8. Heat pumps, heat pipes, cold thermal energy storage

Transcription

1 Kylteknik ( KYL ) Refrigeration course # v Heat pumps, heat pipes, cold thermal energy storage Ron Zevenhoven Åbo Akademi University Thermal and Flow Engineering Laboratory / Värme- och strömningsteknik tel ; ron.zevenhoven@abo.fi Åbo Akademi Univ - Thermal and Flow Engineering /24 ÅA Refrigeration / Kylteknik 8.1 Heat pumps Åbo Akademi Univ - Thermal and Flow Engineering /24

2 Heat pumps /1 Using a refrigeration cycle for heating is referred to as a heat pump (mostly based on a vapour-compression cycle) Heat pumps make use of low-temperature (waste) heat, replacing sources of (unnecessarily) high temperature heat (and electricity!) for heating and air conditioning purposes Heat pumps became popular in the 1970s, for combustion-free heating, and air conditioning Picture: 3/24 Unit: W/W U.S.A: (BTU/h)/W Heat pumps /2 Energy balance: Q High temp = W in + Q Low temp COP HP > 1, typically 3 ~ 6, for a typical building 20 ~ 40 kwh electricity gives 100 kwh heat; large units can achieve that with ~ 15 kwh power input Primary energy ratio PER η power work = EER should be > 10 HSPF should be , equals ~ 3.4 COP HP SEER should be , depends on location! COP HP PER EER HSPF SEER Q W H in Primary energy ratio power work QL 1 COPR 1 W total seasonal heating output electrical energy input total seasonal cooling output electrical energy input 1 kw = 1000 W = 3413 Btu/h (Nov. 2014) in COP Energy efficiency ratio HP cooling capacity electrical energy input Heating season performance factor Seasonal energy efficiency ratio Åbo Akademi Univ - Thermal and Flow Engineering /24

3 Heat pumps /3 With the evaporator outside the space to be heated, the options are to use 1) outside air heat, 2) outside ground heat, 3) outside water heat and 4) heat from another indoor space, or 5) waste heat from a process or device COP HP ~ T H / (T H -T L ) COP HP decreases if T H -T L increases Low T L for a given T H, COP HP may become too low (partly because of lower compressor efficiencies!) The cheapest option of using outside air heat can become limiting in winter; in those cases using ground- or watersource heat has a wider range of operation, at somewhat higher costs Picture:: Åbo Akademi Univ - Thermal and Flow Engineering /24 Heat pumps /4 Energy input Table: after D03 Typical heat pump output heat capacities range from a few kw for single family homes, via 100 s of kw for shops and offices up to 30 MW or more for industry Heat delivery temperatures range from 5-10 C for chilled water and cool air to C for hot water and steam A great benefit is that the cycle can be used as a cooling system (air conditioning) by switching a reversing valve see next slide 6/24

4 Heat pumps using v-c cycle A heat pump vapour-compression system with reversing valve for summer / cooling (a) or winter / heating operation (b) Pictures: KJ05 Åbo Akademi Univ - Thermal and Flow Engineering /24 Heat pumps heat sources Table : Input heat sources for heat pumps and their temperature range Table: D03 Top: vertical and horizontal closed loop ground heat Bottom: surface water closed loop, well water open loop Pictures: 8/24

5 Industrial heat pumps /1 Important applications of heat pumps in industrial processes are Space heating Water heating & cooling Steam production Drying and dehumidification processes Evaporation, distillation and concentrating processes; using cooling water, condensate and other liquid effluents, or condenser heat from refrigerator plants as input heat source Diluted solution Concentrated solution Compressor Picture: Ö96 Heat exchangers Condensate Concentrating solutions by evaporation using a heat pump system See also chemical heat pumps, metal hydride heat pumps, thermo-electric heat pumps, absorption heat pumps,... ( for example: D03 chapter 4) 9/24 Industrial heat pumps /2 A very important application: sea water desalination Picture: D03 A mechanical vapour recompression system (MVR) can be seen as an open cycle vapour recompression evaporator The solvent that is removed acts as the operating fluid, the heat of vaporisation is recovered while the vapour is condensed after the compression 10/24

6 Heat pumps in Finland (2013/2014) Numbers give number of units Total capacity (2013/2014): HPs extracting 4 TWh year from around buildings Brine/water = ground-source Source / picture: (accessed: ) Åbo Akademi Univ - Thermal and Flow Engineering /65 ÅA Refrigeration / Kylteknik Heat pump ÅA VST (T&FE) water air 15 l/min 15 l/min 7 l/min 7 l/min Refrigerant: R407c 23% of R32, 25% of R125, 52% of R134a ODP = 0, GWP =1610 Cold side Hot side Åbo Akademi Univ - Thermal and Flow Engineering

7 ÅA Refrigeration / Kylteknik 8.2 Heat pipes note: vacuum tubes for solar thermal energy recovery, like Double Glass Vacuum Heat Pipes (DGVHPs) are not considered here The DGVHP represents a special case of heat pipe: typically, no wick structure is used, and the inner surface where the primary loop working fluid is contained is non-porous glass. The circulation of the working fluid (usually, ethanol) is only controlled by gravity and by the surface tension effects taking place on a flat glass/ liquid interface. (Text and pics: Fiaschi and Manfrida, ECOS2012, paper 312) Åbo Akademi Univ - Thermal and Flow Engineering /24 Heat pipes /1 Picture: D03 A heat pipe is a simple device that can quickly transfer heat from one point to another. They are often referred to as the "superconductors" of heat as they possess an extra ordinary heat transfer capacity and rate with almost no heat loss. (source: SN01) Heat taken up at one end vaporises a liquid, which after moving to the other end, condenses and releases heat. As a result of gravity or capillary forces (using a porous material referred to as wick ) the liquid returns to the evaporator. 14/24

8 Heat pipes /2 Heat pipe main components: The container The working fluid The wick Pictures: D03 A cylindrical heat pipe 15/24 Heat pipes /3 Picture: D03 Suitable heat pipe fluids can cover the range from very low to very high temperatures The pressure inside the heat pipe is the saturation pressure of the fluid at the fluid s temperature; freezing temperatures are not much affected by pressure Most used are water ( C) and methanol ( C) 16/24

9 Heat pipes: Loop heat pipe (LHP) Very important: the compensation chamber, which is a two-phase reservoir that Helps to establish LHP pressure and temperature, Maintain the working fluid inventory Picture: RK06 Åbo Akademi Univ - Thermal and Flow Engineering /24 Heat pipe applications /1 Spacecraft thermal control Electronics cooling CAPL-3 on space shuttle Sources: and RK06 Åbo Akademi Univ - Thermal and Flow Engineering /24

10 Heat pipe applications /2 Computer CPU cooling Pictures: RonZ Åbo Akademi Univ - Thermal and Flow Engineering /24 Heat pipe applications /3 Prevention of permafrost thaw (Alaska) Snow melting, de-icing (Japan, also Russia, USA) using ground source heat! Pictures: RK06 Åbo Akademi Univ - Thermal and Flow Engineering /24

11 ÅA Refrigeration / Kylteknik 8.3 Cold thermal energy storage (cold TES) Åbo Akademi Univ - Thermal and Flow Engineering /24 Cold thermal energy storage /1 Night-time off-peak (cheaper) energy can be stored (batteries) for day-time peak use for air conditioning Cooling capacity can be stored as cold or frozen water, or other materials such as glycol and eutectic salt + water systems Also special phase transition materials (PCMs) were developed Note also that with lower outside (night-time) temperatures, cooling and freezing processes are more efficient! PCMs: see (Accessed Nov. 2014) Picture: Åbo Akademi Univ - Thermal and Flow Engineering /24

12 Cold thermal energy storage /2 An ice CTES system has 18x more capacity per kg than a water CTES system Example of an ice TES system atmosperic ice ball (single tank) system Pictures: DR02 23/24 Sources #8 A11: R. C. Arora Refrigeration and air conditioning, 2nd. Ed. PHI Learning Private Limited, New Delhi (2011) CB98: Y.A. Çengel, M.A. Boles Thermodynamics. An Engineering Approach, McGraw-Hill (1998) D03: İ. Dinçer Refrigeration systems and applications Wiley (2003) DR02: İ. Dinçer, M. Rosen Thermal energy storage Wiley (2002) KJ05: D. Kaminski, M. Jensen Introduction to Thermal and Fluids Engineering, Wiley (2005) RK06: D. Reay, P. Kew Heat pipes. Theory, design and applications Butterworth-Heinemann (2006) SKL06/12: Suomen Kylmäliikkeiden Liitto (2006, 2012) SN01: Shankara Narayanan K.R. What is a Heat Pipe? Ö96: G. Öhman Kylteknik, Åbo Akademi University (1996) Picture: RandomPictures/CameraDownloads/ /ADSC0006.JPG 24/24