Selection of the reference setup

Live steam parameters of subcritical steam power plants are almost attainable with state-of-the-art line-focusing collectors with DSG. Therefore, the variety of possible process arrangements for CSP hybrid systems is large. In order to select one reference setup for the hybrid system, a preliminary energy analysis is conducted to possible pro-cess arrangements (see Chapter 2.3.2) in order to calculate the amounts of solar heat, which could be add to the joint steam cycle. In order to conduct the energy analysis, state point data from the conventional steam power plant modelled in Apros is collect-ed.

Energy analysis is based on the first law of thermodynamics, which is related to the conservation of energy. For an open system with a steady flow process, the first law of thermodynamics can be calculated with Equation 2 (Gupta et al. 2010, p.1229):

+ = + (2)

in which

Ei is energy of mass flow entering the system [W]

Eo is energy of mass flow leaving the system [W]

Qj is heat transfer to system from source at Tj [W]

Wnet is the net work done by the system [W]

The kinetic and potential energy changes are neglected in the Equation 2. The perfor-mance of thermal power plants can be evaluated with the first law efficiency ηI (Equa-tion 3), which is defined as the ratio of estimated energy output to the supplied energy input of the system (Gupta et al. 2015, p.569):

= (3) First law efficiency is often related to the net efficiency of the plant, which includes the calculation of thermal efficiency, the efficiency of the generator and needed reactive power. As generator efficiency and reactive power of the hybrid are not calculated in the energy analysis, the efficiency of the hybrid plant is evaluated through thermal effi-ciency ηth(Equation 4)_,which is defined as the ratio of mechanical work of steam tur-bine divided by the total heat input (Schenk et al. 2012, p.57):

= ℎ

ℎ (4)

Furthermore, the total heat input can be calculated for the hybrid system with Equa-tion 5 (Popov et al. 2011, p.348):

ℎ = _, + (5)

in which

Qth,solar is the thermal power of solar field [Wth] Qboiler is the fuel power of the steam boiler [W]

For the selection and definition of the reference setup, an energy analysis is conducted with two different thermal powers of the solar field in a reference case and five compar-ison cases. In addition, the extraction point for feedwater is selected from three different options: after the condensate pump, after the deaerator and before the economizer. The cases for energy analysis are:

1. Feedwater heating process arrangement, in which superheated solar steam is fed into bled off steam lines and the two HP FWHs are fully replaced (FWHBOS).

This is selected as reference case, since it is achievable for current state-of-the-art line-focusing collectors with DSG.

2. First comparison case is feedwater heating process arrangement, in which heated water from solar field is fed into feedwater line (FWHFL).

3. Second comparison case is the cold reheating line process arrangement, in which superheated solar steam is injected before the reheaters (SuSCRH).

4. Third comparison case is the injection of superheated solar steam at the inlet of HP turbine (SuSHP).

5. Fourth comparison case is the injection of superheated solar steam into inlet of the IP turbine (SuSIP).

6. Last comparison case is the injection of saturated solar steam into steam boiler drum (SaSBD).

= ℎ^, ∗100% = ^,

, + ∗100% (6) By using xsolar, the reactive power and generator efficiency can be neglected. The ther-mal solar shares are comparable only if the power output of the power plant is assumed constant in all the cases. Thus, the power output of the turbines is kept constant in the energy analysis, and the fuel power of steam boiler is decreased directly proportional to the thermal power of the solar field. In order to conduct the energy analysis, a steady state mass flow diagram of host steam cycle is presented in Figure 50. The power output of the turbines is 134.2 MW, and the fuel power of the steam boiler is 402.8 MW with-out solar field. Thus, the reference thermal efficiency of the plant is approximately 33%, which is calculated with Equations 4 and 5.

Figure 50. A diagram of the developed steam cycle in Apros. Adapted from Sengupta et al. 2007, p.17.

Furthermore, the information of state points related to Figure 50 are presented in Table 5, which includes the values of pressure, temperature, enthalpy and mass flow related to the state points in the Figure 50. The state point data for the points 1 to 16 are observed in order to calculate the different cases of energy analysis.

Table 5. State point data of the steam power plant used for energy analysis.

State point Pressure (bar) Temperature (°C) Enthalpy (kJ/kg) Mass flow (kg/s)

1 145.12 550.00 3455.62 109.24

Energy analysis for the different cases is based on values in the Table 5 and on Equa-tions 5 and 6. In addition, the solar field is assumed to be operated with same input and output temperatures and pressures, as in the state points of the steam cycle. Thus, the thermal power of solar field is calculated with Equation 6 (Gupta et al. 2009, p.600):

̇ _, = ̇ × (ℎ − ℎ ) (6) in which

Q_th,solar is the thermal power of the solar field [W_th]

̇ is the mass flow through solar field [kg/s]

hout is the enthalpy of solar steam or heated feedwater [J/kg]

hin is the enthalpy at the extraction point of the feedwater [J/kg]

The results of the first calculation are presented in Table 6, as the solar field produces 18.12 kg/s of superheated steam. This is the equal amount of bled off steam entering the two HP FWHs. Thus, the two HP FWHs are fully replaced in the first calculations. The results include the thermal power of solar field as well as the fuel power of the steam boiler. It is estimated that the fuel power of steam boiler is decreased relative to the thermal power of the solar field. In addition, the thermal solar shares are calculated in all cases by using Equation 6, and the power output of turbines is kept at 134.2 MW for all cases.

feedwater

56.65 15.64 53.12 59.20 61.13 44.39 Fuel power of

boiler (MW) 346.15 387.16 349.68 343.60 341.67 358.41

xsolar (%) 14.06 3.88 13.19 14.70 15.18 11.02

After deaerator Thermal power of the solar field (MW_th)

49.00 7.99 45.46 51.55 53.48 36.74 Fuel power of

boiler (MW) 353.80 394.81 357.34 351.25 349.32 366.06

xsolar (%) 12.16 1.98 11.29 12.80 13.28 9.12

boiler (MW) 361.79 - 365.32 359.24 357.31 374.04

xsolar (%) 10.18 - 9.30 10.81 11.29 7.14

The thermal solar shares are lower than 30% in all the cases in Table 6. In order to in-crease the solar share, more feedwater have to flow through the solar field. In FWHBOS process arrangement, the maximum thermal solar share is 14.06%, when the both HP FWHs are fully replaced, and the feedwater is extracted after the condensate pump.

Thus, this process arrangement is excluded from the following calculations, in which higher solar shares are investigated. In addition, the FWHFL process arrangement is excluded, as the achieved thermal solar shares in FWHFL are much lower than in other process arrangements. Next calculations are conducted with an overall mass flow of 40 kg/s through the solar field in order to achieve thermal solar shares close to 30%, as in the previous calculations the maximum solar share is approximately 15 % with 18.12 kg/s steam mass flow from solar field. The thermal power of solar field, the fuel power of boiler, the thermal solar shares are calculated for the 40 kg/s mass flow (Table 7).

Table 7. The results of the calculation of a hybrid in which the overall mass flow through the solar field is 40 kg/s.

Extraction point

for feedwater Case SuSCRH SuSHP SuSIP SaSBDFWH

After condensate

boiler (MW) 285.55 272.12 267.85 304.80

xsolar (%) 29.11 32.44 33.50 24.33

boiler (MW) 302.44 289.01 284.74 321.69

xsolar (%) 24.92 28.25 29.31 20.14

boiler (MWth) 320.07 306.64 302.37 339.32

xsolar (%) 20.54 23.87 24.93 15.76

The thermal solar share of over 30% can be reached with the mass flow of 40 kg/s if feedwater is extracted after the condensate pump, and solar steam is injected into the inlet of HP turbine or IP turbine. In addition, thermal solar share of 29.11% is reached if the feedwater is extracted after the condensate pump and solar steam is injected into the cold reheat line entering the reheaters. Furthermore, thermal solar share of 29.31% is reached if the feedwater is extracted after the feedwater pump and solar steam is inject-ed into the inlet of IP turbine. Moreover, thermal solar share of 28.25% is reachinject-ed as feedwater is extracted to solar field after the deaerator and solar steam is fed into the inlet of HP turbine. As a conclusion, the thermal solar share over 30% or close to 30%

is reached with mass flow of 40 kg/s if feedwater is extracted after the condensate pump or after the deaerator and solar steam is fed into joint turbine or cold reheat line of steam boiler. The calculated thermal solar shares of all cases are collected in the Figure 51 for comparison.

Figure 51. The calculated thermal solar shares in six cases with two different over-all mass flows through the solar field.

For conclusion, the extraction point for feedwater affects to the reached thermal solar share. If feedwater is extracted after the condensate pump, the thermal solar share is greater than if feedwater is extracted after the feedwater pump or before economizer.

This is due to the preheating of feedwater in solar field and not in the existing FWHs.

On the other hand, the overall size and the size of preheating section of the solar field are greater if the feedwater is extracted after condensate pump than before the econo-mizer. Furthermore, as the one of the objectives of this thesis is to investigate the possi-bilities to reach higher solar shares up to 30%, the selection of the reference case is be-tween the process arrangements, in which higher thermal solar shares can be achieved.

In other words, the selection is between the SuSCRH, SuSHP and SuSIP process rangements, as the solar shares are higher in these arrangements compared to other ar-rangements (Figure 51).

The selected reference setup for the CSP and conventional steam power plant hybrid model is the SuSHP process arrangement, as the full potential of line-focusing solar fields with DSG can be likely achieved with the SuSHP process arrangement in the near future. Furthermore, the efficiency of the hybrid can be greater in SuSHP than in SuS-CRH or SuSIP process arrangements, as the solar steam does not by-pass the HP tur-bine. However, for later work, the selected process arrangement should be compared also against the complexity of the process arrangement as well as the economic feasibil-ity of the process arrangement.