Dr Josef Petek, VTU Energy, Austria
As part of global efforts to reduce the carbon footprint of fossil fuel power generation, various policy instruments have been introduced by a number of governments to subsidize preferred technologies. Feed-in tariffs are intended to help new technologies reach maturity in spite of the initial high cost of generating electricty.
In the case of integrated solar combined-cycle (ISCC) power plants, the solar portion of the overall power output of the plant may be subject to such favourable feed-in tariffs. However, the actual settlement of a power purchase agreement (PPA), including such tariffs, faces the problem that the portion of electricity that is accountable as renewable energy is not accessible to direct physical metering.
Thus, the PPA requires additional model-based procedures besides the production meters to determine the applicable tariff, which is calculated from the portion of the overall power output from the combined-cycle gas turbine (CCGT) plant that can be attributed to the solar field which provides additional thermal energy to the power plant.
This article describes a comprehensive accounting and settlement system that processes a dual-tariff PPA utilizing a reference plant model to determine the solar portion of the overall electrical output. Using a detailed model of an ISCC plant, various operational scenarios are investigated.
Benefits of ISCC
ISCC is a hybrid power technology that brings together a concentrating solar power (CSP) plant with a modern CCGT power plant.
There are two major benefits of this technology compared to a stand-alone CSP plant.
Firstly, since the fossil fuel fired CCGT plant can operate continuously, the start-up and shutdown losses of the solar plant can be minimized, and secondly the incremental costs for a larger steam turbine in the CCGT plant are less than the overall unit costs in a CSP plant.
In addition, the larger plant capacity, as well as the shared operation and maintenance costs, has the potential to make ISCC a very attractive option from a commercial point of view.
Nevertheless, ISCC technology still depends on favourable feed-in tariffs which are applied as a policy instrument to promote power generation technologies from renewable energy sources.
Consequently, new complexity is added for the PPA of an ISCC plant by the solar contribution to the energy input of the plant.
In an earlier publication1 the authors compared various methodologies to determine the fraction of the overall power output of an ISCC plant that is attributable to the CSP plant and concluded that a model-based approach is required to correctly assess this value.
Model-based output allocation
The model-based approach to determine the solar contribution to ISCC power output uses a detailed thermodynamic plant model, as shown in Figure 1, that is executed twice, firstly with both solar heat and fossil fuel present as operated, and secondly without thermal input from the solar field but with the same input of fossil fuel as in the first case.
|Figure 1. Screenshot of the Ebsilon heat balance model for the Kuraymat ISCC power plant Source: VTU|
The latter calculation represents a theoretical operating case and is referred to as the Reference Plant. The difference in calculated electrical output between as operated and as determined from the Reference Plant represents the solar contribution to the overall plant output under current operating conditions.
This work uses a heat balance model representing the Kuraymat ISCC power plant in Egypt, one of the first ISCCs in operation. Data for the plant were made available by Fichtner Solar GmbH in a publication by Brakman et al at the SolarPACES 2009 conference in Berlin2.
The model of the plant was generated with the Ebsilon Professional heat balance software3, which includes a library of component models for solar applications that was co-developed with the German Aerospace Centre (DLR).
This library includes parabolic trough and linear Fresnel type collectors, distributing and collecting headers and a special component to calculate the sun position as a function of the location, date and time.
Table 1 lists the results of the baseload operation at plant reference conditions (21 March at solar noon, 700 W/m2 DNI, 20 oC ambient) with and without steam import from the solar field.
|Table 1. Results of the baseload operation at plant reference conditions, with or without steam imports from the solar field Source: VDU|
Under reference conditions the solar contribution to the overall plant output is calculated to be 22 MW which represent 17.5 per cent of the overall baseload output of 125.7 MW.
Fuel demand model
The concept of the fuel demand model (FDM) was developed during the first independent water and power plant (IWPP) projects in the Middle East and is currently being applied to practically all independent power projects within the Gulf region.
The purpose of the FDM is to produce accurate predictions of the fuel consumption of the plant as per the PPA to allow the execution of the performance guarantees in the settlement procedure.
The FDM is a detailed physics-based thermodynamic model of the plant that reflects the contract guarantees with very high accuracy. Since the PPA typically only includes a limited number of guarantee points, most of the operating conditions of day-to-day operation do not coincide with the operating conditions stipulated in the PPA.
Simple interpolation between individual guarantee points is very likely to produce erroneous or even physically improbable results. Thus, the model-based approach of the FDM, which reflects the plant through thermodynamic models of the major plant components, has proven to be a much more reliable measure to assess conformity with well-acknowledged PPA performance guarantees.
Given the quality and accuracy requirements of the FDM as currently applied in Middle East PPAs, the FDM will represent the plant performance guarantees under all possible operating conditions, including periods without steam generation from the solar field.
Plant accounting system
In order to process the PPA it is necessary to collect a variety of data from the plant and the interface points to the grid.
The inputs to the formulae of the PPA comprise both on-line data and manual inputs. They can be categorized as follows: production meters; fuel meters and operational data for the FDM; availability data; start-up data; and contractual and manual inputs.
With regards to ISCC, additional meters to account for the solar portion of the plant have to be considered.
For the assessment of the solar contribution to the overall electrical output of the plant, as described in detail above, the metering of the high-pressure (HP) feedwater flow to the steam generator of the solar plant, as well as the conditions of the steam returning to the steam cycle of the CCGT plant have to be recorded.
A particular requirement of the data acquisition for a settlement system applied to an ISCC plant is the refined granularity of certain data to allow for detection of non-steady operation of the solar field – such as, for instance, the case of cloud passage.
In the case of such short-term discontinuity of thermal input from the solar field, the respective drop in solar share in the overall electricity output needs to be recorded.
As power generation through CSP is, by its essential nature, a transient process – as the level of irradiation continuously changes with every minute – the use of integral metering is recommended to capture the overall production for the settlement period.
All data collected by the plant accounting system have to be accessible to the user through reports that contain not only the values of the data, but also additional information about the status and the validity of the information.
As an example, Figure 2 shows the entry screen to Bahrain’s Al Dur IWPP’s accounting and settlement system, with the navigation bar at the left hand side that allows the user to quickly access various reports.
|Figure 2. Sample screenshot from the plant accounting and settlement system of the Al Dur plant Source: VDU|
The settlement of the PPA includes the consideration of all formulae of the PPA with regards to the following aspects of the plant operation: production of power and combined products such as water, if applicable; plant availability; and plant efficiency.
The invoice is calculated and the necessary supporting documentation has to be prepared. In the case of a special feed-in tariff for the solar share a separate parallel process for the settlement of the solar share of the plant output also has to be applied.
Critical quality criteria
Due to the large amount of data to be processed and the complexity of the procedures, the settlement system should provide as much automation as possible. Advanced logging and alarming features, as well as the provision of analysis tools are essential for the detection and reporting of problems or errors in the most transparent way.
Signal quality is by far the most frequent root cause of problems in the settlement applications. Therefore the system should provide detailed reports on signal problems and offer easy-to-use facilities to replace missing or faulty data. Figure 3 shows a Signal Validation Report as an example of how the system should allow tracing back errors in the invoice generation to the individual signal that causes the event.
Figure 3. Sample screenshot of the Signal Validation Report for the analysis of signal quality during the settlement period Source: VTU
In order to keep such manual interactions transparent to all contract parties, state-of-the-art authorization management and detailed automated logging features should be included.
Finally, it should be noted that since the settlement system is a business critical system for the plant, it is of high importance that the supplier is capable of providing continuous technical support for the system – to ensure its maximum availability and reliability. It is also essential that direct user support and training for the plant’s staff is provided so that the required skill sets can be maintained among the plant’s users, regardless of issue such as staff turnover or fluctuation. MEE
1. J. Petek, P. Pechtl, P. Hartner, Accounting for the Solar Contribution in ISCC Power Generation, POWER-GEN Europe, 2010, Amsterdam, The Netherlands
2. G. Brakman, F. A. Mohammad, M. Dolejsi, M. Wiemann, Construction of the ISCC Kuraymat, SolarPACES 2009, Berlin, Germany
3. R. Pawellek, T. Hirsch, T. Löw, EbsSolar – A Solar Library for EBSILON Professional, SolarPACES 2009, Berlin, Germany
About the authors
Josef Petek is product line manager for VTU Energy. This article was co-authored with Peter Hartner and Peter Pechtl. For more information visit www.vtu.com.