Single stage heat transformers can increase the temperature of approximately 50% of the waste heat energy by ~50 degrees C . They are the simplest and most commonly investigated heat transformer configuration. The thermodynamic performance of a SSAbHT increases with an increase in the temperature of the evaporator, and a decrease in the temperatures of the condenser and the absorber. Art. [#ARTNUM](#article-28423-2051897141) A heat source supplied to the generator is used to separate the more volatile component, the refrigerant, from the absorbent (e.g. a water and LiBr–H2O solution) by evaporation at an intermediate temperature. The refrigerant vapour then flows to the condenser where it is condensed by reducing its temperature, discharging its latent heat to a low temperature heat sink (generally to atmosphere). The condensed refrigerant is pumped to a higher pressure prior to entering the evaporator, where it is once more evaporated utilising an external heat source (generally the same heat source as used by the generator). This refrigerant vapour is then absorbed in the absorber into the concentrated absorbent solution coming from the generator. Some of the heat of absorption liberated is used to maintain the absorber at a temperature higher than that of the evaporator and generator (approximately 30–60 °C hotter), while the remainder of the liberated heat energy is removed as the high temperature heat product. The dilute absorbent solution leaving the absorber is used to preheat the concentrated solution entering the absorber from the generator, prior to having its pressure reduced and returning to the generator. Art. [#ARTNUM](#article-28423-2051897141) **Research findings:** - The majority of all simulated SSHT studies predict COP values of between 0.4 and 0.5, and GTLs (Gross Temperature Lifts) of approximately 50 °C. Experimental SSHT cycles which have been tested generally do not achieve these high levels of energy recovery, however. Art. [#ARTNUM](#article-28423-2051897141)

A double stage heat transformer (DSHT) is essentially the combination of two single stage heat transformers (SSHT) as illustrated in the figure. Intermediate waste heat energy is supplied to the evaporator and generator of one of the cycles, named the low temperature cycle. This low temperature cycle increases the temperature of a fraction of this energy to approximately 145 °C which is released by the absorber of this cycle. This heat energy is used to supply some or all of the energy requirements of the other single stage cycle within the DSHT (termed the high temperature cycle). There are three ways to link the low and high temperature cycles, namely by coupling the absorber of the low temperature cycle to either the evaporator or the generator of the high temperature cycle, or else by coupling the absorber to both of these units. The figure shows a schematic of a DSHT in which the absorber of the low temperature cycle is coupled to the evaporator of the high temperature cycle. In this case, the evaporator of the high temperature cycle is able to operate at an increased temperature, which enables a higher GTL to be achieved. Thus the absorber of the high temperature cycle is capable of reaching temperatures of about 190 °C. The generator of the high temperature cycle is heated by the same heat source as the low temperature cycle. If the absorber of the low temperature cycle were coupled with the generator of the high temperature cycle then the inverse would occur and the generator would operate at an elevated temperature while the evaporator would remain at the same temperature as the evaporator in the low temperature cycle. If the absorber of the low temperature cycle were coupled with both the evaporator and the generator of the high temperature cycle, then both of these units would operate at an increased temperature. Art. [#ARTNUM](#article-28427-2051897141)

A double absorption heat transformer (DAHT) is very similar to a DSHT, except that both the high and low temperature cycles share a common generator and condenser (see figure). All of the units are arranged vertically according to their temperature (as shown by the axis on the left hand side) to allow for easy interpretation. This system consists of 6 basic units, namely a condenser, a generator, an evaporator, an absorber–evaporator, a solution heat exchanger, and an absorber. A heat source supplied to the generator is used to separate the more volatile component, the refrigerant, from the absorbent by evaporation at an intermediate temperature. The refrigerant vapour then flows to the condenser where it is condensed by reducing its temperature, discharging its latent heat to a low temperature heat sink (generally to atmosphere). One fraction of the condensed refrigerant is pumped to a higher pressure prior to entering the evaporator, where it is once more evaporated utilising an external heat source (generally the same heat source as used by the generator). This refrigerant vapour is then absorbed in the absorber–evaporator into an absorbent solution. Some of the heat of absorption liberated is used to maintain the absorber–evaporator at a temperature higher than that of the evaporator. The dilute solution produced in the absorber–evaporator has its pressure reduced and returns to the generator. The second fraction of the condensed refrigerant leaving the condenser is pumped to a higher pressure (greater than the pressure in the evaporator), and is then evaporated by utilising the remaining heat of absorption being liberated by the absorber–evaporator. This refrigerant vapour is then absorbed in the absorber into the strong absorbent solution coming from the generator. Some of the heat of absorption liberated is used to maintain the absorber at a temperature higher than that of the absorber–evaporator (approximately 30-60 °C hotter), while the remainder of the liberated heat energy is removed as the high temperature heat product. Some of the absorbent solution leaving the absorber enters the absorber–evaporator and is used to absorb the refrigerant vapour being produced in the evaporator. The remainder of the solution coming from the absorber flows through solution heat exchanger to pre-heat the concentrated solution entering the absorber prior to having its pressure reduced and returning to the generator. Art. [#ARTNUM](#article-28428-2051897141) **Research findings:** - In this study, the operability of a double-lift absorption heat transformer that generates pressurized steam at 170 °C is studied across a full range of operative conditions. The results demonstrate and clarify the manner in which the system can operate steadily and efficiently when driven by hot water temperature at approximately 80 °C while safely generating steam at a temperature exceeding 170 °C. The conditions yielding maximum system efficiency and capacity are identified, and the obtained experimental results are used to define an optimal control strategy. Art. [#ARTNUM](#article-28428-2623997446) - A pilot plant with 100 kW performance was built and tested in order to test in practice the newly developed heat transformer. Transformation took place here from 80°C to 121°C at a heat ratio of 0.52 on average. Art. [#ARTNUM](#article-28428-2094793557)

A double effect heat transformer (DEHT) is effectively a version of the DAbHT in which two generators are used instead of two absorbers (see figure). The dilute solution leaving the absorber is cooled in two solution heat exchangers prior to entering the low temperature generator. Here, the solution is concentrated by boiling off some its refrigerant. The vapour generated flows to the condenser while the concentrated solution is pumped to the high temperature generator. Waste heat energy is used to boil off more refrigerant and to concentrate the solution further before it is transferred to the absorber. The vapour produced in the high temperature generator flows to the low temperature generator where it is used as the heat source (i.e. it is condensed). This condensate has its pressure reduced and is combined with the vapour produced in the low temperature generator prior to entering the condenser. Zhao et al. demonstrated that while its COP is approximately 20% greater than that of a corresponding SSHT, such a cycle is only suitable in situations where relatively high temperature heat energy is available and small GTLs are required as the COP of the DEHT is shown to fall off much more rapidly with an increase in absorber temperature than that of the SSHT. Once a certain GTL is exceeded, the SSHT is capable of recycling a greater fraction of the supplied heat energy. Art. [#ARTNUM](#article-28424-2051897141) **Patent Findings:** - The doubleeffect lithium bromide absorption heat transformer has the advantages that the condensate of the hightemperature coolant steam enters the condenser after the coolant water heat exchanger performs heat exchange and heats the low temperature coolant water, less heat of the condensate is brought away by cooling water after flash cooling of the condensate entering the condenser, consumption of waste heat resources in the evaporator are decreased, and the waste heat resources can be converted into high temperature heat sources more efficiently. Art. [#ARTNUM](#article-28424-2877493226)

A triple absorption heat transformer (TAHT) can upgrade heat to around 200 degrees. It consists of 9 basic units, namely a condenser, a generator, an evaporator, two absorber–evaporators (at different temperatures), three heat exchangers, and an absorber as demonstrated in the Figure. A heat source supplied to the generator is used to separate the more volatile component, the refrigerant, from the absorbent by evaporation at an intermediate temperature. The refrigerant vapour then flows to the condenser where it is condensed by reducing its temperature, discharging its latent heat to a low temperature heat sink (generally to atmosphere). One fraction of the condensed refrigerant is pumped to a higher pressure (P1) prior to entering the evaporator, where it is once more evaporated utilising an external heat source (generally the same heat source as used by the generator). This refrigerant vapour is then absorbed in absorber–evaporator-1 into the strong absorbent solution coming from the generator. Some of the heat of absorption liberated is used to maintain the absorber–evaporator-1 at a temperature higher than that of the evaporator. The second fraction of the condensed refrigerant leaving the condenser is pumped to a pressure P2 (greater than P1), and is then evaporated by utilising the remaining heat of absorption being liberated by absorber–evaporator-1. This refrigerant vapour is then absorbed in absorber–evaporator-2 into the strong absorbent solution coming from the generator. Some of the heat of absorption liberated is used to maintain the absorber–evaporator-2 at a temperature higher than that of absorber–evaporator-1 (approximately 30–60 °C hotter). The third (and final) fraction of the condensed refrigerant leaving the condenser is pumped to an even higher pressure P3 (greater than P2), and is then evaporated by utilising the remaining heat of absorption being liberated by absorber–evaporator-2. This refrigerant vapour is then absorbed in the absorber into the strong absorbent solution coming from the generator. Some of the heat of absorption liberated is used to maintain the absorber at a temperature higher than that of absorber–evaporator-2 (again approximately 30–60 °C hotter), while the remainder of the liberated heat energy is removed as the high temperature heat product. The weak absorbent solutions produced in absorber–evaporator-2 and the absorber are used to preheat the respective strong solutions entering them from the generator, prior to having their pressure reduced and returning to the generator. Art. [#ARTNUM](#article-27547-2051897141) They are not very well studied. GTL values of 145 with COP of 0.2 have been reported. Art. [#ARTNUM](#article-27547-2051897141)

The ejector is placed at the entrance to the absorber as illustrated in the figure, and the concentrated LiBr–H2O solution entering the absorber is used to entrain the saturated vapour which gives rise to a higher pressure in the absorber compared to the evaporator. With a compression ratio of 1.2, the GTL increased by 5 °C, and the ECOP (exergetic coefficient of performance) by 2.7%. EAHT and determined that the COP is increased by 14%, the GTL by 6 °C, the ECOP is increased by 30% and that the flow ratio is decreased by 57% compared to a conventional NH3–H2O SSHT excluding the ejector. Art. [#ARTNUM](#article-27542-2051897141) By the use of the ejector, the absorber pressure becomes higher than the evaporator pressure and thus the cycle works with a triple pressure level. The ejector has a double function; it enables the pressure at the evaporator to remain low, and upgrades the heat quality obtained in the absorber by mixing the refrigerant vapor with the solution. Art. [#ARTNUM](#article-27542-2885535689)

The open absorption heat transformer (using LiBr–H2O) simply removes the condensed water from the condenser as a product, while the evaporator acts as the water distillation cycle. COP values as high as 1.02 were achieved with a four effect distiller (possible as no external heat is being supplied to the evaporator). Art. [#ARTNUM](#article-27553-2051897141) It is often coupled with water desalination. Art. [#ARTNUM](#article-27553-2624354158)

The Self-regenerated AbHT is an improved heat transformer with a generator–absorber heat exchanger (GAX) cycle applied in the normal heat transformer cycle. As shown in the cycle flow diagram (Figure), the absorber consists of three parts: (1) a solution temperature amplifier (STA) in which refrigerant vapor is absorbed by the super-cooling solution compressed by solution pump (p2) after leaving the generator and the solution gets a raise in temperature; (2) an absorbing temperature amplifier (ATA) in which the heating medium has a temperature lift by receiving absorption heat; (3) an absorbing heat exchange absorber (AHXA) in which absorption heat is taken as generation heat. The generator is made up of two parts; one is the heat exchange generator (HEG) heated by an outer heat source, and the other is an absorbing heat exchange generator (AHXG) heated by absorption heat. The refrigerant, which is condensed liquid in condenser (COND), is compressed by a pump (p1) to the solution heat exchanger (SHE). It exchanges heat with a high temperature rich solution at the end of absorption process and then enters the evaporator (EVAP). Passing through the throttle valve, the rich solution at a decreased temperature flows into the rectifier (REC). Due to the larger temperature lift obtained by the SRAHT cycle, the waste heat is reused as much as possible by reducing the temperature of the waste heat as low as possible through the SRAHT cycle. According to the characteristics of the SRAHT cycle, waste hot water flows in series connection. The waste hot water with a higher temperature goes in HEG first and releases some quantity of heat, then it enters the evaporator to give off further heat in order to recover as much waste heat as possible. Art. [#ARTNUM](#article-28425-1979237495)

Ammonia water AbHT is often used in cooling applications. As a cooler: An ammonia-water mixture is used as the working fluid. According to the picture: Saturated liquid (1) in the absorber is subcooled to state (2), and is then pumped to the high-side pressure at (28). This pumped stream cools the rectifier, rising in temperature to state (29), also designated as state (3). This solution stream gets further heated in the recuperative solution heat exchanger to state (4). The corresponding solution saturated state is designated (3). Waste heat, states (19) to (20), desorbs ammonia-water vapor (6) from this concentrated solution stream, resulting in dilute solution (7). The desorbed vapor (6) is rectified to a higher concentration vapor (8), with reflux liquid exiting at (9). This reflux combines with the dilute solution (7) and flows to the solution heat exchanger at state (16), cooling to state (17) in this heat exchanger as it recuperatively heats the concentrated solution. The rectified ammonia-water vapor is condensed in the condenser to saturated liquid state (10), with the heat of condensation rejected to the ambient stream flowing from state (22) to (23). The refrigerant is subcooled to state (11), flows through the high-temperature side of the refrigerant precooler to state (12), and expands through the valve to the low-side pressure at (13). The refrigerant is evaporated to state (14) across a 4 K temperature glide, in the process cooling the conditioned air from state (24) to (25). The refrigerant is then recuperatively heated to state (15) and flows to the absorber. It then combines with the dilute solution stream at state (17), forming a two-phase mixture at state (18). Heat rejection to the ambient stream, state (26) to (27), accomplishes absorption. This cycle can be reversed for temperature lifts. Art. [#ARTNUM](#article-27545-1992306432) - 120 degrees wasteheat: COP 0.707; footprint of 3.80 m2. - 60 degrees waste heat: COP 0.853; footprint of 0.299 m2. **Research findings:** - Compared to watersalts mixtures, waterammonia allows operating the machine in a lower temperature range, fostering recover of lowgrade heat. Driving temperatures between 60 °C and 64 °C were tested, with condenser temperatures of 8–16 °C. The unit proved able to operate in a stable, reliable and repeatable way in this working range, achieving gross temperature lifts up to 25 °C and thermal COPs in the range 0.400–0.475. Useful effect up to 4.5 kW was achieved, with electric consumption always below 100 W. Art. [#ARTNUM](#article-27545-2746203047)

The Lithium Bromide Absorption Heat Transformer (LBAHT) can upgrade the temperature of waste heat with little electricity using LiBr as the sorbent and water as sorbate. The LBAHT is powered by waste heat. Part of the waste heat whose temperature is upgraded will be reused in industrial processes, and other part will be rejected to low temperature heat sink. Hot water is supplied from absorber or steam is supplied from steam flasher. Art. [#ARTNUM](#article-27548-2465635260) Figure: Saturated liquid at state (1), subcooled to state (2), exits the absorber and expands to the low-side pressure at state (3). From state (3) to state (5), heat is added in the generator from the waste heat stream (18) to (19), desorbing water vapor from the lithium bromide solution. The water leaves at state (5) as a superheated vapor, while the concentrated solution leaving at state (6) is pumped up to the high-side pressure at (7) and returns to the absorber. The water vapor at (5) enters the condenser, reaches a saturated vapor state at (8), then condenses to a saturated liquid at (9) and is further subcooled to state (10). The heat of condensation is rejected to the coupling stream (16) to (17). The subcooled water at (10) is pumped to the high-side pressure at state (11), where it enters the evaporator. The waste heat stream (state (20) to (21)) is used to heat, evaporate and superheat the water (states 12–14). The superheated water at (14) then combines with the concentrated lithium bromide-water solution at (7) to yield state (15), leading to a rise in temperature. As absorption of the vapor progresses to yield dilute solution at state (1), heat is rejected to the stream entering at (22), heating it to state (23), thereby providing the desired higher-grade heat output. Art. [#ARTNUM](#article-27548-1992306432) Temperature lift: - Waste heat: 66 degrees used for 85-100 lift: COP=0.476; footprint: 0.158 m2 - Waste heat: 120 degrees used for 135-150: COP= 0.469; footprint: 2.045 m2 Next to upgrading waste heat, LiBr-water absorbers are often used in airconditioning and refridgeration. They are the state of the art heat transformers, however problems with crystallization are common at lower temperatures.

In the HeCol cycle the heat of a natural water basin or domestic waste heat at temperature TM = 2–20 °C is used as a heat source and the ambient air with ultra-low temperature TL = −40 - −20 °C is used as a heat sink to produce the useful heat at TH = 35–50 °C. Art.[#ARTNUM](#article-27740-2801112808). A HeCol cycle can use most types of sorbents, composites are the most promising. **Research findings:** - Testing a lab-scale HeCol prototype loaded with the LiCl/silica and CaClBr/silica composites demonstrated the practical feasibility of the HeCol cycle. At the heat sink temperature TL = −20°С and the heat source temperature TM varied from 10 to 25°С, a maximal temperature of the released heat of 32–49 °C was obtained with the CaClBr/silica composite that is suitable for warm floor systems. The LiCl/silica allows the higher maximal temperature 34–53 °C, but requires the higher heat source temperature TM = 20–28°С as well. The maximum Specific Heating Power SHP = 1.4–3.6 and 6.0–10.8 kW/kg was reached with the CaClBr/silica and the LiCl/silica, respectively, that is the excellent base for designing compact HeCol units for upgrading the ambient heat temperature in cold countries. Art. [#ARTNUM](#article-27740-2791744877)

The simplest way to achieve temperature lift is the basic thermodynamic cycle, which consists of a reactor, where the solid–gas synthesis or decomposition reaction happens, and a heat exchanger, where the evaporation or condensation of the gas takes place, as shown in the figure. First, in the generating period, the middle-grade heat at is supplied to the heat exchanger (acting as an evaporator) so the liquid in it evaporates; the pressure of evaporator rises to (point 1 and the valve is opened; the high pressure gas enters the reactor to synthesize with the reactive salt, releasing high-grade heat at (point 2); the evaporation and the synthesis reaction continues so the pressure remains steady; when the synthesis reaction in the reactor finishes, the valve is closed. Immediately, the reactor is supposed to be cooled. Then in the recovering period, the middle-grade heat at , which may be different from the former, is supplied to the reactor for decomposition; once the pressure of reactor rises to (point 3), the valve is opened; the released gas transfers to the heat exchanger (acting as a condenser) to be condensed by the coolant, releasing condensation heat at (point 4); when the decomposition reaction finishes, the valve is closed. Then the condenser is supposed to be heated before the next cycle, and the cycle continues. Art. [#ARTNUM](#article-29230-2092847094) **Research findings:** - The reversible reaction of SrBr2 anhydrate to its monohydrate has been chosen for further analysis as reaction material for thermochemical heat transformation [4, in preparation]. Using this single step reaction, an energy storage density of 170 kWh/m3 [5] (calculation based on anhydrate and a 50 % powder bed porosity) and heat transformation at high temperature levels (150 – 300 °C) is possible. Art. [#ARTNUM](#article-29230-2608178443) - The reversible reaction of strontium bromide with water vapor is proposed in this work for thermochemical heat transformation: SrBr2(s) + H2O(g) ⇌ SrBr2 x H2O(s) + ΔRH. Driven by 90 °C waste heat, this chemical reaction offers the possibility to “lift” process heat flows to a higher temperature level in the range of 180 °C to 230 °C. Art. [#ARTNUM](#article-29230-2594119990) - This thesis introduces a new pillowplate reactor for a thermochemical gassolid reaction system with indirect heat transfer and integrated storage. The reactor can fit around 1.3 L of powdery material, withstand a temperature of up to 600 °C and support a heat flow rate of 1200W. It is experimentally tested with the reaction system of calcium sulfate and its hydrates CaSO4 x nH2O. The experiments have shown a successful heat transformation from 135 °C (open/closed dehydration at 0.009 bar) to 192 °C (closed hydration 0.96 bar). Art. [#ARTNUM](#article-29230-2626799735) - In this manuscript, experimental and numerical studies on a singlestage metal hydride based heat transformer (MHHT) are presented. A prototype of a singlestage MHHT is built and tested for upgrading the waste heat available from 393–413 K to about 428–440 K using LaNi 5 /LaNi 4.35 Al 0.65 pair. The transient behavior of hydrogen exchange associated with heat transfer is presented for a complete cycle. The effects of heat source temperature and heat rejection temperature on the performance of MHHT in terms of coefficient of performance (COP HT ), specific heating power (SHP) and second law efficiency ( η E ) are investigated. At the given operating conditions of heat output temperature 428 K, heat input temperature 413 K and heat sink temperature 298 K, the experimentally predicted COP HT and SHP are 0.35 and 44 W/kg, respectively. Art. [#ARTNUM](#article-29230-2021056073)

The two-salt cycle system was developed because of the high system pressure in the SSGSHT will cause a safety problem because of the coexistence of vapor and liquid in the same heat exchanger for some working pairs. The configuration of the system comprises of two reactors with different reactive solid salts, shown in the figure. The working principle is more or less the same as the SSGSHT; only the condensation and evaporation in the heat exchanger are replaced by synthesis and decomposition reactions. Art. [#ARTNUM](#article-29232-2092847094) **Research findings:** - The working performance and feasibility of the large-temperaturelift thermochemical sorption heat transformer was investigated and analyzed using a group of sorption working pairs of MnCl 2 SrCl 2 NH 3 . Expanded graphite was employed as the porous additive to enhance the heat and mass transfer of reactive salts. The experimental results showed that the proposed solidgas thermochemical sorption heat transformer is feasible to achieve energy upgrade with a large temperature lift. It has the potential to upgrade the lowgrade heat from 96 °C to 161 °C using MnCl 2 SrCl 2 NH 3 sorption working pairs, and the exergy efficiency and energy efficiency are as high as 0.75 and 0.43, respectively. Art. [#ARTNUM](#article-29232-2623646185) - The working pairs of MnCl 2 /NH 3 -SrCl 2 /NH 3 were employed and expanded graphite served as the additive to synthesize composite sorbents with enhanced heat and mass transfer performance. The thermodynamic analysis based on the coupled relationship of temperature and pressure was firstly carried out. The system performances including energy efficiency, heating power and storage density were investigated. The experimental results showed that the maximum coefficient of amplification and energy storage density reached 1.74 and 444.1 kJ/kg composite sorbent without consideration of sensible heat under the operation conditions of the heat source temperature of 120 °C −150 °C, heat output temperature of 50 °C and ambient temperature of 30 °C. Art. [#ARTNUM](#article-29232-2931276669) - Theoretical analysis showed that the proposed targetoriented thermochemical sorption heat transformer is effective for the integrated energy storage and energy upgrade, and the lowgrade thermal energy can be upgraded from 87 to 171°C using a group of sorption working pair MnCl2CaCl2NH3. Moreover, it can give the flexibility of deciding the temperature magnitude of energy upgrade by choosing appropriate sorption working pairs. Art. [#ARTNUM](#article-29232-1975976065)

The limited temperature lift and system efficiency for single-stage heat transformers can be overcome by two-stage or multi-salt systems. Various combinations are possible, most are still in the research phase. Art. [#ARTNUM](#article-29233-2092847094)

Vapor-compression evaporation is the evaporation method by which a blower, compressor or jet ejector is used to compress, and thus, increase the pressure of the vapor produced. Since the pressure increase of the vapor also generates an increase in the condensation temperature, the same vapor can serve as the heating medium for its "mother" liquid or solution being concentrated, from which the vapor was generated to begin with. If no compression was provided, the vapor would be at the same temperature as the boiling liquid/solution, and no heat transfer could take place. In case of compression performed by high pressure motive steam ejectors, the process is usually called thermocompression or steam compression. [[Wiki]](https://en.wikipedia.org/wiki/Vapor-compression_evaporation) The vapour recompression technique consists in increasing the pressure of the vapour produced in a liquid food evaporation process, so that it can be re-employed as heating fluid in the process itself. This results in substantial savings of live steam consumption and therefore of fuel costs, other remarkable advantages include reduction of both condensation water requirements and environmental impact. Thermal Vapour Recompression (TVR) is performed by means of a steam ejector, in which the low-pressure vapour is sucked and compressed by high-pressure live steam. Thermal vapour (re)compression is also used in combination with absorption for refridgeration/ airconditioning. Art. [#ARTNUM](#article-33577-2032737333); [#ARTNUM](#article-33577-2060579385)