in summer,
and at significantly reduced output in winter.
SUMMARY AND CONCLUSIONS
The updraft solar tower works on a simple proven
principle, its physics are well understood. As thermodynamic
efficiency of the plant increases with tower height, such plants
have to be large to become cost competitive. Large plants mean
high investment costs, which are mostly due to labor costs. This
in return creates jobs, and a high net domestic product for the
country with increased tax income and reduced social costs (=
human dignity, social harmony), and in addition no costly
consumption of fossil fuels. The latter reduces dependence on
imported oil and coal, which is especially beneficial for the
developing countries releasing means for their development.