Abstract
The development of efficient long-term heat storage systems could significantly increase the use of solar thermal energy for building heating. Among the different heat storage technologies, the absorption heat storage system seems promising for this application. To analyze the potential of this technology, a numerical model based on mass, species, energy, and exergy balances has been developed. The evolution over time of the storage imposes a transient approach. Simulations were performed considering temperature conditions close to those of a storage system used for space heating coupled to solar thermal collectors (as the heat source), with ground source heat exchangers (as the cold source). The transient behavior of the system was analyzed in both the charging and discharging phases. This analysis highlights the lowering of energetic and exergetic performance during both phases, and these phenomena are discussed. The thermal efficiency and the energy storage density of the system were determined, equal to 48.4 % and 263 MJ/m3, respectively. The exergetic efficiency is equal to 15.0 %, and the exergy destruction rate is 85.8 %. The key elements in terms of exergy destruction are the solution storage tank, the generator, and the absorber. The impact of using a solution heat exchanger (SHX) was studied. The risk of the solution crystallizing in the SHX was taken into account. With a SHX, the thermal efficiency of the system can reach 75 %, its storage density was 331 MJ/m3, and its exergetic efficiency and exergy destruction rate was 23.2 and 77.3 %, respectively.
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Abbreviations
- Ex :
-
exergy (J)
- Ėx :
-
exergy flux (W)
- ex :
-
specific exergy (J/kg)
- h :
-
specific enthalpy (J/kg)
- m :
-
mass (kg)
- ṁ :
-
mass flow rate (kg/s)
- P :
-
pressure (Pa)
- Q :
-
heat (J)
- \( \dot{Q} \) :
-
thermal power (W)
- s:
-
specific entropy (J/(kg.K))
- t :
-
time (s)
- T :
-
temperature (K)
- U :
-
internal energy (J)
- u :
-
specific internal energy (J/kg)
- V :
-
volume (m3)
- v :
-
specific volume (m3/kg)
- W :
-
mechanical work (J)
- Ẇ :
-
mechanical power (W)
- x :
-
mass fraction of lithium bromide (x = m LiBr /m sol ) (kgLiBr/kgsol)
- ε :
-
heat exchanger effectiveness
- μ:
-
chemical potential (J/kg)
- η:
-
efficiency (−)
- ρ :
-
volumetric energy storage density (J/m3)
- τ:
-
exergy destruction ratio (−)
- a :
-
absorber
- c:
-
condenser
- d :
-
destruction
- e:
-
evaporator
- ex:
-
exergetic
- ext :
-
external
- g :
-
generator
- gr :
-
ground
- h :
-
high
- hs :
-
heat source
- hx :
-
heat exchanger
- i :
-
inlet
- int :
-
internal
- ints :
-
intermediate Heat Source
- l :
-
low
- LiBr :
-
lithium bromide
- max :
-
maximum
- min :
-
minimum
- nf :
-
non-flow
- o :
-
outlet
- p :
-
pump
- pinch :
-
temperature pinch
- ref :
-
reference state
- s/i :
-
interface between a heat source and a system component
- sol :
-
water–lithium bromide solution
- syst:
-
system
- tank :
-
solution or water tank
- th :
-
thermal
- v :
-
vapor
- w :
-
water
- ‘:
-
during the discharging phase
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Acknowledgments
We thank the ANR (French National Research Agency) for its financial support under the research projects PROSSIS2 ANR-11-SEED-0011-01.
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Appendix A: Detailed mass, species, power, and exergy balances for the components of the system
Appendix A: Detailed mass, species, power, and exergy balances for the components of the system
A.1 Charging phase
Based on the hypothesis described in section “Main equations and hypothesis of the model”, the mass, species, energy, and exergy balances for the system components can be expressed as following for the charging phase.
Generator:
Condenser:
Solution tank:
Water tank:
Solution pump:
A.2 Discharging phase
Based on the same hypothesis described in section “Main equations and hypothesis of the model”, the balances for the system components can be expressed as following for the discharging phase.
Absorber:
Evaporator:
Solution tank:
Water tank:
Solution pump:
A.3 Values over the whole charging or discharging phase
Heat (Q) and the associated exergy (Ex th), work (W), and exergy destruction (Ex d) are obtained for the whole phases by the integration, over time, of the respective exchange rates.
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Perier-Muzet, M., Le Pierres, N. Modeling and analysis of energetic and exergetic efficiencies of a LiBr/H20 absorption heat storage system for solar space heating in buildings. Energy Efficiency 9, 281–299 (2016). https://doi.org/10.1007/s12053-015-9362-2
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DOI: https://doi.org/10.1007/s12053-015-9362-2