An Investigation of Hybrid Solar-Steam Power Plant: A case study in Iraq

In this work integrating Al-Zubaydia (Kut-Iraq) thermal power plant with solar thermal system is studied for heating feed water by solar energy to reduce fuel consumption and greenhouse gases emission. A closed type Parabolic Trough Solar Collector (PTSC) is designed, constructed, instrumented, and tested. Its thermal characteristics are reported under Iraq climate conditions for the period extended from June, to September 2017. The collector heat gain, efficiency, absorber temperature and heat exchanger effectiveness (considered as feed water heater) were presented for absorber side flow rates of (0.15, 0.2, 0.3, 0.4, 0.5) lpm of water or oil), and shell side water flow rates of (0.4, 0.5, 0.6lpm). Results show that the maximum obtained thermal efficiency of parabolic trough solar collector was 83.33% for oil working fluid. The maximum obtained oil outlet temperature was 106 C at solar noon for (0.15) lpm. Theoretical results showed that the fuel save mode needs collector area of (32842 m), while that needed for power boosting is (102569 m) for the same thermal cycle efficiency. The fuel save mode reported a reduction in greenhouse emission.


Introduction:
The consumed fossil fuel in power plants causes many serious negative effects on the environment like global warming, ozone layer destruction and air pollution. Solar energy is clean, free, non-depleting source with zero greenhouse gas emissions. Iraq receives annual solar radiation approximately for 4000 hours with daily incident (4.5 to 5.4) kWh/m 2 (NASA 2008). Technically Iraqi power needs is distributed between (65-70%) for heating and cooling equipment, and 30% for lighting and other electrical sets. So there is a need for efficient solar thermal system to convert solar energy to thermal energy by (60-70%).
In a typical parabolic trough collector's power plant, hybridization is the combination of thermal energy obtained from parabolic trough collectors with that obtained from another conventional fuel. [1] Studied the techno-economic feasibility of a parabolic trough concentrator solar thermal power plant. [2] Studied the efficiency of solar aided power generation with different solar replacements of extraction steam. The results show that the solar to electricity efficiency of solar aided power generation are higher than those of a solar alone power plant with the same temperature level of solar input. Also, the low temperature No.29-(12) 2020 Journal of Petroleum Research & Studies (JPRS) E201 heat resource is very hard to be used for power generation in other types of solar power plants. [3] investigated the energy and economic advantages of developed solar aided model. The thermal solar aided (in the temperature ranged from 250 to 901) was coupled with 200 and 300MW typical, 1000MW ultra-supercritical and 600MW, 600MW subcritical and 600MW supercritical fuel power plant. The solar aided is used to replace the steam extraction in fuel saving and power boosting modes.
The reported solar aided power generation is appropriate to be adopted in supercritical and subcritical plants compared with ultra-supercritical plants. The solar energy contributed (up to 20%) significantly for boosting electricity. [4] proposed and analyzed a novel integrated solar combined cycle with direct steam power plant. To increase solar share in the global output power, they used two stage solar inputs.
The results show that utilizing solar energy to supply latent heat for vaporization of feed water is better than that used for sensible heating purposes. Also the efficiency of solar to electricity for two stages was 30% higher than that for one stage of integrated solar combined cycle. [5] studied the thermal performance of the integrated solar/North Benghazi united. Cycle under Libyan climate conditions. Parabolic trough solar is used as solar thermal system with direct steam generation. The proposed solar assisted plant was analyzed as power boosting and fuel saving modes for the same area of solar field. The obtained results show that the yearly the natural gas consumption saving with approximate 3001.56 tons and CO 2 emission is 7972.25 tons for fuel saving mode. For power boosting mode, the yearly solar to. Electricity is 93.33 GW-h.
The benefit per cost ratios for power boosting modes is 1.30 and for fuel savings is 1.74 for the same area of solar field. So, the fuel saving mode is found more beneficial. E202 field and power block to evaluate solar to electrical efficiency and solar energy efficiency. The influences of operation condition and structure layouts on the solar hybrid coal-fired power plant performance are disclosed. It is reported that the solar radiation has slight influence when receiver temperature is 290 o C and the concentration ratio is higher than 60.
Flow rate in the range from 0.5 m/s to 4 m/s has inconsiderable effect on the performance of solar receiver. [9] established a correlation to describe the effect of collector efficiency, turbine efficiency and upgraded energy level of solar heat on plant thermodynamics. Results show that the moderate-temperature solar and coal-fired power plant hybridization can provide a promising direction for efficient utilization of low-grade solar heat. [10] simulated proposed kalina cycle system 11 (KCS11) assisted with indirect solar heating system ,parabolic trough solar collectors with oil heating fluid is used to superheat Ammonia -water binary mixture (75% mass fraction ).the KCS11 is integrated with (500MW) subcritical coal fired steam power plant to assist power generation, the evaporation of (Ammonia -water ) mixture is accomplished by waste heat recovery of coal fired flue gases in the kalina cycle ,this process reduces the temperature of flue gases by (10k), a thermodynamic modeling algorithm is adopted to analyses the proposed system at different environmental conditions. Results show that the power generated from kalina (KCS11) cycle is increased from 389.51KW to 515.37 KW by integrating solar energy. The cycle efficiency is increased from 5.946% to 6.988%, there by annual emission can be reduced daily by 1008.28 ton. [11] proposed integrating solar-assisted pressure-temperature swing adsorption (PTSA) into an 800MW electrical coalfired power plant, to avoid energy consumption by turbine steam extraction. They found lower gas mission than that of the referenced power plant.
In regenerative Rankine cycle, steam is extracted to heat feed water from 40 o C (condenser outlet) to 300 o C (boiler inlet) to increase the overall plant thermal efficiency while the power generated by turbine is reduced due to bled steam mass. By replacing part or all the extracted steam by solar thermal energy for heating feed water gives a good solution to increase the generated capacity with the same or lower consumption of fuel. Up to the author knowledge, this work is unprecedented for Iraq climate conditions. The objective of this work is to evaluate theoretically and experimentally the hydrothermal characteristics of solar thermal system used for Al-Zubaydia thermal power plant in Kut -Iraq feed water heating. Depending on the performance of this solar thermal unit, area of solar heating units will be proposed.

Solar collector
The collector useful heat gain (W) is evaluated as: (1) where A ap : aparture area (m 2 ).
F R : heat removal factor.
I : incident solar radiation taken as an average value as (750 W/m 2 ).
T in : inlet temperature of the working fluid (K).
C r : the concentration ratio. It is calculated as: U L is collector heat loss coefficient (W/m 2 K). It is calculated by [12]: (3) in which, A abs : absorber surface area (m 2 ). h w : convective heat transfer coefficient due to wind speed V w (W/m 2 .K).
h r,g-amb : heat transfer coefficient due to radiation between glass cover and ambient (W/m 2 .K) h r,abs-g : heat transfer coefficient due to radiation between the absorber and glass cover (W/m 2 .K) all the variables presented in eq. 3 are calculated as cited by [13] The value of correction factor or heat removal factor (F R ) is (0 ), it can be interpreted as the ratio of the actual useful heat to that which would be collected if the entire absorbed surface is at the fluid inlet temperature, and it can be evaluated as: The collector efficiency factor (F ) can be calculated as [13]: The solar collector thermal efficiency (ƞ th ) depends upon the operating conditions namely: (solar radiation flux, inlet fluid temperature, as well as the ambient temperature) and on the concentrator design. It is evaluated as:

Heat Exchanger
Shell and U tube heat exchanger is designed and instrumented in this study, its thermal analysis involves the determination of the heat transfer coefficient of shell side and tube side.

Heat Transfer in tube side
The internal heat transfer coefficient h t in tube side is calculated as: For laminar flow in circular pipes (Re < 2100), The Nusselt number was calculated by using [14]: The Reynolds and Prandtl numbers of the tube side flow can be determined by: Pr = The tube side flow area pass is calculated by:

The overall and shell side heat transfer coefficient
The shell side heat transfer coefficient can be calculated as: Heat transfer from U tube to the shell side is calculated as: = The overall heat transfer coefficient of the heat exchangers is evaluated as: U= (15) The effectiveness of the heat exchanger is calculated as [14]: where: NTU is Number of Transfer Units, it is calculated as:

NTU = (17)
C ratio is the ratio between the minimum (C min ) to maximum (C max ) heat capacity, such that: The heat capacity of hot fluid (C h ) and cold fluid (C c ) can be determined as: If C h , then C min = C c and C max = C h . If C c , then C min = C h and C max = C c .

Power plant theory
Al-Zubaydia thermal power plant consists of one reheater and eight feed water heaters.
To calculate the advantage or the efficiency of the solar energy utilization, the solar percentage (P solar ) is defined as: Where: and solar to power efficiency (ƞ se ) is defined as: where is defined as the difference in the output works between the hybrid power plant (W hyb ) (after replaced the extracted steam energy by solar energy) and only steam power plant (W Tur ). It can be calculated as: Where: is the exergy of collector solar heat and determined as: in which, is the Carnot efficiency corresponding to the receiver temperature.  Table (2). A counter flow heat exchanger is used for the experimental test. A novel heat exchanger is used in this work; the cross-sectional area of its shell is elliptical with (210mm and 100mm) major and minor diameters and (600mm) length.
The copper tube is formed as S shape coil shown in Figure (

Ambient conditions and temperature variation of solar collector
Figure (4) presents that solar radiation on (13 th June, 30 th July, 30 th August and 11 th September, 2017) rises to reach its maximum value at noon, and then ceased to vanish at sunset. The maximum solar radiation was (996W/m 2 ) at (12:00 pm) on 30 th July and decreases after that.
The ambient temperature follows the solar radiation as illustrated in Figure (

Heat removal factor of solar collector
Figure (6) shows that the heat removal factor increases slowly with the increasing of mass flow rate. Heat removal factor for the solar collector with circulating water is higher than that when oil was the thermal circulating fluid. This is due to the differences in thermo physical properties between them as given in Table (3).

Al-Zubaydia thermal power plant
The given data from the Al-Zubaydia thermal power plant (theoretical model) are analyzed as a power boosting mode and a fuel saving mode for the same increase in the cycle thermal efficiency. It is found that the work of the turbine is 338.6MW, work of the pump is 7.85 MW, work net is 330.8MW, the heat added in the boiler is 727.7MW and thermal efficiency is 45.45%. Three Cases were studied namely: x x CASE I) Six closed feed water heaters are replaced by solar thermal energy.
x CASE II) The feed water heaters of LP Turbine (feed water heaters number 8, 7, 6, and 5 as presented in Figure (1)) are replaced by solar thermal energy.
x CASE III) The feed water heater of IP Turbine (feed water heater number 3 as presented in Figure (1)) is replaced by solar thermal energy.
For each case two sub cases are investigated, these are A refers to power boosting and B refers to fuel saving. Per results given in Table (

Comparison of this work with published data
The reported average efficiency of parabolic trough solar collector with reflector of (1.8 x 2.8m) dimensions was 61% by [15] when synthetic oil in evacuated receiver was used. The [16] Analyzed the solar aided steam power system for power boosting mode. They simulate parabolic trough solar field. They viewed that this type of power cycle can achieve higher cycle efficiency compared with lignite fired power cycle for fuel save mode with maximum solar field of (120000 m 2 ). In this work, Al-Zubaydia thermal power plant needs collector area of (32842 m 2 ), since higher values of incident solar radiation and ambient temperature are recorded for Iraq compared with Greece.

Conclusions:
According to the previous discussion of the obtained results, it can be concluded that the maximum thermal efficiency of parabolic trough solar collector PTSC is (80.3%) and (83.33%) for water and oil working fluid respectively. The maximum outlet temperature of the absorber tube was (106 o C) for solar radiation of (973 W/m 2 ) and ambient temperature (47. The results for Al-Zubaydia thermal power plant show that the fuel save mode has higher benefit than power boosting for the same increase in cycle efficiency, because the first requires less the collector area less (32,842 m 2 ) than the second (102,569 m 2 ) and the fuel save mode reduces greenhouse gas emission.