WO1984004153A1

WO1984004153A1 - Heat pumping process based on the principle of absorption

Info

Publication number: WO1984004153A1
Application number: PCT/SE1983/000143
Authority: WO
Inventors: Mats Westermark
Original assignee: Scandiaconsult Ab
Priority date: 1983-04-18
Filing date: 1983-04-18
Publication date: 1984-10-25
Also published as: FI844979L; AU1518783A; BR8307710A; FI844979A0

Abstract

Phosphoric acid, especially orthophosphoric acid, is employed as the absorption medium in an absorption heat pumping system using water as its working medium. The acid may contain additions of corrosion inhibitors and/or boiling point elevating agents if desired.

Description

HEAT PUMPING PROCESS BASED ON THE PRINCIPLE OF ABSORPTION

This invention is concerned with a process for heat pumping based on the absorption principle and operating with water as its working medium.

By definition a heat pump will convert low tempe¬ rature heat to useful heat of a higher temperature. This requires a supply of power in the form of mechanical energy or heat energy. Up to now it has been common prac¬ tice, both for heating purposes and for cooling purposes, to employ mechanical heat pumps which require mechanical power for operation; but in some cases absorption heat pumps are employed, these pumps being heat operated, for instance by using heat obtained from steam or flue gases. Since energy costs have been rising for a number of years heat pumps have become increasingly economical, for example for heating rooms and other indoor premises. A disadvantage of mechanical heat pumps is that the power necessary for their operation has to be valuable mechani¬ cal power derived either from electric motors or from die- sel engines. It appears that the costs of both electric power and oil will sharply rise in the future, and in view of this prospect it may be worthwhile to consider the ex- pedient of using absorption heat pumps which allow for a broader choice among different sources of power for their operation. For example, wood chips, peat, coal etc. may be employed as a fuel providing the necessary driving power. However a disadvantage inherent in the absorption heat pumps hitherto employed is their low heat factor; this is significantly lower than the heat factor of mecha¬ nical heat pumps. The "heat factor" is here defined as being the ratio of the recovered amount of heat to the amount of energy supplied for operation of the pump. Fur- thermore, difficulties may arise in achieving sufficient¬ ly high temperatures in e.g. remote heating systems (muni-

_OM£ - cipal heating networks etc.) .

Originally heat pumps of the absorption type have been utilized substantially^* only for cooling purposes: To obtain a cool (air-conditioned) indoor atmosphere the pair of media employed have been most commonly, water and lithium bromide; for more vigorous refrigeration the pair of media employed have mainly been ammonia and water. But in view of the rising energy prices during the last years it has now become economically sound to also produce heat by means of an absorption heat pump; and consequently se¬ veral apparatus manufacturersare developing adaptations of known media and equipments to this entirely new tempe¬ rature range. Preparatory experiments are also under way for employing water and sulfuric acid as the heat pump media, this pair of media being a useful means for che¬ mical heat storage because sulfuric acid is a cheap mate¬ rial.

If absorption heat pumps are to ^"be used for heating e.g. houses in residential areas the following four re- quirements should be fulfilled:-

(1) Sufficient pumping height (temperature level difference) implying that the heat pump should be able to take up low^* temperature heat at about 0 C and raise the temperature to 60-70 C in order to be capable of pro- viding heat to a central heating network;

(2) High heat factor so as to minimize the consump¬ tion of power in operation;

(3) Low hazard media and equipment: the medium should be as harmless as possible and should not be capable of leaking out into the environment;

(4) Maximum chemical stability and minimum corrosive- ness of the medium.

When reviewing the absorption systems of known heat pumps in the light of these requirements one will note the following: If the lithium bromide and water system is employed the temperature increase obtainable is only about 30°C. Unfortunately this is too low for most applications. Such a system will be useful only in cases where, for in¬ stance, a 30 to 40° surplus heat (waste heat) is available and is to be raised to say 60-70°C by the pumping opera- tion. It may be noted, though, that this ab.sorption medium does present a high heat factor, about 1.65, furthermore a low hazard, and acceptable corrosion properties.

The known ammonia + water system while satisfying the requirement of a 60-70°C pumping height has a low heat factor of only about 1.3 - 1.4; and this medium may be said to be fairly hazardous - in view of the fact that ammonia has a working pressure of about 20 - 40 bars. This means that if a breakdown occurs ammonia is liable to escape into the environment and since the ammonia is then in its gaseous state it cannot be collected but will spread with the winds. Although corrosion problems are negligible if ammonia is employed the equipment will ne¬ vertheless have to be an expensive one since its dimen¬ sions must be such as to be suitable for high operational pressures.

The absorption system using sulfuric acid and water has a sufficient pumping height and a high heat factor of about 1.70. But sulfuric acid has obvious drawbacks inas¬ much as it gives rise to major corrosion problems and is rather volatile. At the temperatures prevailing in the ge¬ nerator it is necessary to employ heat exchangers of glass or impregnated graphite. Due to the volatility of sulfuric .acid, gases containing this acid will be liberated if leaks occur in the generator cycle. These gases may cause damage immediately, affecting the lungs of the personnel. Presum¬ ably, therefore, authorities will be reluctant to adopt the sulfuric acid + water system in view of its hazardous environmental implications in larger scale operations.

Consequently the absorption heat pump system in or- der to become better suited to its new purpose of heat production will need a better absorption medium: that is,

OMPI _. a medium giving a high raise in temperature and a high heat factor without being excessively corrosive or hazar¬ dous.

It is an object of the present invention to provide a process for heat pumping according to the absorption system which largely avoids the drawbacks involved with the earlier types of absorption media employed. This is achieved according to the invention substantially in that the absorption medium empolyed is phosphoric acid, espe- cially orthophosphoric acid, optionally with an addition of corrosion inhibitors and/or boiling point elevating agents.

Careful investigations have shown that with the phosphoric acid and water absorption system according to the invention it is possible to obtain heat factors of about 1.7 to 1.9 when an ordinary one-step evaporation is performed in the generator, while in the case of two- step evaporation the heat factor attainable will be about 2.3 to 2.5. This is much more advantageous than the con- ditions prevailing in the known systems, and is beneficial to the economy of operation. Moreover the solubility of phosphoric acid in water is so good that the temperature difference between the hot side and the cold side can be increased, if necessary, to at least 70°C. The absorption medium moreover has the advantage of being readily avail¬ able, of being cheap and of involving litte fire hazard; furthermore there is normally no risk that crystals will precipitate, since even a 100% phosphoric acid is in a liquid state at temperatures above +39°C. The phosphoric acid normally employed has a less than 100% concentration and therefore has an even lower melting point.

Using the term "phosphoric acid" in this context means that the mixture of absorption media contains ortho- phosphoric acid {H₃Pθ!,) as its main component plus optio- nally boiling point elevating agents and water in propor¬ tions suitable for providing a desired degree of boiling point elevation. A suitable phosphoric acid concentration is between 70% and 100%, preferably 90% to 100%. The phos¬ phoric acid employed may be either orthophosphoric acid alone or mixtures of orthophosphoric acid and polyphospho- ric acid. Theboiling point elevating agent may be sulfuric acid in proportions of preferably 1% to 25%, or salt so¬ luble in phosphoric acid (dihydrogen phosphates) . These latter are in the first place salts of ammonia and alkali metals. It is moreover advisable to add corrosion inhibi- tors in order to minimize the corrosiveness of the medium. The main component of the medium in the evaporator (work¬ ing medium) is water; optionally this working medium may contain additions of freezing point depressant and corro¬ sion inhibitors. In terms of physics, the good absorption medium pro¬ perties of phosphoric acid may be explained as being due to the extremely low heat of dilution (dilution enthalpi- city) prevailing within the concentration range through which the medium has to pass during a working cycle. The heat of dilution affects the amount of energy consumed in the evaporation process and thus also affects the heat factor. A substance giving off heat when diluted, e.g. sulfuric acid, requires the corresponding extra amount of heat when it is being concentrated. Phosphoric acid on the other hand has the property of generating very little heat when it is diluted from 100% H3PO4 to 70% H_.PO^ (this being the concentration range considered to be of interest in heat pumping contexts) . Consequently, when a given amount of steam is to be stripped off from a solution of phosphoric acid by evaporation of the solution the heat required is substantially only the heat of vaporization. As against this the systems empolying sulfuric acid or li¬ thium bromide or ammonia require major amounts of extra energy due to the considerable heat of dilution and heat of dissolution inherent in these systems. Therefore, when the pair of media empolyed are phosphoric acid + water the

O resultant heat factors will be higher than in the older known systems. This pair of media is very satisfactory also in respect of other important parameter requirements such as good solubility of the acid, high boiling point elevation, good chemical stability and low volatility. As regards the boiling point elevation parameter it can be shown that a 30° and 70°C elevation will occur at 80% and 95% by weight of H₃POt_* respectively. In prin¬ ciple it is possible to employ solutions of up to 100% H₃PO1_. corresponding to an about 110°Cboiling point eleva¬ tion. The surprisingly high degree ofboiling point eleva¬ tion obtained at phosphoric acid concentrations of between 90% and 100% means that heat at 60 to 80 C or higher can be generated also from readily available low temperature sources (water, ambient outdoor air etc.) . This is a ma¬ jor advantage over the water and lithium bromide system in which the temperature raise obtainable is often too small to permit the utilization of low temperature sources with a temperature range of plus 0-20 -C. In this respect, the system of the present invention is much more advan¬ tageous than the system employing lithium bromide and water.

In order to obtain high heat factors the generator portion is sometimes operated as a mul istep evaporation unit, that is, involving two or more evaporation steps. The steam stripped off in one step is utilized for eva¬ porating the working medium from (an) adjacent step(s). In such an embodiment of the process the low heat of di¬ lution of phosphoric acid constitutes a particularly great advantage over other media inasmuch as it will pro¬ vide energy savings in all (both) of the evaporations steps.

Salts of iron, arsenic, copper are examples of cor¬ rosion inhibitors for phosphoric acid which may be employed to give a good protection from corrosion on e.g. stainless or acidresistant steels in heat exchanger equipments. Good results may also be obtained with additions of organic nitriles or C₆-C₂₀ primary amines together with iodide salts or iodate salts.

Various commercially available materials may be used as freezing point depressants and corrosion inhi¬ bitors in the water phase; examples of these are glycol and chromic acid respectively. It is important, however, that these materials be non-volatile substances. Also phosphoric acid in concentration of up to 60%, prefer- ably 10% to 30%, is a very useful freezing point depres¬ sant.

The phosphoric acid system may be employed for many different purposes in connection with heat pumping or re¬ frigerating processes. The phosphoric acid system can be used within the temperature range of from about -10 C to about +300 C, with temperature raisings of up to about 100 C. The attached Figs. 1 and 2 of the drawing are dia¬ grams showing examples of operations wϊϊere the system of the present invention is particularly advantageous. Fig. 1 illustrates conditions that may be encountered when the medium is used in a refrigeration apparatus having a two- stage generator (two-step evaporation generator) . Evapo¬ ration is assumed to take place at +10°C and absorption is assumed to take place at +30°C. In the hottest generator stage evaporation proceeds as indicated by points A-B-C-D in the diagram. In the colder generator stage evaporation proceeds as indicated by A-Bχ-Cι-D.

Fig. 2 shows an example where the medium is employed in a heat pump with two-step absorption in the absorber. Evaporation is assumed to take place at +5°C. The hottest absorber stage produces heat at 70°C (point E) while the colder absorber stage operates at 60 C (point A) . Evapora¬ tion is carried out in one step and proceeds as indicated by the points A-B-C-D. in actual practice the process of the invention may be performed using essentially the same technical equip-

C ____- ent as that employed for the lithium bromide plus water system. Appropriate consideration will however be given to the choice of materials in parts exposed to contact with liquid, as well as to constructional details. In the process of the present invention special ad¬ vantages are obtained - high heat factor or high refrige¬ ration factor - if the generator is of the multi-stage type (two or more steps of evaporation) . A generator por¬ tion equipped for two-step evaporation is known in connec- tion with the lithium bromide + water system. It will give a refrigeration factor of about 0.99 as compared to 0.65 in the case of single-step evaporation; this means that in the case of two-step evaporation the refrigeration ob¬ tained is increased by about 52%. When the phosphoric acid system is employed the refrigeration factor under comparable conditions of temperature will be increased to 1.55 as compared to 0.88 in the case of single-step evapo¬ ration. Consequently here an about 76%.'-'increase of refri¬ geration can be attained by choosing the two-step proce- dure. It will thus be noted that the combination of phos¬ phoric acid and two-step evaporation provides special ad¬ vantages, due to the fact that phosphoric acid has a low dilution enthalpicity. As regards commercial considerations, these will have to weigh the increased cost of two-step evaporation against the energy savings obtained: It will then be readily apparent that the economical prospects for two-step evaporation are particularly good if the present phosphoric acid system is employed.

Claims

What We claim is : -

1. In a heat pumping process according to the absorp¬ tion method, the use of phosphoric acid, especially ortho¬ phosphoric acid, as absorption medium; the working medium comprising water.

2. Heat pumping process according to claim 1 wherein the phosphoric acid absorption medium additionally comprises corrosion inhibitors and/or boiling point elevating agents.

3. Heat pumping process according to claim 1 or 2 wherein the phosphoric acid has a concentration of between 70% and 100%, preferably of between 90% and 100%.

4. Heat pumping process according to claim 2 wherein the phosphoric acid contains sulfuric acid as boiling point elevating agent.

5. Heat pumping process according to claim 2 wherein the phosphoric acid contains as boiling point elevating agents dihydrogen phosphates of ammonium or alkali metal which are soluble in the phosphoric acid.