Final Research Collaboration Agreement Reports-UQ

RATCH-Australia

Collinsville Solar Thermal Power Station

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www.ratchaustralia.com

Table of Contents

Introduction ........................................................................................................................ 3

Research Collaboration Agreement Reports by the University of Queensland ................. 4

Yield forecasting and Yield Analysis ................................................................................ 4

Lessons learned ............................................................................................................... 4

Cleaning Requirements ................................................................................................... 5

Lessons learned ............................................................................................................... 6

Optimisation of the Operational Regime ........................................................................ 6

Lessons learned ............................................................................................................... 7

Power System Stability and Transient Effects ................................................................ 8

Lessons learned ............................................................................................................... 8

Energy Economics & Dispatch Forecasting ..................................................................... 9

Lessons learned ............................................................................................................... 9

Fossil Fuel Boiler Integration ........................................................................................ 10

Lessons learned ............................................................................................................. 10

Broader Lessons for the Solar Thermal Industry .............................................................. 11

Conclusion ......................................................................................................................... 12

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Introduction

RATCH-Australia Corporation Limited (RAC), in partnership Transfield Infrastructure Pty Limited and The University of Queensland (UQ), is undertaking all the preparatory development work to assess the viability of converting an existing 180MW coal fired power station to a 30MW hybrid solar thermal / gas power station at the Collinsville Power Station (CPS) in Queensland (the Project).

As part of the Project, RAC examined the feasibility of using Novatec’s Supernova Linear Fresnel Solar Thermal technology to generate superheated steam to be supplied directly to a steam turbine to provide grid connected electricity. The dual-fuel boiler will also be designed to use natural gas to enhance grid reliability from the Project.

The Australian Government, through an Australian Renewable Energy Agency’s (ARENA) Emerging Renewables Program Funding Agreement, is partly funding the feasibility study.

As a part of Research Collaboration Agreement, UQ has partnered with RAC to provide valuable research in a number of key project-based issues for the early stage commissioning and optimisation of the plant, including:

• Yield forecasting; • Yield Analysis; • Energy Economics & Dispatch forecasting; • Cleaning requirements; • Optimisation of the Operational Regime; • Power System Stability and Transient Effects; and • Fossil Fuel Boiler Integration.

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Research Collaboration Agreement Reports by the University of Queensland

Yield forecasting and Yield Analysis

Energy yield evaluation is essential for secure and competitive financing of a concentrated solar thermal power (CSP) project. Energy yield of a CSP system depends on solar radiation and to a lesser extent on temperature, humidity, atmospheric pressure and wind speed. Weather events such as passing clouds or storms introduce variability in CSP generation profiles in the short term, whereas large scale weather events such as climate cycles could introduce variability in energy yield for larger or inter-annual time scales. Therefore, energy yield analysis of any proposed CSP project using several years of data is important.

Often the yield analysis of the renewable system has been carried out using a typical meteorological year which excludes the worst case weather events which would represent a realistic long term climate. This analysis might be suitable for a pre-feasibility study but may not be suitable for gaining a power purchase agreement (PPA) or attracting investment. Moreover, using multiple year historical data to model the long term performance of the system presents the performance prediction which accounts for the potential worst case years. Therefore it is essential to analyse the energy yield of any proposed renewable energy projects by using multiple year data sets.

As the Collinsville site has only one year of data available for the performance prediction of the proposed 30 MWe CSP plant, this study investigated the performance of the 30MWe LFR system in Collinsville by using historical data from the nearby location of Rockhampton. The yield results obtained by using the historical data sets can be used to test the effectiveness of the weather model at Collinsville for yield analysis.

Lessons learned

• Accurate and ongoing measurement of solar data on site is important in order to forecast the long-term yield with accuracy for any particular site.

• At the time of installation of the solar monitoring equipment, no single international standard existed for solar measurements meaning that there was neither a standard equipment list nor an accepted method to follow for solar measurement. This means that projects are reliant on the expertise and recommendations of individual installers and upon products such as data loggers and power supplies that are not specifically designed for solar measurements and have caused various issues with data measurements, leading to some loss of data and a high expense for ongoing maintenance of the measurement equipment.

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• As solar monitoring for solar farms becomes more prevalent, standard and reliable equipment will become more common and it is expected that solar monitoring will become more straightforward, more reliable and less expensive.

• There is not a long history of ground-based solar measurements available in Australia so it is somewhat difficult to find high quality and long-term reference data to use for the correlations needed to find long-term solar measurements and forecast long-term yields.

• Satellite-derived solar measurements can be used for correlation with site data to determine long-term yield, however the various commercial datasets available provide somewhat different results and there is not yet a well-tested methodology to account for various factors such as rain and cloud cover for these datasets.

• It was found that modelling the weather with limited datasets from the site produce greater yield predictive power than using the historically more complete datasets from nearby sites.

• It was found that a 7-year modelling period (selected due to the amount of solar measurement data available for the area from the Bureau of Meteorology) was not long enough to accurately predict long-term solar yield as it is shorter than the El Nino Southern Oscillation cycle.

• The effect of climate change on the gross yield of the solar plant is expected to be small, however the effect of climate change on electricity demand and wholesale electricity prices is unknown. This could have an impact on the profitability of the plant.

• It was found that that the BoM does adequately adjust for cloud coverage in their DNI satellite data set, however raw satellite data overestimates effective DNI.

• Cleaning regimes for the solar measurement data can alter the effective-satellite DNI ratio.

• Effects such as variability in DNI, the presence or absence of a sea breeze play a more important role in determining yield than the role played by changing elevation.

• Most of the years under study experienced a high deviation from the mean energy yield (i.e. there is a high variability in solar output from year to year).

Cleaning Requirements

A dust monitor was installed at the Collinsville Power Station to collect the data of dust rates, relative humidity, wind condition and temperature for about a year in this study to determine the level of soiling at the site. Dust samples were collected on filters which were inserted in the monitor. By analysing the real time dust data as well as the dust samples collected on site, the dust characterization was conducted.

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Lessons learned

• At Collinsville, the highest density of airborne dust was about 31.5µg/m3 in October and the highest frequency was 898 for the density of 23µg/m3. On average, the dust was about 20µg/m3 in summer and 10µg/m3 in winter.

• Regular mirror cleaning forms an important part of the operation and maintenance (O&M) activities in solar thermal power plants to maintain high reflectivity of the mirrors.

• Dust composition, particulate size of the dust, relative humidity, rainfall, wind, temperature, and the materials of the mirrors used are the most important parameters in optimizing solar mirror cleaning.

• Wind speed does not seem to have an effect on the dust concentration at the Collinsville site.

• Selecting a cost-effective cleaning method and frequency for CSP plants is very site-specific since the density of the dust deposited on solar mirrors depends on the rate and nature of dust accumulation, particulate size of the dust, the materials of the mirrors used, and the ambient conditions like relative humidity, rainfall, wind and temperature.

• The interaction of dust with different mirror materials needs to be quantified in the future work.

• Future study on the most widely used high pressure jet washing system should be focused on the optimum selection of parameters such as water pressure, water flow rate and nozzle type etc.

Optimisation of the Operational Regime

This study investigated the dynamic characteristics and performance of the proposed 30 MWe hybrid direct-steam generation (DSG) solar thermal power plant to replace the pre-existing Collinsville coal-fired power station in North Queensland, Australia. A dynamic model of the solar field in the proposed power plant was developed and simulations were conducted to understand steam generation characteristics during transient events affecting solar heat input such as passing cloud cover and heat ramps during start-up and shutdown. Flow instability in the parallel evaporator tubes of the solar field was simulated and a control strategy for stabilising steam output quality in the evaporator tubes was also demonstrated using simulations.

Preliminary investigations into optimisation of the operational regime at the proposed Collinsville solar thermal power plant included the development of a dynamic model of the solar field in Dymola. The identification and implementation of appropriate flow-

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regime based heat-transfer correlations in the dynamic model along with pressure drop and heat-loss models for the evaporator and superheater solar fields were also conducted. The need for consideration and resolution of two-phase flow instabilities in the parallel evaporators of the solar field was highlighted using steady-state analyses and dynamic simulations.

Future work could include investigation of solar field dynamic performance at low operating pressures and with integration of the superheater solar field and turbine group. Improvement of the proposed control strategy could also be investigated.

Lessons learned

• Designing a solar field for operation at high steam qualities is risky. This is due to the high possibility for mist flow or dry-out occurring in the tubes causing excessive tube-wall temperatures in the solar field.

• The estimates of pressure drop by two different software modelling packages (Thermoflex and Dymola) are different. It is possible that the cause of the difference is due to a difference in the specific two-phase pressure drop correlation used in modelling.

• The calculated steady-state heat loss from the receiver modelled by the Dymola software compares well with results from tests at the National Renewable Energy Laboratory.

• The results demonstrate that there will be a momentary spike in water (or condensate) returning from the separators to the condensate drum with a rise in solar heat input which, if not considered in the design stage, could cause a sudden and significant rise in drum water levels during dynamic operation of the power plant.

• An uneven distribution in solar heat-flux amongst adjacent evaporators in the solar field would have the potential to trigger undesirable two-phase flow instabilities in the tubes. It was found that a control strategy will be required for mitigating both static and dynamic flow instabilities occurring in the parallel evaporator sections.

• Results from the study at both low and high operating pressures have indicated that the thick-walls of the solar collectors have a stabilising effect on steam generation and flow during solar heat fluctuations and uneven distribution of solar heat across the solar field. The solar field is however highly susceptible to flow maldistribution and therefore potential resulting damage. The possibility for certain evaporator tubes to be entirely superheated and others to be entirely subcooled due to uneven heat distribution are highlighted in the simulation results. A control strategy based on a cascade scheme comprising the actions of a central pump and control valves situated directly upstream of each evaporator tube was been proposed to mitigate potential boiling instabilities identified using the simulations. This proposed control

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strategy showed promising results in the simulations to meet the objective of delivering predictable steam quality and flow through the solar field during fluctuations in solar heat input and for operation with uneven solar heat distribution across the solar field.

Power System Stability and Transient Effects

Integration of renewable energy generation into medium voltage (MV) and low voltage (LV) networks have been encouraged in Australia to meet the mandatory Renewable Energy Target. Among different renewable energy sources, concentrated solar thermal (CST) is considered to be one of the promising resources for medium to large scale deployment. However, medium scale deployment (1 MW – 30 MW) possesses integration challenges such as voltage regulation due to its inherent power generation profile. Owing to the variability and uncertainty in CST generation, it is important to assess the low or medium voltage distribution network performance in detail before making a final decision for interconnection. With this perspective in mind, the main objective of this research was to investigate the impact of CST on distribution system voltage regulation and stability.

The power output profile of the CST plant was evaluated by the Novatec Linear Fresnel Reflector (LFR) model available in NREL-System Advisor Model (SAM). A time series analysis method was used to quantify the impact of CST in a distribution system. The main advantage of this method is that it can provide information about the power fluctuation from a CST plant. However, this method requires extensive analysis and simulation, making it impractical for a utility to study the system with long historical data sets. Therefore, a clustering technique was used to reduce the power profile datasets by keeping the required information of the data series.

The performance of the system was assessed for both nominal and 6% load growth conditions. System voltage regulation, static and dynamic voltage stability performances were assessed and presented in this report.

Lessons learned

• Placing a CST in the vicinity of a strong high voltage bus does not significantly affect the steady state voltage profile of a low voltage network.

• Regarding voltage stability of a network, a 30 MWe CST plant in a distribution system has a noteworthy impact on small- and large-disturbance voltage stability however it does not pose any threat of voltage instable operation.

• System loading margin or voltage security margin was found to be altered significantly with the changing power profiles of the CST plant.

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• Due to the difficulty of obtaining network details from the distribution utility, this study was conducted on a generic model of a distribution system. Better access to network data from the utility companies would allow analysis to be carried out on the actual distribution system in which the plant would operate.

• Due to the current reactive power regulation act, renewable based distributed generators such CST may be required to operate in fixed power factor mode, which could lead to a reactive power imbalance in the system. It was found that due to the CST insertion in the system the steady state voltage profiles of the system buses experienced significant variations during the midday and late afternoon as compared to the morning due to higher real and reactive power demands during those time windows of the day and no reactive power injection to the grid from CST plant. Voltage control mode operation of the CST plant would minimize these voltage variations during the midday and late afternoon, if reactive power regulation was able to be optimised with the network operators.

Energy Economics & Dispatch Forecasting

The primary aim of this study was to determine the half-hourly dispatch of the gas component of the proposed hybrid gas-LFR plant at Collinsville and the associated half-hourly wholesale spot prices for the plant’s node on National Electricity Market (NEM) given the yield from the solar thermal component of the plant and a fixed total dispatch profile. This report determined these half-hourly wholesale spot prices and dispatch profiles for the gas component of the plant for the lifetime of the plant. The half-hourly yield profile for the solar thermal component of the plant was determined in a separate yield report. These three profiles: solar thermal yield, gas dispatch and spot price, would be utilised to help to negotiate a Power Purchase Agreement (PPA) and to evaluate the proposed dispatch profile. The proposed profile would maintain a 30 MW dispatch during the weekdays by topping up the yield from the LFR by dispatch from the gas generator to imitate a baseload function currently provided by coal generators.

Lessons learned

• It was found that operating the gas component only on weekdays, Monday to Friday, is well justified.

• It was found that the proposed plant is a useful addition to the NEM but the proposed profile is unsuitable because the gas component is loss making for four months of the year. This is because the high short-run marginal cost of the gas component in a baseload role loses the flexibility to respond to market conditions and contributes to loss instead of profit and to CO2 production during periods of low demand.

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• A modified dispatch profile was proposed in order to only use gas when the demand (and hence the wholesale price of electricity) justifies it. The alternative profile would retain the advantages of the proposed profile but would allow the gas component freedom to exploit market conditions.

• The flexibility of the gas component may prove more advantageous as the penetration of intermittent renewable energy increases.

Fossil Fuel Boiler Integration

A gas-fired steam boiler can provide significant flexibility to the proposed Collinsville solar thermal power plant as it also allows operation of the power plant during periods of low or no solar insolation and evening peak periods. This report presented investigations which are complimentary to the dynamic modelling and simulation work presented in the ‘Optimisation of Operational Regime’ report with focus on investigating the requirements for integration of a fossil-fuel fired steam generator or boiler into the solar thermal power plant. A fundamental understanding of the time required for the gas-fired boiler to react to transients in solar insolation is important in the design and operation of the solar thermal power plant. Hence, an analysis of the time necessary for the gas-fired boiler to react for steam delivery to the turbine during solar insolation transients including start-up and shutdown was determined through analysing the time-constants of the evaporator solar field using dynamic simulations. The time-constant of the evaporator in the solar field is defined by the time taken for steam generation to start and cease during a rise or drop in solar insolation to the solar field. The amount of steam generated in the evaporator also governs the required reaction time for the gas-fired boiler as well as the reaction time for any control action such as defocusing required on the superheating section of the solar field to avoid tube overheating during plant start-up.

Simulations were conducted in Dymola using the dynamic model of the evaporators in the solar field developed and described in the ‘Optimisation of Operation Regime’ report. The model includes flow-regime based heat-transfer coefficients, pressure-drop, and heat-loss from the absorber and the evacuated-tube to the environment. The overall dynamic model in Dymola comprises both pre-existing sub-component models from the Thermal Power and ModelicaStandard library along with newly developed sub-component models.

Lessons learned

• The required response time of the fossil-fuel boiler is governed by the time-constant of steam generation or cessation in the evaporators of the solar field.

• Time-constants of the evaporator solar field were found to be influenced by the mass-flow rate delivered to the solar field and by the magnitude of

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solar insolation received. The time-constants of the evaporators and therefore required response time of the fossil-fuel boiler range from approximately one to three minutes depending on the specific operating condition and are shorter for start-up or heat-rise conditions when compared to shut down or heat-drop conditions.

• These time constants will be used in the specification of the gas-fired boiler to ensure that the dynamic response of the gas-fired boiler is fast enough to account for the changing output of the solar boiler due to prevailing solar conditions.

Broader Lessons for the Solar Thermal Industry

The UQ research reports have provided a number of broader lessons for the solar thermal industry. These include:

As there is not a long history of ground-based solar measurements available in Australia it is somewhat difficult to find high quality and long-term reference data to use for correlations to forecast long-term yields. The various commercial satellite-derived solar datasets provide somewhat different results and there is not yet a well-tested methodology to account for factors such as rain and cloud cover for these datasets. It was found that a 7-year modelling period (selected due to the amount of solar measurement data available for the area from the Bureau of Meteorology) was not long enough to accurately predict long-term solar yield as it is shorter than the El Nino Southern Oscillation cycle. As such, accurate yield predictions for Australian solar thermal projects will be challenging until ground-based measurements have been in operation for a number of years. It was found that modelling the weather with limited datasets from the site may produce greater yield predictive power than using historically more complete datasets from nearby sites.

Further, some interesting points noted in the research were that raw satellite data overestimates effective DNI and that at the site studied there was a high variability in solar output from year to year, such that most of the years under study experienced a high deviation from the mean energy yield.

Regarding soiling, there appears to be very little, if any, information about dust concentrations at various solar sites, and scarce information about the impact that dust concentration or dust composition would have upon output of CST plants. Although dust concentration was measured at the Collinsville site, there was no data from other sites with which to compare it, nor any public information about the effect of dust concentration on solar output. While collection of dust data is not specifically useful at this stage, is expected that this information will be useful once a critical mass of projects are underway and dust monitoring data is available for a number of different sites.

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Regarding the operational regime of a solar thermal plant, it was found that designing a linear Fresnel solar field for operation at high steam qualities is risky due to the high possibility for mist flow or dry-out occurring in the tubes causing excessive tube-wall temperatures in the solar field. The research made clear a number of factors that would need to be considered in the detailed design of the project and its control system and it appears that detailed software modelling of any linear Fresnel project at an early stage is highly valuable.

In terms of power system stability, the research demonstrated that placing a CST in the vicinity of a strong high voltage bus does not significantly affect the steady state voltage profile of a low voltage network and it does not pose any threat of voltage instable operation. Early discussions with the relevant Network Service Provider about reactive power and modes of operation for a solar thermal project would be likely to benefit the outcome of the project on the local power distribution system.

For hybrid solar thermal plants it was found that the operating profile of the gas component of the plant would ideally be to only use gas when the demand (and hence the wholesale price of electricity) justifies it in order to exploit market conditions. There is no financial benefit in running the plant as a baseload power plant.

Conclusion

The UQ research reports have provided valuable insights and inputs into the design and operation of the proposed 30MW hybrid solar thermal / gas power station at the Collinsville Power Station. The yield forecasting and analysis has shown the importance of high quality long-term site measurements in order to add certainty to yield forecasts. Existing satellite-derived datasets and measurements available from other sites do not provide the same accuracy as on-site measurements. The amount of dust in the air and regular cleaning of the solar mirrors will have an effect on the output of the plant. Density of dust at the Collinsville site was measured and analysed however at this stage there is a lack of comparison data from other sites to determine the effect of this dust on the solar output. It was found that the dust concentration was not affected by wind speed at the Collinsville site. The investigation and findings concerning dynamic characteristics of the hybrid solar thermal plant during transient events will influence the detailed design of the solar thermal plant and its control strategy. In terms of power system stability, it was found that placing a concentrating solar

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thermal plant in the vicinity of a strong high voltage bus does not significantly affect the steady state voltage profile of a low voltage network; however system loading margins and voltage security margins would be affected. It was also found that operation of the solar thermal plant in voltage control mode would improve the reactive power performance of the network if such operation could be agreed with the operators of the network. These results would be used to inform discussions and negotiations with the distribution network operator regarding network connection for the proposed plant. From an economics perspective, the investigation into energy economics and dispatch forecasting has shown that the dispatch model should be re-examined to make use of the gas component of the power station only when the demand (and hence the wholesale price of electricity) justifies it. Otherwise the high short-run marginal cost of the gas component in a baseload role loses the flexibility to respond to market conditions and contributes to loss instead of profit for some months of the year.

Finally the time constants determined by the boiler integration studies will be used in the specification and control of the gas-fired boiler to ensure that the dynamic response of the gas-fired boiler is fast enough to account for the changing output of the solar boiler from prevailing solar conditions.

Overall, the research has improved understanding of a number of key project-based issues for the detailed design, early stage commissioning and optimisation of the plant.

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