Thermo-economic Optimization of Hybrid Solar Combined Power Cycles Using Heliostat Field Collector (PDF)

Electricity has an essential role in our daily life. However, with the ever increasing cost of fossil fuels and natural gas, power generation with higher efficiency and lower capital cost is in high demand. Nowadays, global warming and climate change have become vital issues prompting investigations into increasing the share of renewable sources of energy implementation in power generation. Solar energy is arguably the most favorable solution for a greener power generation technology. With solar technology’s current level of maturity, solar energy cannot provide a significant contribution to the world’s energy demand due to intermittency and storage issues. A possible solution to the aforementioned difficulties is power plant hybridization. In particular, concentrated solar power technologies are displaying significant potential for electricity production. The United Arab Emirates’ hot, sunny climate is an indication of the great potential it possesses for hybrid and solar only power plant implementation. In this research work, the feasibility of a 50 MWe hybrid (solar and natural gas) combined cycle power plant with a topping gas turbine cycle and four different bottoming cycles are assessed. Power plant hybridization is accomplished by employing a solar tower collector (Heliostat field collector). Three rather unconventional bottoming cycle configurations have been chosen including gas turbine (air bottoming cycle), water injected gas turbine (humid air bottoming cycle), and the Maisotsenko cycle (Maisotsenko bottoming cycle). These three configurations along with the conventional combined cycle power plant (steam bottoming cycle) are optimized by conducting thermo-economic and transient analyses in MATLAB to identify the most economically justified plant configuration for the United Arab Emirates. Additionally, two different heliostat field layouts are taken into consideration including the radial-staggered and spiral layouts. Moreover, thermo-economic evaluation is accomplished by utilizing five different economic approaches, i.e. net present value, payback period, life cycle saving, Knopf objective function, and levelized cost of electricity.

Conventional Combined Cycle

The conventional combined cycle consists of topping gas turbine and bottoming steam turbine cycles. Ambient air is drawn into the topping cycle compressor (1) where it is compressed adiabatically (1-2). After compression, air enters the central tower where it is preheated with the available solar flux at the receiver (2-3). Next, air enters the combustion chamber where the air fuel mixture is further heated in an isobaric process (3-4). In the last step, the flue gases departing the combustion chamber are expanded adiabatically in the gas turbine to produce mechanical work (4-5) which is converted into electricity by the generator. Exhaust gases from the turbine go through the heat recovery steam generator (HRSG) where they will be cooled down by generating steam in the bottoming cycle (5-6). Afterward, cooled exhaust gasses are released to the atmosphere. In the bottoming cycle, saturated liquid water is pumped adiabatically to the HRSG (7-8) where water is heated up and steam is generated in an isobaric process (8-9). Depending on the requirements, steam can be either saturated or superheated. Afterward, steam is expanded adiabatically in the steam turbine (9-10). Expanded steam is condensed to the saturated liquid water in the condenser (10-7) to complete the bottoming cycle.

Combined Cycle Configurations

Air Bottoming Cycle

The air bottoming cycle consists of a topping gas turbine and a bottoming air turbine cycle. Ambient air is drawn into the topping cycle compressor (1) where it is compressed adiabatically (1-2). After compression, air enters the central tower where it is preheated with the available solar flux at the receiver (2-3). Next, air goes through the combustion chamber and heat is added to the air fuel mixture in an isobaric process (3-4). In the next stage, flue gases departing the combustion chamber are expanded adiabatically in the gas turbine for power generation. Exhaust gases from the topping gas turbine go through the air heat exchanger where a portion of the available waste heat is recovered by the bottoming cycle air flow (5-6). In the bottoming cycle, ambient air is drawn into the bottoming cycle compressor (7) and it is compressed adiabatically (7-8). Afterward, compressed air goes through the air heat exchanger where it is heated by waste heat available in the topping cycle turbine’s exhaust gases (8-9). Heated air is expanded in the bottoming cycle air turbine producing additional power and enhancing the plant’s overall efficiency (9-10).

Maisotsenko Bottoming Cycle

Maisotsenko bottoming cycle consists of a topping gas turbine and a bottoming Maisotsenko (humid air) cycles. The main difference between Maisotsenko bottoming cycle and humid air bottoming cycle is in their waste heat recovery approaches. Maisotsenko bottoming cycle utilizes an air saturator which replaces both HRSG and air heat exchanger. This reduction in the number of equipment may result in a reduction of the capital investment cost. Ambient air is drawn into the topping cycle compressor (1) in which it is compressed adiabatically (1-2). After compression, air enters the central tower where it is preheated with the available solar flux at the receiver (2-3). Next, air goes through the combustion chamber and heat is added to the air fuel mixture in an isobaric process (3-4). In the last step, the flue gases departing the combustion chamber are expanded adiabatically in the gas turbine to produce mechanical work (4-5), that is converted into electricity in the generator. Exhaust gases from the turbine go through an air saturator where they will be cooled down by the bottoming cycle air flow (5-6). Moreover, it will humidify the bottoming cycle air flow. In the bottoming cycle, ambient air is compressed adiabatically (7-8). After the compression, the compressed air enters the air saturator where it is heated and humidified by the exhaust gases from the topping cycle gas turbine (8-12). In principle, air saturator utilizes the waste heat available in the topping cycle gas turbine exhaust gases to increase the air wet bulb temperature. In this basic operation, the compressed air is divided into three streams in the lower section of the air saturator. These streams are cooled down sensibly to their dew point temperatures by indirect evaporation of water (8-9). Two of these streams mix together while the third stream is fed backward to the lower section of the air saturator (9-10). The third stream's humidity increases as it travels through the lower section of the air saturator while it cools down the incoming air streams. The combined stream enters the upper section of the air saturator where it is heated up by the exhaust gases supplied from the topping cycle turbine (9-11). Moreover, its moisture content increases as it travels across the upper section of the air saturator. At a specific point, humid air stream at state 10 is mixed with the other humid air stream at state 11 before leaving the air saturator (12). In this operation, one of the major benefits of the air saturator is its ability to control the moisture content of the supplied air to the second air turbine (expander). By controlling the required amount of water in the air saturator, the air humidity ratio and its temperature can be adjusted. Then, the humid air is expanded adiabatically in the turbine to produce mechanical work (12-13), which is converted into electricity in the second generator.

Humid Air Bottoming Cycle

Humid air bottoming cycle consists of a topping gas turbine and a bottoming humid air turbine cycles. In order to increase the cycle efficiency, an evaporator can be employed in the bottoming cycle in order to further cool down the topping cycle exhaust gases. Ambient air is drawn into the topping cycle compressor (1) where it is compressed adiabatically (1-2). After compression, air enters the central tower where it is preheated with the available solar flux at the receiver (2-3). Next, air goes through the combustion chamber and heat is added to the air fuel mixture in an isobaric process (3-4). In the last step, flue gases departing the combustion chamber are expanded adiabatically in the gas turbine to produce mechanical work (4-5) which is converted into electricity in the generator. Exhaust gases from the turbine go through the air heat exchanger where they will be cooled down by the bottoming cycle air flow (5-6). In the HABC, ambient air is drawn into the bottoming cycle compressor (7) and is compressed adiabatically (8-9). After compression, it goes to the evaporator where water is added to the air (8-9). As a result, the compressed air is cooled down and humidified simultaneously. Humid air goes through the air heat exchanger and recovers a portion of waste heat available in the topping cycle turbine’s exhaust gas (9-10). Hot and humid air is expanded in the bottoming cycle turbine to produce extra electricity which results in an increase in the total cycle efficiency with respect to a simple gas turbine cycle (10-11).

Radial-Staggered

Radial-staggered configuration is the most popular and commonly used heliostat field layout which has been adopted vastly in many heliostat field projects. In radial-staggered configurations, back to back rows of mirrors do not share the same azimuth angles resulting in a significant improvement of the blocking factor. Additionally, the distance between two adjacent mirrors increases as they get further away from the tower. In consequence, as the rows get further away from the tower, the space between their heliostats increases. 

Phyllotaxis Spiral 

There is another promising heliostat field layout proposed by a group of scholars from MIT. The authors argue that the transition between the high and low density areas within the field is not continuous in radial-staggered layout. Thus, a new heuristic is presented based on the spiral patterns of the phyllotaxis discs. It is reported that the new pattern replacing radial-staggered configuration will improve the field optical efficiency and reduce the land area. The proposed field layout have the advantage of continuous density function unlike radial-staggered configuration.

Heliostat Field Layouts

Net Present Value

One of the methods which is considered for economic analysis of this study is the commonly utilized and popular net present value (NPV) approach. This method evaluates the potential of an investment by determining the investment worth growth during plant life time. Consequently, an investment is only advisable as long as its net present value is positive. Negative net present values imply that the required initial investment is greater than the positive cash flow during plant life cycle.

Payback Period

It is decided to consider another economic analysis method to further evaluate the plant performance. A rather simple but informative economic indicator is the payback period. For the payback period calculation, the ratio of the required additional investment over the first year saving is determined. Thus, the payback period for the integration of the heliostat field with an already existing plant is investigated.  Additional investment associated with solar components are calculated. First year saving, in forms of fuel reduction savings and additional operating and maintenance cost, is determined. Taking into account that payback period can provide an approximate ranking of different investments which are comparable in terms of duration and functionality .

Thermo-economic Assessment Approaches

Life Cycle Saving

An economic model which presents a similar methodology to the payback period method is life cycle saving. Like the payback period method, an already existing air bottoming cycle plant operating with natural gas only is considered. Therefore, the reference plant configuration is an air bottoming cycle with natural gas only. This method is utilized to investigate the economic advantages of water injection and hybridization on an already operating air bottoming cycle plant. Thus, additional investment required to hybridize the plant and provide steam injection in the bottoming cycle,  in forms of Humid air bottoming cycle and Maisotsenko bottoming cycle configurations, are studied. In this method, steam bottoming cycle configuration is not considered for the analysis. Only the terms which are different between a simple air bottoming cycle and hybridized air bottoming cycle, hybridized Maisotsenko bottoming cycle or hybridized Humid air bottoming cycle must be considered.

Knopf Objective Function

Knopf presented an objective function to improve the thermodynamic optimization of cogeneration power plants. His proposed objective function is selected to further analyze the plant thermo-economic performance and enhance the optimization process. This method calculates the total cost rate including fuel consumption, equipment capital investment and maintenance in US$/s. Additionally, this method will convert capital investment cost into annual basis by employing capital recovery factor method making it more convenient to comprehend and follow. Knopf recommended objective function is modified to better represent the situation in this research work.

Levelized Cost of Electricity

The most popular economic indicator for power plants thermo-economic analysis is the levelized cost of electricity. This approach determines the minimum cost of electricity generated by the power plant. Understandably, lower levelized cost of electricity implies that the investment is more cost-effective. In principle, levelized cost of electricity is the cost of electricity sale which makes the net present value of the investment equal to zero. If the actual cost of electricity sale to the grid is lower than the levelized cost of electricity, investment will not be profitable and net present value will be negative. Therefore, there is an internal connection between the net present value and levelized cost of electricity appraches. However, levelized cost of electricity is a much more convenient economic indicator to understand and interpret.

© 2015 by Mohammad Saghafifar

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