Mitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)
1Development of a Concentrated Solar Power
Generation System with a Hot-Air Turbine

HIROMI NAKATANI*1

TOSHIYUKI OSADA*2

KAZUTA KOBAYASHI*3 MASAHARU WATABE*4

MASASHI TAGAWA*5

As part of the overall utilization of solar thermal energy, the application of concentrated
solar power generation systems is highly anticipated in the Sunbelt. Mitsubishi Heavy Industries,Ltd. (MHI) is the world’s leading developer of high-temperature air-turbine power generation
osystems, which concentrate insolation with heliostats to raise the air temperature to 850 C with asolar receiver, and generate electric power via an air turbine. The system offers advantages suchas high conversion efficiency, and no need of water for power generation. This report describes theresults of the pilot test of the solar receiver, which is one of the key components of the system, as
well as some other items that govern system performance, including the design of thehigh-temperature turbine and solar field.

|1. Introduction
The need for alternative energy sources and environmental concerns are growing due tovarious reasons. In this regard, the utilization of solar energy has become increasingly important.
Photovoltaics systems convert insolation directly to electricity, although the effective
wavelength is restricted. They have been widely installed even in Japan, where direct insolation is
not abundant because diffusion light can also be utilized and can generate power at a reasonablecost.
On the other hand, as concentrated solar power systems have high conversion efficiency anddo not lose efficiency due to temperature increase unlike photovoltaics, the market for such systems
that can utilize only direct insolation is expected to expand in the so-called Sunbelt region.
It is generally believed that existing commercialized technology, based on a parabolic troughwith a line focus collector and low light concentration, is difficult to improve in terms ofefficiency/cost. Hence, attention is currently being focused on tower systems, in which the light
concentration and efficiency can be increased by the use of a point focus collector, thereby
reducing construction costs. This report describes the development of a tower concentrated solar
power generation system based on the Brayton cycle. It does not require water cooling for power
generation, which is very attractive for regions where water resources are quite limited.

|2. Market and technical trends for concentrated solar power
generation
In regions where sunny weather is not abundant and the insolation intensity is relatively low,
photovoltaics systems, which can generate power from diffusion light, are more appropriate thanconcentrated solar power generation, which requires direct insolation. However, in regions wherethe weather is stable and the insolation intensity is high, concentrated solar power generation offers*1Project Manager, New Energy Systems Department, Power Systems*2

Deputy Chief Engineer, Power Systems Project Management Division, Engineering Headquarters*3

New Energy Systems Department, Power Systems*4

Deputy General Manager Sustainability Energy & Environment Strategic Planning Department, Engineering Headquarters*5

Chief Engineer, Power Systems

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)2

higher efficiency and capacity factor. Figure 1 shows a map of the regions where concentratedsolar power generation is suitable. In particular, the Sunbelt, where direct insolation is abundant
and the climate is dry, is appropriate for this type of power generation. The Sunbelt includes
various areas such as the U.S., Southern Europe, Australia, Africa, the Middle East and India.
As Figure 2 indicates, the market size is forecast to be 25 to 45 GW of world installedcapacity in 2020. The annual construction capacity is expected to be 2 to 5 GW. In addition to theinstallation plans of individual countries, huge global cooperative projects are in progress. One ofthese is called DESERTEC1, in which the electricity generated in North Africa will be supplied toEurope through a submarine cable, and will cover approximately 15% of European consumption.
The market size is expected to expand beyond the above forecast if the project is fully realized.
A comparison of concentrated solar power generation systems is presented in Figure 3. The
current mainstream generation system utilizes a steam turbine rotated by the thermal energy
converted from collected sunlight. In particular, the trough type (Figure 3 (1)) has a standardizedoreceiver, and is widely used. The collected heat temperature is on the order of 400 C. A tower type(Figure 3 (2)) has recently been introduced as a means of increasing the collected heat temperaturefor improved plant efficiency.3 A steam turbine (Rankine cycle) is used in both types of systems.
A hot-air turbine system (Figure 3 (3) tower type, Brayton cycle), which will lead to further
efficiency enhancements by operating at higher temperatures than those of steam turbine systems,
is currently under development. This system enables power generation without water cooling, andis attracting attention because of its applicability to dry areas suitable for concentrated solar powergeneration.

Figure 1 Distribution of global solar radiation
intensity

Figure 3

Figure 2 Market size forecast ofconcentrated
solarpower
generation2

Comparison of concentrated solar power generation systems

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)
3|3. Development of the new concentrated solar power generation
system3.1

Features of the system

We are developing an unprecedented hot-air turbine solar power generation system (Figure 3(3)) utilizing our accumulated expertise and experiences in gas turbine, boiler, heat exchanger andother technologies. Air extracted from the atmosphere is compressed and heated in the solar
receiver. The hot air drives the turbine to generate electric power. The heat of the exhaust air istransferred to the receiver inlet air via a recuperator, and the exhaust air is discharged into theatmosphere to form an open-cycle system. As high-temperature design is relatively easy even under
low-pressure conditions in the case of an air based cycle, higher plant efficiency is attainable with
the new system compared to a conventional steam turbine system. Also, auxiliary equipment suchas a condenser and water supply pump is not required, and the simple structure facilitates
maintenance. The system offers the advantages of easy handling and waterless operation, and thus
is suitable for dry areas where abundant solar energy can be utilized.

3.2

Solar receiver

The solar receiver, which is one of the key components of a hot-air turbine power generating
system, is a heat exchanger that draws thermal energy from the solar energy collected by an arrayof heliostats. The solar energy enters the receiver through an aperture of the casing and reaches theotubes inside the receiver. This energy heats the air passing through the tubes4, and hot air at 850 C
is efficiently generated to drive the turbine. Since the metal temperature of the receiver exceeds
o900 C, heat-resistant nickel alloy is used. The design is suited for high-temperature operation,
taking into account thermal fatigue, creep strength, and other factors, and the length, diameter,
thickness, and pitch of the tubes are optimized. Since the operating temperature is high, andradiation loss significantly affects the performance, a cavity type receiver is adopted, with only asingle aperture to admit the solar energy. This design permits the thermal radiation generated in thehigh-temperature casing to be effectively used to raise the temperature of the operating air, andradiation loss from the aperture is minimized. The aperture is located at the focal point of theheliostat-reflected light, and the size of the opening is minimal.
To validate the design, the three-dimensional heat radiation and heat transfer of the flow in apipe were simultaneously analyzed using general-purpose thermal-fluid analysis software. The
boundary conditions were the air inflow rate and temperature. The intensity distribution of the solar
heat inlet, which was separately calculated, was applied to the heat-receiving surface. Figure 4shows the air temperature distribution as an example of the analysis results. The low-temperature
airflow at the heat receiver inlet is heated by solar energy as it passes through the tubes, reaches aohigh temperature of 850 C at the outlet of the receiver, and then flows into the turbine.
To verify the receiver performance predicted by the above analysis, a prototype of a 200
kWe-class receiver was built, and a verification test was conducted in May 2011 in collaboration
with the Commonwealth Scientific and Industrial Research Organization (CSIRO5) in Australiao(Figure 5). The results show that the receiver outlet temperature was raised to the target 850 C bysolar heat alone, and conform well to the radiation analysis predictions. The same considerations
and analysis procedures will be applicable to future designs.

3.3Hot-air turbine

We are currently developing a hot-air turbine applicable to the system. The heat source is thesolar energy collected by the heliostats. Improved efficiency of the hot-air turbine system isimportant, since reducing the required number of heliostats will reduce the overall cost of thesystem. Although the system is operated at a relatively low temperature compared to the gas
turbines on which it is based, the technology used in our most advanced high-efficiency J-class gas
turbines is being applied to each element, with the aim of designing a high-circumferential-velocityocompressor and turbine, capable of attaining high efficiency even at the 850 C air temperature atthe receiver outlet.

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)
4

Figure 4
Results of threedimensional heat radiation
analysis (air temperaturedistribution)

3.4
(1)

Solar field
Heliostats
A heliostat is a device that reflects light to the receiver by coordinating mirror
movements with solar diurnal movements, using two control axes. The devices are used
exclusively in large-scale solar power plants. Various mirror sizes and drive systems are being
developed by a number of manufacturers. The settings of the two axes vary, but the mostcommonly used configurations are the altazimuth mount which consists of a vertical pivot and atilt axis rotating around the pivot, and the equatorial mount which consists of a polar axis andan orthogonal declination axis (Figure 6). Various mirror sizes are currently available, thesmallest of which is 1 m2, while the largest exceeds 100 m2. The number of heliostats used in a
typical system is enormous, and the cost impact is significant. Hence, the development of alow-cost, high-accuracy heliostat is a necessity.

Figure 6(2)Figure 5 Verification test of the heat receiver at CSIRO

Examples of heliostat construction

Optical path calculation
Optical path calculation is a tool to analyze the passage of light reflected by the heliostat,and it is used to study design factors such as the target angle and the heat input to the receiver.
Optical path calculation is therefore necessary for plant design, including heliostat arrangement
and tower height. Many manufacturers and research institutes develop unique light pathcalculation software exclusively for solar power plants.
Figure 7 shows the efficiency of each of the heliostats arranged around a tower at theculmination of the spring equinox. The efficiency includes the effects of cosine loss (effective
area reduction due to the slanted incidence angle) and blocking and shadowing loss by adjacentheliostats. The heliostat arrangement is very important, since it affects the output power
differently even in cases where the same number of heliostats is used. Figure 8 shows a plant
image of the heliostat arrangement.

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 1 (March 2012)
5Figure 8
Plant image showing heliostat
arrangement
Figure 7 Example of a heliostat efficiency
evaluation
The light reflected by the heliostats is supposed to be focused at the receiver aperture.
However, the focused light cannot be reduced to a dimensionless point, but must occupy somespace, due to the apparent size of the sun and the reflection inaccuracy. For this reason, thereceiver aperture must be of an appropriate size. However, the larger the opening, the larger
will be the loss due to radiation and convection of the high-temperature air inside the receiver.
Conversely, the smaller the opening, the greater will be the loss due to spillage. The shape andsize are determined in accordance with the optimum angle and size, based on the above
considerations. The incoming heat intensity distribution is calculated at the heat-receiving
surface in the receiver, based on the optical path calculation. The thermal analysis of thereceiver is then conducted.

|4. Conclusion
To largely commercialize a renewable energy system, it is necessary to generate electricitywith low operating, construction and maintenance costs. The concentrated solar power generating
system introduced in this report has a very simple configuration with a hot-air turbine, is wellmatched with market demand and practical implementation is already anticipated in many
countries.
A power generation test of a 200 kWe-class system will be conducted in collaboration with
Australia/CSIRO, and after the demonstration project has been completed, commercialization isplanned. We hope to contribute a new solution for global environmental and energy issues with ourconcentrated solar power system.

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