April 1, 2009

Fossil Fuels + Solar Energy = The Future of Electricity Generation

Dave Ugolini and Dr. Justin Zachary, Bechtel Power Corp., and Hyung Joon Park, Bechtel Enterprises

Options for Integrating Solar with Combined Cycles

Discussions concerning integrated solar combined-cycle plants have been going on for many years, and several plants have been proposed, are in development, or are under construction (Table 2). For our discussion, we select a hypothetical new combined-cycle in a 2 x 1 arrangement using F-Class gas turbines, unfired three-pressure, reheat heat-recovery steam generators (HRSGs), a reheat steam turbine with throttle conditions of 13.1 MPa (1,900 psia)/566C (1,050F) and reheat temperature of 566C, and an air-cooled condenser.


Table 2. Partial list of proposed integrated solar combined-cycle plants.Source: Bechtel Power Corp.

Determining how best to integrate steam generated by a CSP technology with a combined cycle is obviously highly dependent upon the steam conditions possible from the different CSP technologies. You will recall that all the power generated in the steam cycle of a combined cycle is "free" from a fuel perspective. That is, steam cycle power is produced without burning any additional fuel (all steam cycle power is a function of the energy remaining in the gas turbine exhaust gases). Replacing the free energy in the gas turbine exhaust gases with energy from solar power to generate steam isn’t productive. The design objective must be to optimize both sources of energy.

Solar Road Map

Determining how to technically integrate each of these three solar technologies into the hypothetical combined-cycle plant (or a conventional steam plant) is the first step in the design process. A close second step is calculating overall plant economics. To simplify the technical analysis, we have categorized each technology based on fluid temperature capability (high temperature: >500C (932F); medium temperature: 400C; and low temperature: 250C to 300C). We begin with medium-temperature technology, as it is the most proven of the three.

Medium-Temperature CSP. The most common medium-temperature solar technology is the parabolic trough. State-of-the-art parabolic trough systems produce up to approximately 380C (716F) saturated steam. That steam is then mixed with the saturated steam generated in the HRSG high-pressure (HP) drum (Figure 4).


4. Medium temperature integrated solar combined cycle. Source: Bechtel Power

Integrating HP saturated steam into an HRSG is very common in integrated gasification combined-cycle (IGCC) plant designs; heating feedwater is an example. An engineering company familiar with the integration issues of IGCC can easily manage the integration involved for ISCC.

Note that it is extremely important to take the feedwater supply to the solar boiler from the proper location in the steam cycle. The most convenient place to take the feedwater is from the discharge of the HP feedwater pump. On most modern combined-cycle systems, the feedwater pumps take suction from the low-pressure (LP) drum, typically ~0.5 MPa (72.5 psia) in a three-pressure reheat system. The feedwater temperature at the pump discharge is ~160C (320F). The heat-transfer fluid leaves the steam generator at ~290C (554F) for a parabolic trough plant similar to those used in the SEGS (solar electric generating systems) plants. Allowing for a reasonable approach temperature, feedwater temperature should be ~260C (500F) to maximize the heating of feedwater in the HRSG and minimize the heating of feedwater in the solar field.

We conclude that it is far more beneficial to take the solar feedwater after some heating in the HRSG HP economizers, as doing so maximizes the recovery of gas turbine exhaust energy to heat the feedwater and minimizes the size of the solar field for a given increment of power. For the same steam flow provided from the solar field, the field would have to be ~30% larger if feedwater were taken from the HP feedwater pump discharge as opposed to downstream of the HP economizer. Alternatively, for the same solar MWth input, the change in net output would decrease by 11% if feedwater for the solar field were taken from the HP feedwater pump, compared to the HP economizer exit (Table 3).


Table 3. Effect of feedwater supply temperature on combined-cycle performance for with a medium temperature CSP. Source: Bechtel Power Corp.

Alternatively, solar thermal input to an ISCC can be used to reduce the plant’s fuel consumption for a given power level. Note that reducing gas turbine fuel consumption also reduces gas turbine power and exhaust energy. For the same plant net output with 100 MWth of solar energy input, plant fuel consumption is reduced by ~8%.

Note that some Fresnel systems also fall into the medium-temperature category, although pressure limitations of that design prevent their use in developing HP saturated steam. Given these pressure limitations, the integration of Fresnel systems would be more suitable for a low-temperature system.

High-Temperature Solar Technology. Power tower systems can generate superheated steam at high pressures and up to ~545C (1,013F). These conditions allow the admission of solar-generated superheated steam directly into the HP steam line to the steam turbine. In addition, steam is reheated in the power tower, much as it is in the HRSG (Figure 5). This configuration requires virtually no HRSG design changes because the solar boiler superheats and reheats the solar-produced steam.

As with the medium-temperature technology, the location of the feedwater takeoff to the solar field is an important parameter to consider in the plant design.


5. High-temperature ISCC. Source: Bechtel Power

Low-Temperature Solar Technology. Most Fresnel systems fall into the low-temperature category. These systems generate saturated steam up to ~270C (518F)/0.55 MPa (80 psia). This pressure is too low to allow integration into the HP system of the steam cycle. Basically, two options exist: Either generate saturated steam at ~3 MPa (435 psia) and admit it to the cold reheat line or generate steam at ~0.5 MPa (72.5 psia) and admit it to the LP steam admission line (Figure 6).


6. Low-temperature ISCC. Source: Bechtel Power

As with the other solar systems, taking the feedwater supply from the optimum location in the steam cycle is vital to maximize the cycle efficiency. However, in low-temperature systems there is less flexibility in the selection of the feedwater takeoff point because the takeoff temperature must be below the saturation temperature of the steam being generated.