Petter Nekså

CO2-heat pump water heater : characteristics, system design and experimental results

Abstract CO 2 is one of the few non-toxic and non-flammable working fluids that do not contribute to ozone depletion or global warming, if leaked to the atmosphere. Tap water heating is one promising application for a trans-critical CO 2 process. The temperature glide at heat rejection contributes to a very good temperature adaptation when heating tap water, which inherits a large temperature glide. This, together with efficient compression and good heat transfer characteristics of CO 2 , makes it possible to design very efficient systems. A heating-COP of 4.3 is achieved for the prototype when heating tap water from 9°C to 60°C, at an evaporation temperature of 0°C. The results lead to a seasonal performance factor of about 4 for an Oslo climate, using ambient air as heat source. Thus, the primary energy consumption can be reduced with more than 75% compared with electrical or gas fired systems. Another significant advantage of this system, compared with conventional heat pump water heaters, is that hot water with temperatures up to 90°C can be produced without operational difficulties.

CO2 heat pump systems

After the CFCs and the HCFCs were deemed unfit as working fluids in refrigeration, air conditioning, and heat pump applications, there has been a renaissance for carbon dioxide technology. Heat pumps is one of the application areas where theoretical and experimental investigations are now performed by an increasing number of research institutions and manufacturers. This paper gives an overview of some of the current activities in the CO2 heat pump field. Discussed are the important characteristics of the transcritical CO2 process applied to heat pumps, and also discussed are theoretical and experimental results from several heat pump applications. Provided that calculations and system designs are performed on the premises of the working fluid, and that test plants are constructed and operated to fully exploit the specific characteristics of both the fluid and the transcritical process, the results show that CO2 is an attractive alternative to the synthetic fluids. Competitive products may be launched in the near future.

Micro technology in heat pumping systems

Abstract Micro heat pumps, with dimensions in the order of centimetres, may in the future be utilised for the heating and/or cooling of buildings, vehicles, clothing, and other products or applications. A number of issues have yet to be solved, including the construction of a microscale compressor, and determination of micro heat exchanger heat transfer capacities. Test samples of micro heat exchangers and a corresponding test apparatus have been built. Some two-phase experiments with propane (R-290) as refrigerant have been conducted. Preliminary results for a micro condenser with 0.5 mm wide trapezoidal channels of 25 mm length showed that a heat flux of up to 135 kW/m 2 , based on the refrigerant-side area, was attainable. The corresponding overall heat transfer coefficient was 10 kW/(m 2 K), with a refrigerant mass flux of 165 kg/(m 2 s) and a refrigerant-side pressure drop of 180 kPa/m.

Energy-Efficiency Technologies for Reduction of Offshore CO2 Emissions

Summary This paper will discuss novel technologies for increasing the energy efficiency of offshore oil and gas platforms. Three case studies are in progress that are based on actual oil-producing platforms—two on the Norwegian Continental Shelf (NCS) and one in the Brazilian basin. The current focus is on developing compact, novel bottoming cycles for recovery of waste heat from the gas turbine and heat recovery from the compressor train for gas export. The technologies under investigation use steam and alternative working fluids, such as carbon dioxide (CO2) and hydrocarbons. All the fluids investigated in this project are natural working fluids; hence, they will not cause any unexpected environmental issues in the future. A case study was performed that considered an 18-year period of operation on an actual platform and a scenario in which one gas turbine was removed and replaced with a CO2 bottoming cycle by use of the exhaust heat from a different gas turbine. The beauty of this scenario is that it would not increase the weight on the platform because the crate containing the gas turbine to be removed was of a weight similar to that of the crate containing the CO2 bottoming cycle. The substitution would not affect the ability to cover the heat demand on the platform because a waste-heat-recovery unit (WHRU) could be installed on the platform’s other gas turbine. The case study indicates a significant reduction in CO2 emissions of 22% (63 000 t/a), and does not involve adding additional weight or volume to the platform. If operating on the NCS, the annual savings in reduced fuel costs and CO2 tax from implementing this scenario would be USD 17 million, although much lower in other territories.

Flexible Combined Heat and Power Systems for Offshore Oil and Gas Facilities With CO2 Bottoming Cycles

In this work different concepts are investigated for combined heat and power production (CHP) from offshore gas turbines. Implementation of such technology could improve energy efficiency of offshore oil and gas production and lead to reduced fuel consumption and resulting CO2 emissions. Offshore electric power is in most cases generated by gas turbines operating in a simple cycle. However it would be desireable to increase energy efficiency by adding steam or CO2 bottoming cycles to produce power from the exhaust heat. However part of the heat from the gas turbine exhaust is normally used for onboard process heat for the oil/water separation process among others, this must be taken into consideration when estimating capacity for additional power production. Different CHP concepts will be evaluated at different operating conditions while running the turbines in both design and off-design mode The results show that it is possible to produce an additional 6–8 MW of electrical power from a 32 MW turbine (depending on the conditions) while using 15 MW of heat from the exhaust for on-board processing.Copyright

Design and Optimization of Waste Heat Recovery Unit Using Carbon Dioxide as Cooling Fluid

This paper describes design and optimization of a Waste Heat Recovery Unit (WHRU) for a power cycle which uses CO2 as a working fluid. This system is designed for offshore installation to increase gas turbine efficiency by recovering waste heat from the exhaust for production of additional power. Due to severe constraints on weight and space in an offshore setting, it is essential to reduce size and weight of the equipment to a minimum. Process simulations are performed to optimize the geometry of the WHRU using different objective functions and thermal-hydraulic models. The underlying heat exchanger model used in the simulations is an in-house model that includes the calculation of weight and volume for frame and structure for the casing in addition to the thermal-hydraulic performance of the heat exchanger core. The results show that the for a set of given process constraints, optimization with respect to minimum total weight or minimum core weight shown similar results for the total installed weight, although the design of heat exchanger differs. The applied method also shows how the WHRU geometry can be optimized for different material combinations.Copyright

Enabling Efficient Heat Recovery from Aluminium Pot Gas

In the present work, previous studies carried out by the Norwegian aluminium industry and research centres with the aim of recovering heat from aluminium production off-gas, are reviewed. The main challenge in improving heat recovery is the fouling phenomena, which is due to the presence of particulate matter and corrosive gases in the off-gas. Fouling can occur due to particle deposition, condensation of corrosive acids and scaling reactions, which in turn can build up hard layers, particularly, on heat exchanger surfaces. The review focuses primarily on fundamental studies (theoretical and experimental), which address off-gas composition characterization, particle size distribution and particle deposition phenomena in laboratory and industrial environments. Moreover, it presents commercial concepts already implemented in industry applications. Upcoming activities in regards to the scaling phenomena, which include the design of a cold-finger for laboratory and industrial measurements, as well as mathematical modelling using CFD, are also discussed.