San Francisco State University
School of Engineering

Power Tower

12/11/2015
Engr 469
Fall Semester

Instructor: Professor A. S. (Ed) Cheng

Team:
Sanjeev Gupta
Abit Baral

Abstract: In this article different aspects of Power Tower has been investigated. Power Tower or Heliostat power plant is one of the most promising and affordable Concentrated Solar Power technology. Different parts of the Power tower namely heliostats, central receiver, heat transfer fluid, turbine and storage tanks has been examined. Also current situation of Power Tower have been researched and compared with other Solar technology.

Introduction:
Power towers have a potential of being one of the lowest cost and environmental friendly solution to our power needs. Today power tower systems are often considered to be the most promising Concentrating Solar Power (CSP) technology for the future. In power tower concentrating solar power systems, numerous large, flat, sun­tracking mirrors, known as heliostats, focus sunlight onto a receiver at the top of a tall tower. A heat­transfer fluid heated in the receiver is used to generate steam, which, in turn, is used in a conventional turbine generator to produce electricity. Some power towers use water/steam or molten salt as the heat transfer fluid. Different part of these types of power plant are analyzed in detail below.
Components of CSP power tower.
1. Heliostats
2. Tower with receiver
3. Heat­transfer fluid
4. Turbine/Generator
5. Molten salt energy storage system
6. Cold storage tank 1. Heliostats:
The word ‘heliostat’ comes from the Greek words helios (sun) and stat (stationary), meaning a device that tracks the sun from a stationary point. [1] Heliostats are sun­tracking mirrors that reflect the sunlight onto a receiver at the top of a tall tower. CSP tower systems consists of arrays of heliostats. Each heliostat is composed of several mirror module panels rather than a single large mirror. The mirrors are usually thin glass with very high reflectivity and form concave surface. The heliostat focal length is approximately equal to the distance from the

receiver to the farthest heliostat[3]. Number of heliostats in a CSP tower can range from a few hundred to many hundred thousand depending on the power generation required. Each mirror is initially adjusted so they each reflects light onto receiver. The tracking system of each heliostat is controlled by motors which receive signals from a central control computer that precisely points the mirror normal halfway between the sun and the receiver.

Figure(1): Heliostats­ Ivanpah Power Plant
Ivanpah Solar Electric Generating System in California has 173,500 heliostats, each with two mirrors focusing sunlight onto three towers. This power plant has cumulative rated power generation capacity of 390 Megawatts. [2] 2. Receiver
The receiver captures the energy being reflected on it from the heliostat field and transfers it into a heat transfer fluid by increasing the temperature of the fluid. The receiver is a placed at the top of a tower which is located at a point where reflected energy from the heliostats can be received most efficiently. The tower range from 250­600 ft and is dependent upon weight and size of receiver and area of the heliostat field. The two types of receivers commonly used are
i. External Receivers

ii. Cavity Receivers i. External Receivers:
These types of receivers are comprised of panels of many small (0.2­0.7inch) vertical tubes welded side by side to approximate a cylinder. The bottom of each tube is connected to headers that supply heat transfer fluid which is heated and collected out of the top of the tubes.

Figure(2): External receiver of Solar one facility of Barstow
Figure (2) shows the receiver used at the Solar One facility in Barstow, Ca which is an external type receiver. It is located at the top of a 253 ft tower and comprises 24 panels, each 45 ft high, consisting of 70­1/2 in diameter tubes. Six of these panels are used for preheating the water and 18 for producing steam. The overall receiver diameter is 23 ft. This power plant produces
10 MW of electricity. ii. Cavity Receivers:
In this design the flux absorbing surface is placed inside of an insulated cavity, thereby reducing the convective heat losses from the absorber. The flux from the heliostat field is reflected through an aperture onto absorbing surfaces forming the walls of the cavity. The aperture is

sized according to the same dimensions as the sun’s reflected image from the farthest heliostat.
Figure(3) shows a cavity receiver with four cavity. Heat transfer to the working fluid is done via air convection in the cavity.

Figure(3): Cavity type receiver
Efficiency of the receiver is determined by their operating temperature. To reduce heat losses from the external part of receiver, it’s temperature needs to be less for higher efficiency.
Figure(4) shows a graph on the relationship between the efficiency of the receiver and their working temperature.

Figure(4): Receiver Thermal efficiency vs external Temperature

3. Heat Transfer Fluid:
The choice of the heat transfer fluid to be pumped through the receiver is determined by the operating temperature of the turbine, cost­effectiveness of the system and safety considerations. High temperature heat transfer fluid are more efficient and produce more power but are more expensive and require intensive safety precautions. Most popular heat transfer fluids are described below.
a. Steam:
Steam is the most cost efficient of all other heat transfer fluid. Maximum operating temperature is around 540°C with pressure about 10 MPa to produce a high boiling temperature. Freeze protection is necessary for ambient temperatures less than 0°C .
In order to prevent scale buildup on the inner walls of the receiver heat transfer surfaces, the water used in the receiver must be highly deionized. Another disadvantage is use of water as a high­temperature storage medium is difficult because of the high pressures involved. b. Heat­transfer Oil:
These are the fluids with the lowest operating temperature capabilities. They include both hydrocarbon and synthetic­based oils. Their operating temperature is around 425
°C. Advantage is that there vapor pressure is significantly low at these temperatures and can be used for thermal energy storage. Drawback is at low temperature heat must be supplied to make these oil flow. Another disadvantage is they are highly flammable thus require special safety systems when used at high temperatures. They cost about
$0.77/kg.

c. Nitrate salt mixtures:
Nitrate salt mixtures can be used as both a heat transfer fluid and a storage medium at temperatures of up to 565°C. However these heat transfer fluid freeze at temperatures around 140 to 220°C and thus require high temperature to operate. They have a good storage potential because of their high volumetric heat capacity. The cost of nitrate salt mixtures is around $0.33/kg ($0.15/lb).
d. Liquid sodium:
These heat transfer fluid can also be used as both a heat transfer fluid and storage medium, with a maximum operating temperature of 600°C. Because sodium is liquid at this temperature, its vapor pressure is low and can be used for thermal energy storage.
However, it solidifies at 98°C, thereby requiring heating on shutdown. The cost of liquid sodium is about $0.88/ kg ($0.40/lb). This system is expensive than others since cost of sodium is higher. 4. Turbine/Generator:
After the focused energy from receiver heat the fluid at approximately 553°C, the converted steam is fed under high pressure into the turbine using simple rankine cycle. The rotating shafts inside the turbine are connected with generator where the mechanical energy generated by shaft is converted into electricity.The Generator at Ivanpah Solar Electricity generates approximately 932 MW of electric power. In the article, Energy and exergy analysis of Solar
Power Tower Plants” by Xu, C Zf Wang, X Li and Fh Sun, they have found that the overall efficiency of the energy generated from a simple Concentrated Power plant is 22.9%. The efficiency of the Power tower is lower due the radiation loss from the receiver and corrosion of the molten salt mixture

5. Molten salt Storage System:
There is no consistent on solar energy received from the sun due to the rotation as well as due to revolution of the Earth. There is always a necessity for storying the solar energy when there is excess and use that energy to convert into electricity when there is no direct sunlight such as during night time. So A solar Energy storage system is always a very important component of in the Power Tower.
In Solar Thermal Energy Storage Technology developed by Solar Reserve, a molten salt is circulated inside the piping in the heat exchanger during the day and hold in a storage tank at
566⁰
C and 1atmospheric pressure during the night. Since the molten salt has the life cycle of over 30 Years and is used as both heat receiver and solar energy storage, it minimize the number of tanks and amount of salt required to the run the Power Tower, in turn saving construction and maintenance cost. When the life cycle of molten salts of Nitrate mixture is over they can be used as an agricultural fertilizer.
6. Cold Storage Tank:
It is a storage tank where excess energy from sunlight is reserved on a molten salt at 259 degree celsius. When there is no sunlight molten salt from cold storage Tank is pumped into the steam generator where molten salt is superheated and passed through another generator to produce electricity.
Simple diagram of energy storage system is given below Fig.5

fig5 Current Situation and Conclusion
:
The popularity of power tower seems to be fading. Technologies like parabolic trough, CSP photovoltaic and photovoltaic Solar panels are leading in power production that power tower.
The reason behind this is the construction cost and maintenance of these power towers.
Furthermore all these different electricity generating technologies have similar efficiencies.
Power tower is lagging because the initial installation is very complex and requires substantial amount of money compared to solar pv panels or parabolic trough.
Currently there two major power tower which have been approved but are at a stall. The Rice
Solar energy project which was approved by the California Energy Commission in 2010, is at a standstill unbuilt on its site east of Joshua Tree National Park in Riverside County. Similarly
500MW Hidden Hills Solar electric generating system proposed for eastern Inyo county in april
2013 has not moved in forward directions.
But nonetheless power tower is still one of the cleanest form of energy generation and if companies are willing to take risk on building it, it can be of huge profit to them as well as can be very beneficial for the environment.

Reference:
[1]
http://www.nrel.gov/csp/solarpaces/project_detail.cfm/projectID=62
[2]
http://www.brightsourceenergy.com/brightsource%E2%80%99s­heliostat­technology#.VloQu
0fF_T8
[3]Stine, William B., and Michael Geyer. " ." Power From The Sun. Online, n.d. Web. 11 Dec.
2015.
[4] http://www.babcockpower.com/products/boilers­boiler­services/riley­power­inc/products/solar­ st eam­generator [5]Ho, Clifford K., and Brian D. Iverson. "Review of High­temperature Central Receiver Designs for Concentrating Solar Power." Renewable and Sustainable Energy Reviews 29 (2014):
835­46. Web. http://energy.gov/eere/energybasics/articles/concentrating­solar­power­tower­system­basics http://www.azocleantech.com/article.aspx?ArticleID=24
Xu, C, ZF Wang, X Li, and FH Sun. "Energy and Exergy Analysis of Solar Power Tower Plants."
Applied Thermal Engineering, 31.17­18 (2011): 3904­3913.
Ramos, A, and F Ramos. "Strategies in Tower Solar Power Plant Optimization." Solar Energy,
86.9 (2012): 2536­2548. u, C, ZF Wang, X Li, and FH Sun. "Energy and Exergy Analysis of Solar Power Tower Plants."
Applied Thermal Engineering, 31.17­18 (2011): 3904­3913.