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There is a substantial market for nuclear energy in non-electric applications such as hydrogen production or water desalination. Among the Generation IV reactor concepts, the very high temperature reactor (VHTR) with a reactor outlet temperature close to 1000 oC and a power conversion efficiency of approximately 50% is believed to be the most suitable concept for cogeneration of process heat. Its high coolant exergy would enable centralize d hydrogen production and other process heat applications. In this paper it is shown that a reactor with lower coolant outlet temperature or another nearterm heat source can also meet the VHTR objectives which are high power conversion efficiency and capability to deliver high temperature process heat in the narrow temperature window required by thermochemical hydrogen production cycles.
The approach was to separate the requirement for high temperature process heat production from the nuclear part of the plant, in other words the nuclear part of the power plant would run at acceptably low te mperature while the high te mperature heat production via a heat pump system would be limited to a conventional external circuit, thus avoiding nuclear constraints. The separation of these high temperature constraints from the reactor would avoid massive R&D requirements on materials, components and fuel with uncertain outcome thus unnecessarily delaying introduction of this other wise very attractive reactor concept. We then show that the proposed technology is equally suitable for the generation of cold (e.g. for air conditioning) and for desalination of seawater.
Introduction
The sustainability of nuclear power increases linearly with power conversion efficiency and can be further raised by cogeneration of electricity and heat for various process heat applications. As an example, the Generation IV very high temperature reactor (VHTR) concept aims at approximately 50% power conversion efficiency and, according to the prevailing opinion, a temperature close to 1000 ◦C would be required to enable centralized hydrogen production. These high temperatures put severe constraints on materials, components and fuel and implymassive R&D requirements with uncertain outcome thus unnecessarily delaying introduction of this otherwise very attractive reactor concept.
The first objective of this paper is to develop an alternative option to both reach the VHTR power conversion efficiency target and the capability to deliver high temperature process heat with a reactor or other heat source with lower temperature output that would be feasible in the near-term. The approach was to separate the requirementfor high temperatureprocess heat production from the nuclear part of the plant, in other words the nuclear part of the power plant would run at acceptably low temperature while the high temperature heat production would be limited to a conventional external circuit, thus avoiding nuclear constraints. Two such methods would be feasible: electric superheating to the desired temperature level or the use of heat pump (HP) technology via compression to the desired temperature level. While the heat thus generated is then delivered to a heat exchanger, some of the compression work can be recovered in a turbine, which makes this option energetically more efficient than electric heating.
The second objective of this paper is to show how the same heat pump technology can be used for combined desalination and district cooling which corresponds to market needs in arid regions. While classical gas-cooled reactors are proven technology, the GIF VHTR still requires a strong R&D effort. Based on already available technology, the power conversion options proposed here would introduce a near-term solution to provide nuclear produced high temperature process heat.
Reverse Brayton cycle
The principal objective of a heat pump (Angelino and Invernizzi, 1994) is to remove heat from a low temperature environment and to release it to a high temperature environment. This transformation requires mechanical work, which is exactly the opposite of the working process. To achieve that goal, a fluid is used as energy carrier: it undergoes transformations to be colder than the lowtemperature environment (to remove heat there) and then hotter than the high temperature environment (to release heat there).
If the energy gained fromthe cold source is considered as a loss that would otherwise be simply rejected, energy input into a heat pump is the compression work only. Both heat andcompression
work increase the energy in the fluid: The efficiency of this system is then higher than unity. In this study, the cold energy source cannot be considered as a loss such that efficiency is <100%,
This method can be applied in a wide temperature range and reduced losses compared to a reverse Rankine heat pump or even electric heating. Although the compression work is much higher than for a liquid, a large part of it (around 60%) is earned back in the turbine. This enhances efficiency. Shortcomings are:
• Poor heat transfer to and from the gaseous fluid implying high mass flows. As the sensible heats are smaller (per unit masson a unit mass), latent heats, mass and volume flow are larger.
• The gas state, which implies non-isothermal heat exchanges.
In a reverse Brayton cycle, both mechanical power and lowtemperature power are transformed into high temperature power. As the goal is to maximize the contribution of the low temperature power compared to the valuable mechanical work, a reverse Brayton cycle with single compression and no recuperation (RB1CNR) is needed for the heat pump system. This implies production of process heat over a large temperature span. A reverse Brayton cycle with multiple recompression more would lead to an efficiency decrease but would provide process heat in a narrow temperature window
Conclusions
This paper discussed the use of the reverse Brayton cycle as a heat pump to reach VHTR process heat and efficiency objectives. This opens opportunities to design innovative, flexible and efficient power conversion cycles that can be adapted to virtually all GIF reactor concepts as well as to existing or near-term reactor designs such as the advanced gas-cooled reactor (AGR), HTR or other heat sources. While so far only HTR technology was investigated as the primary heat source, it is planned to pursue efforts towards other concepts and to enable feasible configurations with significantly less stringent technology requirements, in particular for the VHTR.
Based on this enabling technology and the expectation that switching to a hydrogen ormethanol economywould requiremuch more high temperature process heat than produced by current VHTRdesigns, innovative layoutswere developed enabling delivery of high temperature process heat in the required narrow temperature window.
This study showed the feasibility of nearer term hydrogen production and other high temperature process heat applications while avoiding costly and time consuming R&D of VHTR. The induced penalty for replacing VHTR by AGR is approximately 17% points of overall efficiency with the resulting efficiency still exceeding 50%.
Finally, a short feasibility study for combined cooling and desalination based on the technology developed in the previous objectives was performed. For these combined applications, a coefficient of performance significantly larger than unity was calculated. This figure can be further raised but complicates the cycle layout. In all three objectives, an economical study is required to assess whether these technical achievements make economically sense
References
Angelino, G., Invernizzi, C., 1994. Supercritical heat pump cycles. Int. J. Refrigeration 17 (8), 543–554, 1994.
Dostal, V., Hejzlar, P., Driscoll, M.J., 2006. High performance supercritical carbondioxide cycle for next-generation nuclear reactors. Nucl. Technol. 154 (265), 2006.
GIF, 2002, A technology roadmap for the generation IV nuclear energy systems, GIF-002-00, December 2002.
IAEA-TECDOC-1444, 2005, Optimization of the coupling of nuclear reactors and desalination systems, Final report of a coordinated research project, 1999–2003, June 2005.
Jeong, Y.H., Kazimi, M.S., Hohnholt, K.J., Yildiz, B. 2005, Optimization of the hybrid sulfur cycle for hydrogen production, Nuclear Energy and Sustainability Program, Massachusetts Institute of Technology Technical Report MIT-NES-TR-004, May 2005.
Li, X., Le Pierres, R., Dewson, S.D., 2006. Heat exchangers for the next generation of nuclear reactors. In: Proc. ICAPP’06, Reno, NV, USA, June 4–8, 2006.
Matzner, D., Kriel, W., Correia, M., Greyvenstein, R., 2006. Cycle configuration for a PBMR steam and electricity production plant. In: Proc. ICAPP 2006, Reno, NV, USA, June 4–8, 2006.
Shropshire, D.E., 2004. Lessons learned from Gen I carbon-dioxide cooled reactors. In: Proc. ICONE-12, Arlington, VA, USA, April 25–29, 2004.
Vitart, X., Le Duigou, A., Carles, P., 2006. Hydrogen production using the sulfur-iodine cycle coupled to a VHTR: an overview. Energy Conversion Manage. 47 (October (17)), 2740–2747.
Woudstra, N, 2006, Cycle-Tempo Release 5.0, Delft University of Technology, http://www.cycle-tempo.nl/.
Yildiz, B., Kazimi, M.S., 2006. Efficiency of hydrogen production systems using alternative nuclear energy technologies. Int. J. Hydrogen Energy 31 (January (1)).
Yildiz, B., Hohnholt, K.J., Kazimi, M.S., July, 2005. Hydrogen production using high-temperature steam electrolysis supported by advanced gas reactor with supercritical CO2 cycles. Nucl. Technol. 155.
By : A. Marmiera, M.A. Futterer
Source : LINK
Introduction
The sustainability of nuclear power increases linearly with power conversion efficiency and can be further raised by cogeneration of electricity and heat for various process heat applications. As an example, the Generation IV very high temperature reactor (VHTR) concept aims at approximately 50% power conversion efficiency and, according to the prevailing opinion, a temperature close to 1000 ◦C would be required to enable centralized hydrogen production. These high temperatures put severe constraints on materials, components and fuel and implymassive R&D requirements with uncertain outcome thus unnecessarily delaying introduction of this otherwise very attractive reactor concept.
The first objective of this paper is to develop an alternative option to both reach the VHTR power conversion efficiency target and the capability to deliver high temperature process heat with a reactor or other heat source with lower temperature output that would be feasible in the near-term. The approach was to separate the requirementfor high temperatureprocess heat production from the nuclear part of the plant, in other words the nuclear part of the power plant would run at acceptably low temperature while the high temperature heat production would be limited to a conventional external circuit, thus avoiding nuclear constraints. Two such methods would be feasible: electric superheating to the desired temperature level or the use of heat pump (HP) technology via compression to the desired temperature level. While the heat thus generated is then delivered to a heat exchanger, some of the compression work can be recovered in a turbine, which makes this option energetically more efficient than electric heating.
The second objective of this paper is to show how the same heat pump technology can be used for combined desalination and district cooling which corresponds to market needs in arid regions. While classical gas-cooled reactors are proven technology, the GIF VHTR still requires a strong R&D effort. Based on already available technology, the power conversion options proposed here would introduce a near-term solution to provide nuclear produced high temperature process heat.
Reverse Brayton cycle
The principal objective of a heat pump (Angelino and Invernizzi, 1994) is to remove heat from a low temperature environment and to release it to a high temperature environment. This transformation requires mechanical work, which is exactly the opposite of the working process. To achieve that goal, a fluid is used as energy carrier: it undergoes transformations to be colder than the lowtemperature environment (to remove heat there) and then hotter than the high temperature environment (to release heat there).
If the energy gained fromthe cold source is considered as a loss that would otherwise be simply rejected, energy input into a heat pump is the compression work only. Both heat andcompression
work increase the energy in the fluid: The efficiency of this system is then higher than unity. In this study, the cold energy source cannot be considered as a loss such that efficiency is <100%,
This method can be applied in a wide temperature range and reduced losses compared to a reverse Rankine heat pump or even electric heating. Although the compression work is much higher than for a liquid, a large part of it (around 60%) is earned back in the turbine. This enhances efficiency. Shortcomings are:
• Poor heat transfer to and from the gaseous fluid implying high mass flows. As the sensible heats are smaller (per unit masson a unit mass), latent heats, mass and volume flow are larger.
• The gas state, which implies non-isothermal heat exchanges.
In a reverse Brayton cycle, both mechanical power and lowtemperature power are transformed into high temperature power. As the goal is to maximize the contribution of the low temperature power compared to the valuable mechanical work, a reverse Brayton cycle with single compression and no recuperation (RB1CNR) is needed for the heat pump system. This implies production of process heat over a large temperature span. A reverse Brayton cycle with multiple recompression more would lead to an efficiency decrease but would provide process heat in a narrow temperature window
Conclusions
This paper discussed the use of the reverse Brayton cycle as a heat pump to reach VHTR process heat and efficiency objectives. This opens opportunities to design innovative, flexible and efficient power conversion cycles that can be adapted to virtually all GIF reactor concepts as well as to existing or near-term reactor designs such as the advanced gas-cooled reactor (AGR), HTR or other heat sources. While so far only HTR technology was investigated as the primary heat source, it is planned to pursue efforts towards other concepts and to enable feasible configurations with significantly less stringent technology requirements, in particular for the VHTR.
Based on this enabling technology and the expectation that switching to a hydrogen ormethanol economywould requiremuch more high temperature process heat than produced by current VHTRdesigns, innovative layoutswere developed enabling delivery of high temperature process heat in the required narrow temperature window.
This study showed the feasibility of nearer term hydrogen production and other high temperature process heat applications while avoiding costly and time consuming R&D of VHTR. The induced penalty for replacing VHTR by AGR is approximately 17% points of overall efficiency with the resulting efficiency still exceeding 50%.
Finally, a short feasibility study for combined cooling and desalination based on the technology developed in the previous objectives was performed. For these combined applications, a coefficient of performance significantly larger than unity was calculated. This figure can be further raised but complicates the cycle layout. In all three objectives, an economical study is required to assess whether these technical achievements make economically sense
References
Angelino, G., Invernizzi, C., 1994. Supercritical heat pump cycles. Int. J. Refrigeration 17 (8), 543–554, 1994.
Dostal, V., Hejzlar, P., Driscoll, M.J., 2006. High performance supercritical carbondioxide cycle for next-generation nuclear reactors. Nucl. Technol. 154 (265), 2006.
GIF, 2002, A technology roadmap for the generation IV nuclear energy systems, GIF-002-00, December 2002.
IAEA-TECDOC-1444, 2005, Optimization of the coupling of nuclear reactors and desalination systems, Final report of a coordinated research project, 1999–2003, June 2005.
Jeong, Y.H., Kazimi, M.S., Hohnholt, K.J., Yildiz, B. 2005, Optimization of the hybrid sulfur cycle for hydrogen production, Nuclear Energy and Sustainability Program, Massachusetts Institute of Technology Technical Report MIT-NES-TR-004, May 2005.
Li, X., Le Pierres, R., Dewson, S.D., 2006. Heat exchangers for the next generation of nuclear reactors. In: Proc. ICAPP’06, Reno, NV, USA, June 4–8, 2006.
Matzner, D., Kriel, W., Correia, M., Greyvenstein, R., 2006. Cycle configuration for a PBMR steam and electricity production plant. In: Proc. ICAPP 2006, Reno, NV, USA, June 4–8, 2006.
Shropshire, D.E., 2004. Lessons learned from Gen I carbon-dioxide cooled reactors. In: Proc. ICONE-12, Arlington, VA, USA, April 25–29, 2004.
Vitart, X., Le Duigou, A., Carles, P., 2006. Hydrogen production using the sulfur-iodine cycle coupled to a VHTR: an overview. Energy Conversion Manage. 47 (October (17)), 2740–2747.
Woudstra, N, 2006, Cycle-Tempo Release 5.0, Delft University of Technology, http://www.cycle-tempo.nl/.
Yildiz, B., Kazimi, M.S., 2006. Efficiency of hydrogen production systems using alternative nuclear energy technologies. Int. J. Hydrogen Energy 31 (January (1)).
Yildiz, B., Hohnholt, K.J., Kazimi, M.S., July, 2005. Hydrogen production using high-temperature steam electrolysis supported by advanced gas reactor with supercritical CO2 cycles. Nucl. Technol. 155.
By : A. Marmiera, M.A. Futterer
Source : LINK
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