TECHBriefs www.burnsmcd.com A Burns & McDonnell Publication 2009 No. 4

Concentrating Solar Trough Modeling: Calculating EfficiencyMethod Incorporates Technological ImprovementsBy Benjamin Munz and James Hays, PE, LEED® AP Concentrating solar trough field technology is an exciting area of renewable energy development. While the flat photovoltaic panels of more-familiar solar energy systems convert the light of the sun to energy, the parabolic mirrored glass panels of a concentrating solar trough field collect and concentrate the sun’s heat. Because thermal energy can be stored,the technology offers the promise of large-scale, stable power production with no emissions.

A large array of trough-shaped mirrors make up the solar field (see Figure 1). Pipes containing the thermal fluid — a special type of synthetic oil — are positioned at the focal points of the mirrors. Concentrating the heat of the sun increases the temperature of the thermal fluid from 291°C to 393°C (555°F to 740°F), which in turn heats water to make steam.

This article outlines the method for calculating the solar-to-electric efficiency of a concentrating solar trough field system. Sample values and calculations within this article are for a typical 49.4-megawatt (MW) net power plant in the Imperial Valley of Southern California, south of the Salton Sea.

Major Componentsand Systems The basic component of the solar field is the solar collector assembly (SCA). The project in the Imperial Valley will use 92 loops for the solar field; each loop includes six SCAs. Each

SCA includes 224 mirrors, an independent positioning system and local control system. The Imperial Valley example project will be modeled on the Luz System 3 design of SCAs.

No Smoke; Just MirrorsThe parabolic trough solar collector is a mirrored glass reflector that focuses direct solar radiation on an efficient evacuated receiver, or heat collector element (HCE).

The reflector mirrors are made up of hot-formed mirrored glass supported by a truss system, which gives the SCA its structural integrity. The aperture, or width, of the parabolic reflector is 5.76 meters and the overall SCA length is 95.2 meters (net glass). The mirrors are sized at 330,016.5 square meters for the 92 loops.

There are 224 reflector panels (mirrors) on each SCA. Each panel is 3.2 millimeters thick,

Figure 1: A concentrated solar trough field as installed.

TECHBriefs 2009 No. 4 2 Burns & McDonnell

covering an average of 2.669 square meters in area. The system efficiencies are identified in the waterfall analysis presented in Table 1. The focal point of the HCE (parabola) is 1.71 meters.

Each HCE, manufactured by Schott for this example, consists of 70 millimeter-diameter stainless steel tubing through which the heat collector fluid is pumped. The external surface of the pipe is ceramic metal-coated, or CERMET-coated, for high absorptivity, then encapsulated under a vacuum inside a glass tube. The CERMET selective surface has an absorptivity of 0.96 for direct-beam solar radiation and a design emissivity of 0.19 at350 degrees Celsius (662 degrees Fahrenheit). The glass tubes are treated internally and externally with an anti-reflective coating.

Soaking Up the RaysDuring operation, the metal bellows take up the difference of the thermal expansion between the hot absorber tube and the cool outer glass envelope as the HCE heats up from the standby temperature, typically in the morning before solar field startup, to the normal operating temperature.

Other primary components of an SCA include the support structural components, the heat collector elements and the tracking system (drive, sensor, controls).

Periodically, commands are sent to the hydraulic drive system to position the SCAs to track the movement of the sun. Solar tracking is achieved using a closed-loop sun tracking system by optical position sensor to maintain

accurate alignment. The SCA moves from the maximum stow position (minus 30 degrees below sunrise horizon) to plus or minus 2 degrees above the sunset horizon, for a maximum angle deployed of 178 degrees.

Site LayoutSolar arrays are aligned parallel with the north-south axis. Design clearance betweenthe ends of each mirror assembly is approximately 1.2 meters for a total design length of approximately 30.5 meters. Layout space for the hot oil headers is 6.1 meters from the ends of the mirror assemblies. Center-to-center of mirror rows is 17.4 meters, providing maximum generation.

As the parabolic trough collectors that compose the solar field individually track the sun from east to west on a single axis, focusing the solar energy on the pipe containing the heat transfer fluid, the fluid reaches 393 degrees Celsius (740 degrees Fahrenheit). It is then pumped through a series of conventional heat exchangers to generate superheated steam powering a conventional steam turbine generator.

The solar fields for the Imperial Valley were sized based on experience gained though the design and operation of previous facilities, coupled with knowledge of design and operational improvements implemented by

Benjamin G. Munz is an assistant mechanical engineerin the Burns & McDonnellSan Diego office. His specialty is in machine and mechanical system design, heat transfer and HVAC design. He received his bachelor’s degree in mechanical engineering from North Carolina State University.

For more information, please e-mail: jhays@burnsmcd.com or bmunz@burnsmcd.com.

James P. Hays, PE, LEED® AP, is manager of engineering in the Burns & McDonnell San Diego office. He has more than 25 years of experience in project and program management, design and design leadership, and engineering execution. He is a registered professional engineer in 12 states.Calculating the projected

efficiency of a field in a specific location, with the specific equipment used in the system,is an important step in sizinga new field and producing cost estimates.

The solar fields for the Imperial Valley were sized based on experience gained though the design and operation of previous facilities, coupled with knowledge of design and operational improvements implemented by suppliers of key equipment components including mirrors, collectors and the steam cycle.

Burns & McDonnell 3 TECHBriefs 2009 No. 4

Table 1: System efficiency is a product of efficiency of each step of the conversion process. System modeling can estimate the effects of incremental improvements in component performance.

Optimal Efficiency

Factor Energy Unit*

Expected Efficiency

Factor Energy

Number of loops 92 92

Gross curved mirror area 330,017 m2 330,017

Aperture area 91.83% 303,054 m2 91.83% 303,054

Insulation 1,000 watts per square meter 1000.00 303,054,188 W 1000.00 303,054,188

Reflection 92% 278,809,853 W 87% 263,657,144

Glass envelope transmission losses 96% 267,657,459 W 90% 237,291,429

Receiver spillage 95% 254,274,586 W 86% 204,070,629

Receiver absorption 95% 241,560,857 W 95% 193,867,098

Radiated and convected receiver losses 86% 207,742,337 W 86% 166,725,704

Piping and storage losses 95% 197,355,220 W 95% 158,389,419

Oil temperature to reboiler F 731

Oil-to-steam heat exchanger 95% 187,487,459 W 95% 150,469,948

Steam turbine efficiency 36.69% 68,785,722 W 36.69% 55,204,673

Electrical generator efficiency 98% 67,410,007 W 98% 54,100,580

Auxiliary power, HTF pumps: 1 million watts approximately

98.52% 66,410,007 W 98.15% 53,100,580

Auxiliary power, other:approximately 3 million watts

95.48% 63,410,007 W 94.35% 50,100,580

Net solar-to-electric efficiency 20.92% 16.53%

Plant availability 97.1% 61,571,117 W 97.1% 48,647,663

Overall annual solar-to-electric efficiency 20.3% 16.05%

suppliers of key equipment components including mirrors, collectors and the steam cycle.

Net Solar-to-Electric Efficiency CalculationsJust as operators of gas- or coal-fired power plants strive to achieve the greatest production of electrical power per unit of fuel consumed, designers and operators of solar-powered generation facilities are interested in achieving the highest solar-to-electric efficiency possible. Calculating the projected efficiency of a fieldin a specific location, with the specific equipment used in the system, is an important step in sizing a new field and producing cost estimates.

Solar trough modeling is used to calculate the efficiency of concentrating solar trough field systems. The efficiency of the field is calculated using the total winter on-peak — the amount of heat energy available on the shortest day of the year with the sun in the optimum position (directly overhead) to produce — in this case — 49.4 MW out of the steam turbine generator.

The overall system efficiency is a product of the efficiency of the collection of sunlight, called solar optical efficiency (the product of receiver solar absorbance and transmittance efficiencies) and the efficiencies of each step of the heat-to-electrical-power conversion process.

* m2 = square meters W = watts F = degrees fahrenheit

TECHBriefs 2009 No. 4 4 Burns & McDonnell

Table 2: This sequence of equations uses known and estimated factors to calculate overall system efficiency.

Solar reflectance and optical efficiency The calculation of solar efficiency at 79.7% was based on typical optimal data for mirror reflection, glass envelope transmission losses, receiver spillage and receiver absorption losses.

92% x 96% x 95% x 95% = 79.7%

Thermal efficiencyReceiver thermal efficiencies represent the radiated and convected receiver losses. Typical losses are approximately 86%. Piping and storage losses based on experience and sound engineering insulation design and construction practices are approximately 95%. Oil-to-steam thermal losses utilized in the industry are assumed to be 95%.

86% x 95% x 95% = 77.6%

Calculation of solar-to-thermal efficiencyThe total solar-to-thermal efficiency is calculated as the product of the above efficiencies.

79.7% x 77.6% = 61.8%

Turbine and generator efficienciesThe solar industry utilizes the efficiencies of 36.7% for the steam turbine and 98.0% for the electrical generator.

36.7% x 98.0% = 36.0%

Auxiliary power loads and plant availabilityPlant design studies and good engineering practice utilize approximately 1% auxiliary load for heat transfer fluid pumping, approximately 5% for all remaining miscellaneous loads and plant availability at 97.1%.

(100% -1%) x (100% - 5%) x 97.1% = 91.3%

Overall solar-to-electric efficiencyThe overall efficiency is the product of the above three efficiencies.

61.8% x 36.0% x 91.3% = 20.3%

The sequence of equations used to calculate intermediate efficiencies and finally, overall solar-to-electric efficiency, are presented in Table 2. The factors in the equations are derived from system modeling results as shown in table 1 on page 3. Together, these procedures form the basis for determining the size of the proposed solar trough field in this example.

ConclusionThe calculations in Table 2 represent a condition in which all processes are optimized. Incremental efficiency improvements in each of these areas will be required to significantly

improve overall efficiency. Table 2 provides one example of the calculation, along with the calculations for expected field conditions.

As development of thermal solar systems continues, higher solar-to-electric efficiencies are likely to be achieved. Incentives for implementation of large-scale renewable energy will also likely increase. The technique described in this article for modeling systems by adjusting existing system data for anticipated technological improvements may be useful in determining costs for funding purposes, as well as in sizing and estimating for the purpose of design.

Burns & McDonnell 5 TECHBriefs 2009 No. 4

Boosting Power Output — Cool!Options for Inlet Cooling in Gas Turbine InstallationsBy John Langaker, PE, and Zachary Loehr Many power generating gas turbine installations in the U.S. and worldwide have an untapped resource for extending power output: inlet air cooling. These turbines can run at higher efficiency and output using cooled inlet air compared to hot ambient air. Cooler (or denser) air increases mass flow per unit volume. In single-speed, constant-volume machines such as most large gas turbines, that translates to higher output.

As demand grows, conditioning the turbine inlet air is an excellent way to expand a fleet’s ratings in the critical summer demand season since this method does not necessarily trigger a new source of air-polluting emissions. A gas turbine owner needs to recognize cooling gains available, the options to get them and ultimately adopt a solution tailored to the owner’s needs, independent of manufacturer preferences or biases.

GainsAnyone who owns or operates gas turbines knows that performance improves as the units consume cooler air. Trends utilizing evaporative coolers are summarized in Figures 1 and 2. In general, single-speed frame machines that do not change the running-flow path geometry

all share roughly the same margin for gain in comparing hot ambient operation to cooler.Aero-derivative gas turbines, which operate using multiple-speed, multi-shaft designs, offer a slightly different gain profile with cooler inlet air compared to hot.

All gas turbines will approach a point of concern, however, as ice forms in the inlet, which threatens first-stage compressor damage. Therefore, an ideal air inlet temperature will avoid temperatures too close to this threshold yet still offer significant payback for the cooling system investment.

Existing Cooling SystemsPartial gains with existing systems mayalready be the optimum solution, particularlyif an economic cost-benefit analysis wasdone initially.

Figure 1: Performance of gas turbines improves as units consume cooler air.

Frame Gas Turbine Typical Gain

Meg

awat

ts

180

170

160

150

140

130

Aero-Derivative Gas Turbine Typical Gain

Meg

awat

ts

6050403020100

Figure 2: Cooling gains are less dramatic for auto-derivative gas turbines.

50°F100°F with evaporative cooling

100°F without evaporative cooling

As demand grows, conditioning the turbine inlet air is an excellent way to expand a fleet’s ratings in the critical summer demand season.

50°F100°F with evaporative cooling

100°F without evaporative cooling

TECHBriefs 2009 No. 4 6 Burns & McDonnell

Many gas turbines are purchased and installed with economical solutions directly from the turbine manufacturer. These systems, while well-proven, tend to sacrifice the maximum potential of the turbine in presenting a standardized system at minimal cost. An unbiased review of existing system design on a gas turbine should review the benefits of the system as-is compared to other viable cooling system options and using the operator/owner’s own values for cost and returns on the investment. Among other factors, consider whether the owners are happy with what they have (i.e., operability).

New Turbine Installation OpportunitiesWhen a utility is in the market to purchase installed capacity by means of one or more gas turbine generator sets, it is wise to understand the payback opportunity of various systems in relation to initial cost, complexity and value of the additional capacity and efficiency. It is also important to understand the advantages and disadvantages of the different cooling systems. Should a risk factor be used in comparing the options, so availability is considered? Absolutely.

Cooling Air Methods: Advantages and ChallengesIn addition to packaged systems, methods of cooling gas turbine intake air include evaporative cooling, fogging, online chilled-water cooling and thermal energy storage.The methods vary in cost and complexity —and each has advantages and challenges. Summarized below from the simplest and most familiar to the most complex, these cooling methods increase gas turbine efficiency.

Evaporative CoolingAlso known as swamp coolers, because they achieve swamp-like conditions of near-full humidity in the air passing through, evaporative coolers are the most common cooling system that gas turbine manufacturers feature on new units. The air entering the turbine is pulled through media saturated with good-quality water (potable or service-grade). Heat from the air is transferred to the wetted surfaces of the media and converts the liquid water to vapor phase. This removes heat effectively equal to the

latent heat of vaporization of the water at the conditions present.

Advantages: A simple system that operates with easily-obtained water quality.

Challenges: Water’s latent heat of vaporization is the dominant method to transfer heat from the passing air. Thus, the wet-bulb temperature is the absolute limit to which the air can be cooled. Nominal- effectiveness performance losses bump up the final air temperature slightly higher. Typical effectiveness is approximately 85%.

(1) Clean,Warm Air

How It Works1. When the filtered air passes through the saturated evaporative cooling media, water evaporates off the wet media. This evaporation is the process that reduces the air temperature. 2. Excess water that does not evaporate is directed downward so as not to be carried along with the cooled air.3. Cooled air then passes through the integral mist eliminator, where leftover water droplets are removed.4. Clean, cooled air is then directed into the turbine inlet.

(1) & (2) Cooling Media(3) Mist Eliminator

(4) Cooled Air to Turbine Inlet

Evaporative Cooling

Cour

tesy

Don

alds

on C

o. In

c.

Figure 3: Evaporative cooling systems are simple and fairly effective, but depending on conditions, may not achieve optimum results.

Burns & McDonnell 7 TECHBriefs 2009 No. 4

FoggingWhereas traditional evaporative cooling has intrinsic limitations in effective heat transfer, fogging promises to fully absorb water injected into the air as a fine mist or fog. There is no blow-down or discharge from a fogging system; all is ingested into the gas turbine.

Advantages: Maintained correctly, both water consumption and the cooling effects (compared to the ambient wet-bulb temperature of the air as it enters the filter house) can be optimized.

Challenges: Water quality must be demineralized grade and fog nozzles must be maintained to ensure droplets are properly atomized as they pass into the inlet air structure, or costly damage to the gas turbine can result.

Online Chilled Water CoolingTo cool the inlet air to less than the ambient wet-bulb temperature, a chilling system can be installed to circulate coolant in a closed loop that includes inlet air coils mounted inside or in front of the filter house to extract heat and chillers to reject the same heat. Depending on fluid type used, the work required to chill the coolant is offset in power output gains to varying degrees.

Advantages: Gas turbine performance can be truly optimized by conditioning the inlet air regardless of the ambient wet-bulb temperature.

Challenges: The chilled-liquid loop and chiller systems are more complex than evaporative cooling or fogging and present inherent trade-offs between efficiency and maintenance costs. Mechanical chillers also require higher auxiliary loads compared to other cooling options.

Thermal Energy StorageAs the use of online chilling has gained popularity for gas turbine enhancement in hotter ambient conditions, thermal storage has emerged to further reduce the cost of the power boost of cooler inlet air.

The tonnage (size) of the chillers can be reduced when incorporated into an insulated tank system that is exhausted and replenished in cycles suited to the optimal periods of cooled gas-turbine operation. Lower tonnage equates to lower auxiliary electrical load that otherwise deducts from plant net output. Timing of the auxiliary load can also be managed to mitigate the cost, since electricity billing rates may vary according to the time of day.

In addition to allowing timing of chiller loads, thermal storage may present the opportunity to draw power for smaller chillers from renewable energy sources, such as wind at night. The stored thermal energy can then be used to chill gas turbines or supply chilled-water systems during dayime peak-demand periods.

Advantages: This is an approach that may quickly pay for itself and further enhance the power-output net gains of online chilling with its concurrent auxiliary-plant electrical loads.

Challenges: Thermal energy storage presents the highest degree of complexity for plant operation compared to its counterpart technologies and is most sensitive to changing rates of return on the investment made.

Packaged SystemsWhether you are building a home for yourself or constructing a greenfield power plant, packaged solutions are alluring for several reasons.

John Langaker, PE, is an engineer in the Burns & McDonnell energy group. He received his bachelor’s degree in mechanical engineering from Vanderbilt University. He has nearly 15 years of experience as lead mechanical engineer in power plant design and construction, and is a licensed professional engineer in Kansas and Florida.

For more information, please e-mail: jlangaker@burnsmcd.com or zloehr@burnsmcd.com.

Zachary Loehr receivedhis bachelor’s degree in mechanical engineering from the University of Evansville. He is an engineer in training in the Burns & McDonnell energy group. His area of specialization is coal and gas turbine plant development.

To capitalize on available gains, conditioning inlet air should be studied early in any utility’s plan to expand a fleet’s ratings. Owners should recognize what cooling gains are available and the options to achieve them.

TECHBriefs2009 No. 4

• ConcentratingSolarTrough Modeling: Calculating Efficiency

• BoostingPower Output — Cool!

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To begin with, a packaged system will marry equipment and logic that is (hopefully) time-tested in other installations, which imparts a degree of reliability and ease of operation.

It should be recognized that a packaged gas-turbine inlet air cooling system has taken measures to unify its mode of construction and operation among many different customers’ needs. For example, is the pressure drop across the cooling media or coil bundle optimized for the loss in power output and gain in heat rate?

The answer depends on owners’ specific costs of power. Among other factors, the individual results of a packaged system depend on how closely an owner’s economic profile matches the packager’s typical customer.

ConclusionOwners of gas turbines will enjoy higher efficiency output with cooled inlet air when the hotter seasons arrive. However, there are many installations in the U.S. and the world where gas turbine inlet-air cooling is an underutilized or altogether untapped resource for extending power output.

To capitalize on available gains, conditioning turbine inlet air should be studied early in any utility’s plan to expand a fleet’s ratings. One should recognize what cooling gains are available and the options to achieve them. The solution should then be tailored to the owner’s needs, independent of manufacturer preferences or biases.