Review on sorption materials and technologies for heat pumps and thermal energy storage

TL;DR: Sorption is used for absorption/adsorption heat pumps (sorption refrigeration) and sorption for thermal energy storage (TES) as mentioned in this paper, where the operation principle of the technology and the materials used or in research are listed and compared.

About: This article is published in Renewable Energy.The article was published on 2017-09-01 and is currently open access. It has received 167 citations till now. The article focuses on the topics: Renewable heat & Thermal energy storage.

Summary

Introduction

• 1    Sorption is used for absorption/adsorption heat pumps (sorption refrigeration) and sorption for 11  thermal energy storage (TES).
• Absorption heat pumping and refrigeration research is today more 18  focussed in the decrease of unit costs and increase of energy efficiency, adsorption is focussed 19  in finding more efficient working pairs, and storage is testing the first prototypes and designing 20  new ones with different or enhanced storage materials and new reactor concepts to optimize 21  energy output.
• N absorption a orking pair ethanol ater /water /water romide/wat or several on the spe which exch id physisorp he basic ad 6]. tion (adsorpt n (absorption) ison between a nd adsorption s er reactors, i.e cific cycle a ange refrige tion is base sorption cyc ion) refrigerat refrigeration bsorption and systems [5,21 Coefficient 0.1 0.2 0.2 0 .
• The c oling deliver , QEV denot the heat sou on the the -effect, half e performan simplest typ two-stage o OP of this e-effect abso absorber a ciple of soli king pressur on between operating ure, chemic librium dro orption refri rption cycle rejected to same heat d loop, pas assumed tha GE is suppli or (sorption ssure.

2.2.1. Absorption systems

• 34    Table 9 shows the newly presented adsorbent-refrigerant pair, developed to achieve optimized adsorption working systems.
• These advanced sorption refrigeration cycles can significantly improve the thermodynamic efficiency and thus enhance the working performance.

3.1. Technologies

• Most of the technologies implementing sorption materials with storage are purposed for building comfort applications.
• Mainly air conditioning, heating, and DHW are the fulfilled requirements.
• Until now, these technologies have been designed tested as single equipment for a dwelling or as a buffer in district heating network.

3.1.1. Adsorption

• Adsorption thermal energy storage is a promising technology that can provide an excellent solution for long-term thermal energy storage in a more compact and efficient way.
• Usua the reactor [ needs to be stem, being , the open nser is not rage system ed, and the st be carefu good vapour ng trend an ation, which n be stored heat transfe ion storage mical heat p ds therefore storage sys ion, the deso (left) vs. closed has no ma working flu evaporation lly closed s 330].

3.2.3. Chemisorption

• A quite recent review shows an exhaustive screening between 125 salt hydrates to be used as sorption storage material.
• Furthermore, the thermal efficiency that would result from the storage process based on salt hydrates without condensation heat recovery appears also to be very low (lower than 40% and typically 25%) [364].
• To determine the reaction rate, further research has to be done in micro- and macro-scale to understand the reaction kinetics and mass transfer processes, especially with respect to the different hydrates of magnesium sulphate [366].
• Referred as composites, salt hydrates mixed with additives and combination of physic and chemisorption materials are considered.
• Mauran et al. [368] reported thermal conductivities of about 10-40 W/m·K for CaCl2-expanded natural graphite.

3.2.4. Absorption

• The most characterized absorption pairs are listed in Table 13 along with its main properties under given boundary conditions.
• A cascade absorption system with aqueous LiBr thermal energy storage and R-123 chiller in series were tested.

4. Conclusions

• Sorption has advanced very much due to the immense amount of research carried out around heat pumping and solar refrigeration.
• Moreover, sorption and thermochemical heat storage attracted considerable attention recently since this technology offers various opportunities in the design of renewable and sustainable energy systems.
• The operation principle of the technology is presented and the materials used or in research are listed and compared.
• The review shots that heat and mass transfer remains currently the main limitations.
• Given that the applications are in different states of development, also today’s research is focussed in different aspects.

Figures

Table 3. Specific prices of different cooling cycles (thousands €/kW).

Table 6. Overview of solar absorption refrigeration studies with LiBr/H2O and NH3/H2O

Table 7. Other working pairs for absorption

Table 11. Main characteristics of central solar heating systems with short and long-term storage (adapted from [6,328])

Table 2. Characteristics of absorption cycles.

Table 10.Thermodynamic reaction values of common metal chloride/bromide/iodide-ammonia sorption pairs [18,316,317]

Figure 17. Fie partment (mi

Table 9. Sorbent and system properties and performance

Table 5.Comparison between the absorption systems with NH3-H2O and H2O-LiBr [1,5]

Figure 5.

Figure 10.

Figure 20. C

Table 4. Main features of solar sorption refrigeration technologies [4].

Table 12. Sorption technologies along with its application and main features

Table 8. Summary of conventional working pairs for solid-gas sorption [1,5,9,19,22]

Table 13. Sorption storage materials along with its main properties under given boundary conditions

Frequently asked questions

Q1. What are the contributions in this paper?

This paper is the first review where the research on both 12 applications is shown together. 

Q2. What are the key parameters for adsorption beds?

directly related to heat and mass transfer regarding the sorption materials itself, thermal conductivity and permeability are key parameters for adsorption beds [360,361]. 

Q3. What is the important factor in enhancing thermal conductivity?

A way to enhance poor heat transfer is to add a high conductive material, such as graphite is by far the most selected additive when developing TCM composites with the purpose of enhancing TCM thermal conductivity. 

Q4. What are the main goals of the composite adsorbents?

Composite adsorbents are developed and studied with mainly two goals [22]: to improve heat and mass transfer performance of chemical adsorbents [131], especially due to the swelling and agglomeration phenomena, and to increase the adsorption quantity of physical adsorbents [132]. 

Q5. What is the cooling output of a water-ammonia absorption system?

The cooling output in most cases is liquid ammonia, separated from the ammonia in the water–ammonia absorption cycle machine, as opposed to the chilled water used in LiBr absorption systems. 

Q6. what is the adsorption reaction between metal chlorides and refrigerants?

The adsorption reaction between metal chlorides and refrigerants is a complexation reaction, and the complex compound is also called coordinated compound [127]. 

Q7. What is the way to increase the reaction rate?

A way to enhance the reaction rate and thus the heat release is to use a physisorption material which might consist on a combination of an active (e.g. zeolite) or a passive (e.g. ceramics), porous, supporting material and salt. 

Q8. How is the adsorption performance of silica gel affected?

If the desorption temperature is too high (above 120 ºC), adsorption performance will drop significantly, even to the point of losing its adsorption capacity. 

Q9. What is the maximum SCP of the adsorption cooling systems?

COP of the adsorption cooling systems achieved a maximum of 0.83 when employing metal hydrides/hydrogen pair, while the maximum SCP is achieved with AC/ammonia pair. 

Q10. What is the temperature lift of the airflow?

During the hydration experiments a temperature lift of the airflow of 14 ºC from 50 °C to 64 °C has been measured in the material bed (relative humidity of the incoming airflow of 40 % at 25 °C). 

Q11. What is the hydration rate of magnesium and copper sulphate monohydrate?

The hydration experiments have shown that for both, magnesium and copper sulphate monohydrate, a high water vapour pressure is needed for a sufficiently high reaction rate. 

Q12. How high is the reaction temperature of copper sulphate monohydrate with humid air?

The60   reaction of copper sulphate monohydrate with humid air is limited by the equilibrium to a maximum operating temperature of 60 °C. 

Q13. What is the volume of pores for zeolites?

The volume of pores for type X and Y zeolites, whose void ratio can be as high as 50% when there is no water adsorbed, is larger than that of other types of pores. 

Q14. What are the promising classes of zeolites?

The two classes that show the most promising features are the aluminophosphates (AlPOs) and the silico-aluminophosphates (SAPOs) [357]. 

Q15. How many effects are produced in a LiBr absorption system?

These absorption systems are categorised by the number of times the solution is heated to produce refrigerant vapours, referred to as the number of effects. 

Q16. How many spherical shapes are available in silica gel?

In addition, the adsorption quantity of this pair is low, about 0.2 kg/ kg. Silica gel is commercially59   available as pellets or beads of quite spherical shape in diameter from 1 to 4 mm [361].