Modeling and analysis of energetic and exergetic efficiencies of a LiBr/H20 absorption heat storage system for solar space heating in buildings

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/ m 3 , 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/m 3 , and its exergetic efficiency and exergy destruction rate was 23.2 and 77.3 %, respectively. Nomenclature General 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) 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 (m 3 ) v specific volume (m 3 /kg)Energy Efficiency W mechanical work (J) Ẇ mechanical power (W) x mass fraction of lithium bromide (x=m LiBr /m sol ) (kg LiBr /kg sol )Greek letters ε heat exchanger effectiveness μ chemical potential (J/kg) η efficiency (−) ρ volumetric energy storage density (J/m 3 ) τ exergy destruction ratio (−)

Identification of best available thermal energy storage compounds for low-to-moderate temperature storage applications in buildings

“…Low discharge capacities as a consequence of a slow kinetics, variable thermal power over time, high cost and long thermal stability under cycling are common issues to be overcome. Among all tested materials for small-scale applications in buildings, Zeolite 13X (solid physical adsorption), have the highest potential for building applications according to the results obtained by different authors (112,125,129,130,145,146,156). Considering all reported properties, MgCl 2 •6H 2 O is the most promising material for energy storage due to its high energy density (~2GJ/m 3 ), relatively low charging temperature (≥130 °C), a discharging temperature sufficiently high to produce water heating (≤60 °C) and low price (<1€/kg).…”

“…A stored energy density of 368 MJ/m 3 was achieved. Perier-Muzet and Le Pierres (146) assessed the efficiency of a LiBr/H 2 O absorption heat storage system integrated with a solar thermal facility for space heating. They analysed the impact of using a solution heat exchanger (SHX), which could improve both the energy efficiency and storage density up to 75% and 331 MJ/m 3 , respectively.…”

Energetic/exergetic study of Kalina and refrigeration bottoming cycles on different solar‐driven organic Rankine cycles

“…where m ˙and c P are the mass flow rate and constantpressure specific heat of HTF in the receiver, respectively. ─ in the KCS) at steady-state condition are expressed as follows 54 :…”

“…Mass, energy and exergy balances are applied to various components of trigeneration systems to evaluate plant performance. The mass balance equations for single‐component streams and two‐component mixtures ($$\mathrm{LiBr}\unicode{x02500}{{\rm{H}}}_{2}{\rm{O}}$$ in the DEARC and the $${\mathrm{NH}}_{3}\unicode{x02500}{{\rm{H}}}_{2}{\rm{O}}$$ in the KCS) at steady‐state condition are expressed as follows 54 : $$$\sum {\dot{m}}_{{in}}-\sum {\dot{m}}_{{out}}=0,$$$ $$$\sum {\dot{m}}_{{in}}{x}_{{in}}-\sum {\dot{m}}_{{out}}{x}_{{out}}=0,$$$where $$\dot{m}$$ denotes the mass flow rate and $$x$$ represents the concentration of lithium bromide or ammonia in the solution. Considering the steady‐state condition and neglecting the changes in kinetic and potential energies, Table 6 presents energy and exergy balance equations for various components of the proposed trigeneration systems.…”