REHOS Product Designs

trading the IP gained by the former R&D company Heat Recovery Micro Systems

Absorption Heat Transformer (AHT) Concepts

Can we really use the stored sensible heat in the ocean knowing that the global average ocean surface temperature is ~ 17°C ? The only way we can use this heat, is when we "upgrade" the temperature by using a heatpump, but it takes a lot of energy to power heatpumps to do that. A typical conventional vapor compression (VC) type heatpump is pictured schematically below:

In the VC-heatpump, a suitable refrigerant vapor (eg. ammonia, written as NH3) is compressed to a high pressure by the compressor and is condensed at the high saturation temperature and pressure, delivering the latent heat of condensation of the vapor to a heat sink at high temperature called the condenser. Condensate liquid pressure is reduced via the Joule-Thompson (JT)-valve and the liquid flashed to vapor in the low pressure evaporator, absorbing the latent heat of evaporation from the evaporator heat source. The difference between the temperature values of the condenser and evaporator is known as the temperature lift of the heatpump.

The VC-heatpump has a compression ratio fixed by the temperature lift, and the electrical energy required to operate the VC-heatpump is strongly tied to the refrigerant mass flow and the compression ratio. The efficiency or coefficient of performance (COP) for any heatpump, is defined as the amount of heat pumped, divided by the compressor work done to pump the heat.                                

In our example above the COP = 4.6 for the chosen temperatures. This means in reality each unit of energy used for compression, (kWh electrical energy in this case) would pump or "upgrade" 4.6 kWh of heat energy from the low input temperature (in this example 35°C), to the heatpump output temperature of 90°C. Heating your household hot water with a standard VC heatpump would therefore cost only ~ 22% of the electricity cost compared to using an electrical element in the geyser, but it is still rather expensive, as electricity is very expensive.

A different type of heatpump we call a "Heat Transformer" work on a different principle, and utilize intermediate temperature heat energy to power the upgrading of heat, instead of expensive electrical energy. In its conventional form it is an absorption refrigerator, operating in reverse, and it makes use of a binary liquid mixture like ammonia-water (NH3-H2O) or Lithium Bromide-water (LiBr-H2O) to name only two example binary combinations. These mixtures are said to be zeotropic, consisting of substances boiling at different temperatures, eg. NH3 is more volatile than H2O, so when a binary mixture of NH3-H2O is heated, the NH3 boil off first at much lower temperature than the water.

The Absorption Heat Transformer (AHT) generate high temperature heat in an absorber by condensing NH3 vapor (latent heat) and absorbing the condensed liquid NH3 into the mixture, releasing heat (Heat of Solution, or as acronym HOS). Some chemicals, like NH3 and H2O, completely dissolve in each other, but release a lot of heat in the process, as the dissolving process is said to be exothermic.

To complete the cycle, NH3 vapor at just above absorber pressure is required to generate the High Temperature (HT) or "upgraded" heat in the absorber.

In the conventional AHT sketched here, the rich high concentration liquid is cooled in an economiser heat exchanger (H/E) and the pressure dropped to the lower desorber pressure, where intermediate temperature heat is used to boil off (desorb) some NH3 vapor from the rich mixture. The remaining lean liquid in the desorber is again pumped back to the higher pressure absorber, while the boiled off NH3 vapor is condensed in a condenser at low pressure by removing the latent heat of condensation with cooling water. From the condenser the NH3 liquid is then pumped to the higher pressure evaporator, at a pressure slightly higher than the absorber. Heat is therefore used to generate the NH3 high pressure vapor, and a very small amount of electricity is required to power the two liquid pumps. The electricity required for pumping is normally ~ 2 orders of magnitude smaller than the heat flow, eg. Pumping power of ~ 100 Watt is used for a heat transformer where the pumped heat amount to ~ 10 kW.

Real COP values for this type of heat transformer is relatively low, however, eg. around 0.35, compared to the VC heatpump example mentioned above with COP = 4.6, but realizing the amount of electricity used by the heat transformer is extremely low as the heat transformer is heat powered, it makes sense to define two different COP values to represent the electrical efficiency, COP_e, and the thermal efficiency, COP_th separately:

This low COP_th of average 0.4 (40%), means that only 40% of the intermediate temperature heat is upgraded to the higher temperature, while the balance is rejected in the cooling water at the condenser, but the high COP_e value indicate that the electricity cost (for liquid pumping) is very low compared to the VC example above. Making the same comparison as before, the amount of electricity used, is only 1/50 or 2% of electricity that would have been used by an electrical element in your geyser. Although this electrical energy consumption is a factor of 10 lower than the VC heatpump example, using the conventional AHT is not very practical for this purpose, as cooling means to remove the latent heat of condensation of the NH3 vapor from the condenser at a temperature much lower than the intermediate temperature would be required. Keeping the AHT principles however, several different means of generating the NH3 vapor at high pressure have been used commercially to make the use of the economy of the AHT operation practical.                                                            

We recognize that the absorption heat transformer (AHT) is actually defined by four criteria, namely:

1.   the fact that it is a closed thermodynamic cycle, which is (at least partially) heat powered to upgrade (or lift) the temperature of the heat from low- to moderate levels up to higher temperature heat;

2.   temperature lift is generated by a vapor absorption process, releasing the heat of solution (HOS), combined with the latent heat of condensation of the vapor into a hot absorber;

3.   the cycle also has a means of producing the vapor at the absorber pressure, although it may be much lower in temperature; and,

4.   means is provided in the absorber to sub-cool the liquid present, eg. heat removal to decrease the temperature below the saturation level, allowing the vapor absorption to take place. (As we know, vapor will not be absorbed into a saturated liquid. It has to be sub-cooled below the saturation temperature before any vapor will be absorbed.).

A different way of creating the high pressure NH3 vapor required for AHT operation, come from the configuration sketched below, called the Hybrid AHT, where the vapor pressure is raised in a vapor compressor to the absorber pressure level. The compressor therefore replace the NH3 condenser and evaporator of the conventional AHT example discussed earlier. The heat pumping is still powered by the intermediate heat input (Q_cold in the sketch below) to boil off some vapor, but it is assisted by the compressor. Note that the compression ratio of 4.76 is much lower than the ratio of the VC example shown earlier of 17.8 and therefore the power requirements is also much lower. The electrical COP_e of this Hybrid AHT calculates to ~ 9.5 which is double the VC heatpump value, but much lower (about 20%) than the 50+ of the conventional AHT.

The configuration of the Hybrid AHT shown here, also demonstrate the very important isobaric temperature glide principle of the zeotropic binary mixture both in the absorber and the desorber. The temperatures on the opposite sides of these heat exchangers differ substantially, and so does the NH3 concentration inside. On the hot (109 Celsius) side of the absorber the ammonia concentration is much lower (37.34%NH3) than the colder (75 Celsius) opposite end (56.72%NH3), clearly highlighting the temperature and NH3 concentration gradients. These concepts have been used extensively in the development of heat exchangers for the Kalina cycle in the late 80's.

This is demonstrated better in the binary NH3-H2O column sketched below. When a vertically held pipe is sealed closed and filled with eg. 60%NH3 in aqua mixture, the NH3 is completely dissolved in the water and the concentration distributed about evenly, making the concentration about the same everywhere in the pipe. The pressure would essentially be the mixture saturation pressure of ~ 5 Bar Abs for 60% mixture at the 26°C ambient temperature shown.

As soon as external heat is supplied to the column, the mixture react by creating an ammonia concentration gradient, with the higher NH3 concentration and lower liquid density moving to the top of the reactor while the higher density, hotter, but leaner NH3 mixture migrating to the bottom of the reactor. Simultaneously the mixture migrating to the top of the reactor is cooled while the reactor bottom is heated, creating a temperature gradient with the hottest area at the reactor bottom. Heat energy is required from external sources to establish this concentration and temperature gradient, but once established, the external heat source may be removed and the gradients would very slowly dissipate as heat is lost by conduction and radiation out from the hot bottom, but no convection, as convection is inhibited due to density differences. During this charging process, the pressure in the column was also raised (in this example to 8 Bar Abs) and the top of the column temperature remained very close to ambient. Note that all points inside the charged column stays in saturation condition, so when some of the vapor present at the top is removed (eg. by a vapor compressor suction inlet), energy is removed and more NH3 would flash off, extracting heat from the top of the binary column. This would drop the pressure in the column, as well as the top temperature, but would not influence the gradients that exists! The amount of energy removed from the column would therefore be a way to decrease the pressure and in the process adjust both top and bottom temperatures simultaneously down or up somewhat. It is therefore not difficult to picture an AHT making use of a vapor compressor and a vertical reactor H/E column, or as we like to call it a "bubble reactor" as sketched below.

A very simple vapor ejector may be used as compressor, as the actual real pressure in the binary column may be relatively high (some 5 - 8 Bar Abs as per example above), but the differential pressure (compression ratio) of the ejector vapor compressor would only be the hydraulic pressure of the liquid column held vertical, and for a 1 - 3 meter binary column, the dP would be only 7 - 20 kPa. This is the ideal dP range for an ejector vapor compressor. The heat required to evaporate the high pressure drive HP vapor powering the ejector, may be extracted from the hot bottom of the column regeneratively by the H/E coil as pictured in the sketch, making it a true Regenerative Heat of Solution (REHOS) Heat Transformer. Only a single pump also decrease the amount of electricity required to upgrade low temperature (in this example 35°C waste heat) to more useful levels with COP_e values much closer to 500, instead of 50 as shown for the conventional AHT we described earlier.

Removing a small amount of energy from the cold side of the reactor, may also be achieved by simply removing high concentration NH3 liquid from the reactor top, creating a flow vertically upwards in the column, while removing heat from the hot reactor bottom with the H/E coil to decrease the pressure (and temperature). This give rise to the bubble reactor heat transformer (BRHT) as shown in the P - T Diagram below. Intermediate (or low) temperature input heat is used in the evaporator to generate vapor at the correct pressure, which is absorbed in the hot bottom section of the bubble reactor, providing a heat transformer with extremely high efficiency, exceeding the conventional AHT in both COP_e and COP_th by far.

With the BRHT performance so high, it is just common sense to utilize this temperature upgrading equipment to provide us with a reasonable heat source at a temperature close to 100°C powered by the sensible heat from available waste heat sources, like the 30°C - 35°C pool of cooling water present below an utility-scale Power Station's wet Cooling tower, as well as ambient sensible heat sources like the surface of the oceans averaging globally ~ 17°C. At the Heat Transformer COP value in excess of 500, the electrical cost to pump heat to some 80°C "upgrade" temperature is extremely small!

Constructing the BRHT utilizing the principle of generating heat at elevated temperature from vapor absorption in a vertical column, create the ideal environment for regeneration, where heat rejected from another cycle or device, in the form of latent heat in a low pressure and temperature vapor, may be re-used regeneratively as vapor input to partially power the Heat Transformer. More about this patented REHOS concept on the pages dedicated to the RAW-Pump, as well as the page for the REHOS Pond, generating power from your swimming pool!

Interesting to note that Heat Transformers are not new, and have been in commercial use for years, but it is not so well known, even though the electrical power for heat pumping is extremely low......but we think that simplifying the AHT construction as sketched in the REHOS Bubble heatpump and the BRHT should change that, however!

The patented Regenerative Heat of Solution (REHOS) cycle is a thermodynamic cycle combining an AHT temperature upgrading sub-cycle, with an Organic Rankine Cycle (ORC) power sub-cycle to generate power from very low temperature waste (and ambient) heat. The power generated may be utilized in various ways, eg. to pump water where the REHOS cycle is used as Prime Mover for powering a pump, or to generate electricity, where the REHOS cycle is utilized as Prime Mover to turn an electrical generator, and many other applications.