Energy Efficiency and Recovery at Large Scale Cryogenic Plants: A Survey

Transcription

1 Energy Efficiency and Recovery at Large Scale Cryogenic Plants: A Survey J. G. Weisend II, P. Arnold, R. Garoby, W. Hees, J. Jurns, A. Lundmark, X.L. Wang November 24, 2017

2 Introduction Cryogenics is an important enabling technology for the superconducting magnets, SRF cavities and hydrogen moderators that are used in modern particle accelerators. Significant levels of refrigeration ( 1 10s of kw) at temperatures between 2 K & 20 K are required The power consumption of these large cryogenic plants is quite big. Saving energy and heat recovery are becoming a fundamental part of modern cryogenic system design

3 Scope of the Problem The ideal (best allowed by the laws of thermodynamics) Refrigeration Cycle is the Carnot Cycle The Coefficient of Performance (COP) for a Carnot Cycle is given by Q COP 1/COP represents the number of watts of work required to remove 1 watt of heat at T C Thus for a cryoplant providing cooling at 4.5 K ( )/4.5 = 66 W of work are needed to remove 1 W at 4.5 K a W net T H TC T C Unfortunately, we can not build a plant that meets the ideal Carnot Cycle Real, state of the art cryoplants operating at LHe temperatures generally only achieve 26 32% of the ideal Carnot COP Note that liquefiers generally have better COPs than refrigerators and that COPs of 2 K systems are worse due to inefficiencies. 3

4 What does this mean for ESS? AT ESS we expect the Accelerator Cryoplant (ACCP) to be capable of removing as much as 9.5 kw at 4.5 K. The FOM of the plant is expected to be 0.26 If the plant operates as expected this means we will need: (66/0.26) x 9500 = 2.4 MW of mechanical power ESS has two other cryoplants: one for the test stand & instruments and 1 for the target moderator All together we plan on using up to 5.1 MW of electrical power to run the ESS Cryoplants; Most of this power will be rejected as heat during the helium compression process 3.4 MW to high temp return (above 70 C) and about 1.1 MW to medium temp return Thus, heat recovery is important for ESS. More details later 4

5 Saving Energy The best answer is not to use the energy in the first place. A common feature of large helium cryogenic plants in scientific facilities is that they usually have widely varying capacity requirements over time. Short time frames: Beam on/off, RF power on/off Longer time frames: future upgrades Thus, these plants today are designed to operate efficiently over a wide range of loads and to match their capacities to changing loads even if this requires greater initial capita costs Use of floating point cycle maintains best COP over a range of operating points Use of VFDs on compressor power systems to allow larger turn down of plant One goal is to minimize the use of artificial heat input at low temperatures to maintain a constant plant load 5

6 Saving Energy at ESS Floating point process as part of plant controls Use of subatmospheric warm compressor in series with cold compressors allows more turn down VFDs on subatmospheric compressors, cold compressors and and low pressure compressor allows better turn down Possible turn down for the ACCP is expected to be about 60% The ACCP is designed to provide cooling under two linac configurations: nominal ( 2kW at 2 K) and contingency (14 more cryomodules 3 kw at 2 K) In order to optimize turn down and reduce energy use, the ACCP has been ordered with 2 sets of Cold Compressors and Turbine inlets, each set is optimized for a given configuration. This results in higher initial capital cost but lower overall energy use and thus lower operating costs. 6

7 Energy Recovery Most of the energy used and removed in the cryogenic cooling process is deposited as heat at the warm end of the plant. Most of this heat occurs at the warm helium compressors Smaller amounts of heat are deposited at the warm end of the turboexpanders and cold compressors This heat is transferred to dedicated water cooling loops. In older plants, the heat would ultimately be released to the environment via cooling towers or ponds At ESS, we recover this heat and use it for other purposes 7

8 Accelerator Cryoplant Schematic System uses 3 cold compressors + 1 warm subatmospheric compressor for 2 K cooling 8

9 Energy Recovery at ESS A high level goal of ESS is to recover 50% of the energy used on the site over the lifetime of the facility In the ESS cryogenics system, heat is mainly recovered from the oil and gas coolers of the Accelerator Cryoplant and Target Moderator Cryoplant. A minor part is recovered from compressor motors and turboexpanders. The recovered heat will be used to heat facility buildings and be deposited into the District Hot Water system. The maximum heat recovered from the cryogenics system is expected to be 4.5 MW out of a total power usage of 5.1 MW 9

10 Heat from screw compressors 100% electrical input power ~ 5% motor losses ~ 6% heat to air ~ 11% to helium cooler ESS high pressure stage: Helium flow: kg/s Oil flow: kg/s Electrical consumption: 1.45 MW Heat into oil cooler: 1.13 MW ~ 78% to oil cooler 10

11 Cooling water system (details to be confirmed) 27 C 37 C Middle temperature Return He from cold box Compr. motor 30 C 39 C He to fine oil removal 40 C 83 C 30 C 39 C 27 C 69 C Middle temperature Return Middle temperature Supply High temperature Return Helium compressor Oil vessel Helium cooler 74 C 49 C 71 C 30 C No elevated oil or helium temperatures out of compressor suppliers specs Dedicated cooling water circuit for cryoplant (quality constraints of available cooling water in the building) Slow temperature control on cooling water side, fast temperature control on oil side Oil cooler Cooler design state of the art e.g. for Kaeser compressors Cooling function has priority over heat recovery return 11

12 District heating in Skåne Several ongoing projects to expand the existing already large grid 12

13 Energy Recovery is Becoming More Common in Large Cryogenic Refrigeration Plants Energy Recovery requires: 1. Investment in equipment to recover the energy 2. A reasonable use for the recovered energy Local conditions are very important (LCLS II example) 13

14 Heat Recovery at Various Large Scale Cryogenic Plants Facility Energy Used Heat Recovered Uses of Recovered Heat ESS 5.1 MW 4.5 MW Building heating on ESS site plus delivery to District Heating System, etc ITER 35 MW 12 MW Heating of all ITER Buildings in the Winter DESY 40 GWh 1) Low Pressure Compressor heat recovery: 2 4 GWh 2) High Pressure Compressor Heat recovery: 6.9 GWh Building Heating on DESY site Comments Recovery from compressor oil and gas cooling & from turboexpander cooling Recovery from compressor oil Recovery from Compressor Oil FAIR 4.5 MW Building Heating in Winter Recovery from Compressor Oil systems 14

15 Energy Recovery in Other Cryogenics Facilities Air Separation Plants Very large scale facilities requiring MW of power, mainly for compression of air at the start of the process Already carefully engineered to use energy from one part of the process to generate boiling or condensation in another part Recent proposals to recover heat from compressors for use in electrical power generation 15

16 Proposal to Use a Secondary Loop to Produce Electrical Power from ASU Compressor Heat M. Anke, M. Wang Applied Thermal Engineering (2015) 16

17 Energy Recovery in Other Cryogenics Facilities LNG Regasification Natural gas frequently is transported from producer to users via ship as LNG to benefit from volume reduction At the receiving terminal, the LNG is generally converted to room temperature gas and distributed via pipelines Regasification is typically accomplished via heat transfer from seawater or air. This approach fails to take advantage of the cold of the LNG. Uses for this cold source have included: Energy reduction in air separation ( Japan) Desalination (USA) Cryogenic CO 2 capture Cold Storage (Japan, South Korea) 17

18 Use Examples of LNG Cold Resource (From B.B. Kanbur et al. Renewable and Sustainable Energy Review 2017) 18

19 Liquid Air Energy Storage from LNG Regasification LNG regasification needs are typically not continuous and the available cold can only be used locally These factors may make direct use of the cold impractical One proposal is to use the LNG cold and Air Liquefaction as part of a Liquid Air Energy Storage System Technologies and economics of this approach TBD 19

20 Summary Cryogenic systems are very energy intensive. Significant improvements in COP (i.e. efficiency) are unlikely. Reliable high performance HTS operating at K would provide a much bigger impact but is in the future (ESS Neutron Science could be critical to these advances) Best approach is to design plants to adjust to varying loads and to recover heat or reuse cold whenever possible. Both heat recovery and cold use are very dependent on local conditions. All significant helium refrigeration systems at European scientific facilities are using or planning heat recovery systems. Saving energy and heat recovery are becoming a fundamental part of modern cryogenic system design. 20