Supercritical CO2 Power Cycle Symposium May 24-25, 2011 Boulder, Colorado

Conversion Cycles with Supercritical Fluids for Nuclear Plants Hajek Petr Quality Assurance Institute ltd. Strzna 222/53 16500 Praha 6, Czech Republic tel., fax: +420 220921476 Email: [email protected]

Abstract The supercritical power cycles are taking advantage of real gas behavior in order to achieve high thermal efficiency. The two most common supercritical cycles perform with water and carbon dioxide. The supercritical water cycle enhances thermal efficiency with rising turbine inlet temperature, while the supercritical carbon dioxide (S-CO2) cycle takes advantage of reduction of compressor input power (in comparison with classic Brayton cycle) due to changing properties close to the critical point (30.98°C, 7.38MPa). The other effort was directed to analyze the real properties of conversion cycles. It is possible to divide this effort into two areas, theoretical studies and experimental verification. The theoretical studies include mainly analysis of different architectures’ of cycles, in comparison with simple Brayton cycle. Preliminary calculation shows the advantages of recompression cycle. The second important part of theoretical studies, are analysis and optimization of key components of the cycle that are heat exchangers, compressors and turbo machineries. Also specific aspects, mainly cycle stability, transient modeling and safety analysis (full power/part load performance, start up, heat up, cooling down, Loss Of Load, Loss Of Coolant Accident, Loss Of Cooling, etc.) are analyzed. The problem of flow oscillations is very important. Safety of all system can be affected with very strong pulsation, which can rise due to relatively high density of S-CO2. The low temperature cycles with special intermediate heat exchanger and with flow acceleration were partially analyzed. Two large experimental devices will be realized in Research Centre Rez. The first one, “small” loop, has 500kW electrical power input, the second “large” loop 10 MW. An “small” experimental S-CO2 loop was built in 1999 in the Czech Republic. The main objective was to obtain experimental data for comparison with previous theoretical studies. This facility was the first of its kind in the world. Its operation and performed measurements have provided many interesting data and thus brought valuable operational experience as well as new objectives for future research and development of S-CO2 cycles. The design and production of the “small” loop is finishing in these days. First operation is planned in 2011. “Large” 10MW loop will be constructed in Centre Plzen. The goal of the “Large” loop is to check the properties of compete cycles at different architectures. The scale 10MW was found as minimal realistic conversion cycle with turbo machineries, allowing the real comparison with real power plants. The operation is supposed at 2014.

1. Introduction One of the main goals in the effort of development of new nuclear reactors is to raise the thermal efficiency. The supercritical power cycles are such candidates and are taking advantage of real gas behavior in order to achieve higher thermal efficiency. There are two main types of supercritical cycles, one uses water and the other carbon dioxide. Great experience is available in the area of supercritical water cycles from classic fossil power energy, the power cycles with supercritical carbon dioxide are currently in the stage of de-velopment, calculations and testing. The main difference is given by the distant positions of the critical points of water and carbon dioxide. Generally speaking, the power cycle with CO2 shows very promising: the calculation with estimative values of components’

Supercritical CO2 Power Cycle Symposium May 24-25, 2011 Boulder, Colorado

efficiency gives high thermal efficiency, low capital costs, short period of construction, non-significant losses caused by corrosion as well as particularly small dimensions of turbo machinery. The direct use of carbon dioxide cycles in nuclear energy is questionable as there is not sufficient world wide experience with this coolant. As a first approach, it seems that the optimal use of carbon dioxide cycles would be in combination with water. Particularly for the case of HPLWR, this solution shows the following advantages: high thermal efficiency of combined cycles; lower capital costs – lower pressure steam part is replaced by CO2; optimal thermal input for CO2 – condensation part of the steam cycle; fewer problems with erosion and corrosion – the low-pressure steam part is omitted. Several research institutes in the Czech Republic (e.g. Czech Technical University in Prague, SVUSS Prague, QAI) have analyzed the possibility of using CO2 as energy conversion medium, where all have started with cycles calculation as input to this issue. A number of possibilities of cycle arrangements were analyzed; in all cases, the results depended on predefined efficiency of machineries and heat exchangers. On the other hand, only few organizations have engaged in the preparation of an experimental loop for testing power cycles with supercritical CO2. One facility of this kind was operated between 1999 and 2000, giving some very interesting results mainly in the part of recuperation. Much effort was spent to find arguments supporting these experimental results. After years, these arguments seem clearer today, thus giving significant occasions for improving the cycles thermal efficiency, especially in the connection with SCWR Projects.

2. Cycles calculation Different types of thermodynamic cycles were analyzed by the above-mentioned partners; input parameters and assumptions were as follows: thermodynamic properties of analyzed medium (predominantly from the NIST Database); efficiency of all compressors and turbines; minimum temperature drop of heat exchanger; pressure losses in the loop. The pressure losses were neglected in most cases, assuming that they are not important for supercritical media. For the combined cycles (SCWR and S-CO2), these input analyses were not made, how-ever, a possibility was examined of using S-CO2 instead of the low-pressure part of a standard PWR. The input data for this calculation were taken from the real operational data of the Czech nuclear cycles in NPP’s Dukovany and Temelin; i.e. the high-pressure part of the turbine was left unchanged and the low-pressure part was substituted by the above mentioned thermodynamic cycle with supercritical carbon dioxide. In this case, the output thermal efficiency has grown up to 39,6%. Next, different power conversion cycles solely with supercritical carbon dioxide were analyzed, while compressor inlet and turbine inlet temperatures were held constant (32°C and 550°C); the results of these calculations are plotted in Fig. 1 (cycle efficiency against turbine pressure ra-tio). Comparison of results for 6 different cycles has shown that the cycle “C” – cycle with regeneration and divided compression – exhibits the highest thermal efficiency.

Supercritical CO2 Power Cycle Symposium May 24-25, 2011 Boulder, Colorado

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Figure 1 - Thermal efficiency against turbine pressure ratio for 6 different cycles A to F with supercritical carbon dioxide.

In these cycles, the heat exchanger plays a very important role due to abrupt changes of most thermal hydraulic parameters through the critical point, e.g. density, viscosity and especially isobaric heat capacity coefficient. As seen in Fig. 2, at low temperatures the thermal capacity is higher for low pressures, while at high temperatures the thermal capacity is higher for high pressures; the point of intersection of the low-pressure and the high-pressure curves will have a major impact on the (intermediate) heat exchanger.

3. Supercritical CO2 loops The second important goal of this action is to carry out research in the field of energy conversion in the secondary circuit with supercritical carbon dioxide. The original concept of a secondary circuit cooled by water has been abandoned because of an extremely fast and exothermic reaction between water and sodium, which occurs whenever the heat exchanger fails abruptly or during leaks between the primary and secondary circuit. Furthermore, supercritical carbon dioxide has better physical-chemical properties than water and this advantage leads to higher efficiency, simplification, and better economic efficiency of the secondary circuit. Even though this thermal cycle has been researched on several occasions in the past, this concept was never realized because there is no suitable technology available in the area of compact heat exchangers and gas turbines. Nowadays, research and development in this area is again being carried out; therefore, the need for experimental facilities is growing again for energy conversion using circuits with supercritical carbon dioxide. With regard to these facts, the goal of this research activity is to develop two technological circuits with supercritical carbon dioxide. The smaller one will be used particularly for studying thermodynamic phenomena, and the larger one in the city of Plzeň will be used for testing the key components.

3.1 Experimental small scale CO2 loop In the first step, a smaller loop with supercritical carbon dioxide of 0.5MW thermal power will be installed in Research Centre Řež Ltd. This highly variable loop will be used primarily to verify the behavior of individual components and their potential testing, including experiments verifying the heat transfer coefficients in the supercritical medium. Fig. 13 shows the scheme of the loop. This loop will use certain components from a previous experimental facility with a similar purpose, which had been implemented at SVÚSS in the city of Praha-Běchovice. New components allowing operation under higher temperatures will have to be designed and manufactured

Supercritical CO2 Power Cycle Symposium May 24-25, 2011 Boulder, Colorado

Figure 2 - Scheme of the small-scale CO2 loop, view

3.2 Experimental large CO2 loop In the second step, it is planned to build a facility in Plzeň. This facility shall be used to test the entire conversion cycle on such a scale (input power of approximately 10MW) so that similar types of technologies can be used for large energy equipment. The circulation will include turbomachinery, turbine and compressors. Fig. 14 shows the planned scheme. This is an entirely new and unique type of experimental facility. Some components will be the first of their kind in the world. The variable experimental loop at Research Centre Řež Ltd. will be used for preliminary testing.

Legend: HTR – high-temperature recuperator K2 – re-compressor LTR – low-temperature recuperator T1 – turbine (main compressor drive) PCL – cooler T2 – turbine (re-compressor drive) T3 – output turbine M – electrical motor G – generator KOM1 – main compressor expander (K1+T1+M) K1 – main compressor KOM2 – re-compressor expander (K2+T2+M)

Figure 3 - Energy conversion loop 10 MW, compressor model

Supercritical CO2 Power Cycle Symposium May 24-25, 2011 Boulder, Colorado

3.3 Experimental goals The overview of goals for both technological circuits is as follows: – verifying the complete secondary circuit using turbo machinery at a minimum output power (according to studies performed so far, the minimum suitable output power would be between 5MW and 10MW), – verifying all turbo machinery and heat exchangers, – verifying the transient states of the entire system under various configurations, – verifying the characteristics of the loop with supercritical carbon dioxide near the critical point, – safety and accident analyses including some experimental verification, – operation with modified critical point by employing a suitable gas mixture, – model operating tests of the fuel clusters, – heat transfer tests, – materials testing, – testing all or parts of the thermodynamic conversion cycles (armatures, purification, etc.). – heat transfer in accelerated flow. – operation avoiding the pinch-point by accelerating the flow 3.4 Specific actions in CO2 energy conversion As mentioned above, specific activities are foreseen, where essential progress is supposed in the area of energy conversion cycles. One of the open issues is the heat transfer into accelerated flow. The effect of flow acceleration has been well described, but not exploited into heat exchange procedures. Change of the speed of flow can be achieved through changes in profile (nozzle) or through changes in density – at constant pressure achieved through changes in temperature. The equation describing this effect is: ∆h = ∆w2/2, where h is enthalpy and w flow rate. Straightforward interpretation of this equation is that a part of thermal energy is changed into kinetic energy. This process is of course connected with pressure drop – the pressure decreases. This can have two consequences: A. Temperature drop If a nozzle is installed at a certain place in the intermediate heat exchanger, the speed increases – if the speed is increased on the high-pressure side, it will cause local decrease of temperature and the heat transfer will be enhanced. B. Pressure drop The second consequence is caused by changes in shape of isobaric heat capacity (induced by the pressure drop) vs. temperature. Medium with lowspeed

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Figure 4 - Description of flow acceleration effect, the heat transfer can be reversed

This effect can be used especially in regeneration in the Brayton cycle, and also other applications can be developed.

Supercritical CO2 Power Cycle Symposium May 24-25, 2011 Boulder, Colorado

4. Summary & Conclusions A large research program on new experimental facilities for Generation IV reactors development has been recently proposed and submitted to the European Commission for evaluation. The planned activities are linked to the already existing experimental facilities and their experimental program. Some of the planned facilities have already been planned into sufficient detail. All the facilities are focused mainly on materials testing under irradiation, thermodynamic and thermal hydraulic tests and development of critical components for the innovative coolants, such as helium or supercritical carbon dioxide.

References 1.

Kulhanek, M.: SUPERCRITICAL CARBON DIOXIDE CYCLES THERMODYNAMIC ANALYSIS AND COMPARISON , Diploma Thesis CVUT 2009, Prague.