AU587831B2

AU587831B2 - Fluid dissociation solar energy collection system

Info

Publication number: AU587831B2
Application number: AU58197/86A
Authority: AU
Inventors: Peter O'Neill Carden
Original assignee: Australian National University
Current assignee: Australian National University
Priority date: 1985-05-06
Filing date: 1986-05-06
Publication date: 1989-08-31
Anticipated expiration: 2006-05-06
Also published as: AU5819786A

Description

FLUID DISSOCIATION SOLAR ENERGY COLLECTION SYSTEM

FIELD OF THE INVENTION

This invention relates to improvements in methods and means for collecting solar radiation. In particular the invention is concerned with a system which uses a fluid which dissociates endothermically and recombines exothermically. BACKGROUND OF THE INVENTION

Prior art methods of collecting and gathering solar energy over large areas of land include the following:

1) Solar energy is collected by hot plate absorbers in which solar energy is converted to heat. The heat either boils a liquid or raises the temperature of a fluid, and by corradiating in either case this hot fluid, energy is brought to a common heat recovery plant.

2) Solar energy is collected by means of parabolic or paraboloidal mirrors and is converted to heat in a focal absorber in which either the temperature of a fluid is raised or a liquid is boiled. In either case the hot fluid is corradiated as in 1.

3) Solar energy is collected by flat mirrors or near flat mirrors by means of which the radiant energy is redirected to a common focal absorber usually elevated. The heat energy is recovered from the

absorber by means of a heat exchanging fluid.

In Australian Patent Specification No 495395 there is disclosed a solar energy collection system in which a fluid dissociates endothermically and subsequently recombines exothermically into an undissociated fluid, comprising: a) means for introducing the undissociated fluid, at elevated temperature, into an absorber-reaction vessel, b) means for focussing solar radiation onto the absorber-reaction vessel, c) means operable from a master control for controlling the orientation of the means for focussing solar radiation, d) means for conducting the fluid from the absorber-reaction vessel through a first counterflow heat exchanger to effect cooling, e) means for conducting the fluid to a reaction chamber wherein partial recombining of the dissociated products occurs with release of heat energy, f) a separator for separating undissociated fluid and dissociated fluid, g) means for returning dissociated fluid to the reaction chamber, 0j

h) means for returning undissociated fluid to the first heat exchanger so as to heat it prior to entry into the absorber-reaction vessel, i) means for maintaining the fluid at superatmospheric pressure, j) means for recovering heat energy from the reaction chamber.

In the above context, a fluid which dissociates is a fluid a component of which undergoes dissociation into simpler constituents, the whole, however, remaining fluid. The simpler constituents subsequently recombine to form the undissociated fluid having the same composition as the original fluid.

More recently research workers at The Australian National University, Canberra, Australia, have published several articles relating to developments in thermochemical energy transfer -

Carden, P.O. (1981). Energy Conversion Technical Report No. 23, Department of Engineering Physics, The Australian National University,

Canberra, Australia. Carden, P.O. and Williams, O.M. (1978). Int. J.

Energy Research 2, 389-406. Williams, O.M. (1978). Solar Energy 20, 333-342. Williams, O.M. (1981). Solar Energy 27, 205-214. Williams, O.M. and Carden, P.O. (1979). Int. J. Energy Research 3, 29-40. SUMMARY OF THE INVENTION

A disadvantage of the system described in our earlier patent is that the fluid lines external to the synthesis terminal line operate at uneconomically high pressures - that is, pressures which are so high that pipe lines have to meet design standards which cause engineering and economic problems - suitable materials are costly, walls have to be thick and joint problems are encountered. As a result of the thickness of the walls of the absorbers necessitated by the high pressures, heat transfer problems occurred.

Furthermore, the reaction kinetics of dissociation are not favoured by high pressures (although synthesis is) .

In the existing system inefficiencies arose due to the need for conducting the heat from the synthesiser into the working fluid of a conventional heat engine i.e. a steam engine.

The objects of the present invention are to overcome the aforesaid disadvantages.

It has been discovered that we improve our existing system by taking work directly from the product stream by expansion turbines instead of 0 ^•>.

extracting sensible heat by conduction.

The present invention involves a new concept whereby the work output is obtained directly from the thermochemical fluid stream by the use of turbines and conversion of the entire exothermic terminal is effected into a heat engine, the rejected waste heat of which is the heat dissipated by the isothermal compressor. Compared to turbines used conventionally for gas and steam, the turbines for direct work recovery operate with quite dense fluids because they do not exhaust to atmospheric or sub atmospheric pressure. Consequently they are unusually compact and can operate at higher speed if this is desirable so as to further reduce size. Thus the present invention concerns an improvement or modification of the invention of our Australian Patent No 495395, the disclosures of which are incorporated by cross-reference. BRIEF DESCRIPTION OF THE DRAWINGS Further details of the invention may be had by reference to the accompanying drawings in which:- with regard to Figure 1 recompression is accomplished by a multistage compressor. Figures 2 and 3 show how the configuration of Figure 1 may be modified to allow for the operation of the fluid lines and endothermic 0 to

terminal at the lower pressure and are examples of the present invention and a modification of Figure 1. DETAILED DESCRIPTION OF THE DRAWINGS

Turning to Figure 1, interstage coolers and an after cooler ensure that compression is practically isothermal. If two phases are involved, as in the case of the ammonia system, the products from each cooler will include a liquid condensate. This must be removed by passing the products into a separator, a vessel large enough to allow the liquid to settle out. The liquid is then compressed separately by a pump. The gas from the separator is used as the feed for the next stage of compression. Thus each stage of compression tends to be smaller than its predecessor because the quantity of gas fed to the stages is successively reduced. An important practical criterion is that condensation must not occur within any of the compressor stages since high speed droplets may cause damage. The fluid property relevant to this criterion is ΔSd/ΔP, the ratio of the increase of dew point entropy to the corresponding increase in pressure, using the saturated fluid state at the input to the compression stage as reference. It is impossible for condensate to form if Δsd/Δp is negative or zero. This is the case for the ammonia system since over the entire regime of interest ΔSd/ΔP is negative (of the order of -1E-5 Kcal/K.Atm.moleNH ).

Considering Figures 2 and 3 which show how the configuration of Figure 1 can be modified to allow for the operation of the fluid lines and endothermic terminal at the lower pressure. This has certain advantages as detailed in the section on trade-offs. The configuration of Fig 2 shows two compressors in parallel, one for saturated vapour and one for the dry gas resulting from fairly complete dissociation at the endothermic terminal. This arrangement allows maximum utilisation of the fluid lines because little of the ammonia is returned from the endothermic terminal undissociated. It also offers high efficiency since losses due to mixing saturated and unsaturated vapours are avoided.

In the configuration of Fig 3 dissociation is less complete. Ideally the mixture returning from the endothermic terminal corresponds to the stream j emerging from the cool side of the heat exchanger

(saturated vapour at ambient temperature) . The two streams are joined and compressed with the one compressor there being no mixing loss. The main advantage of this scheme is that the amount of dissociation catalyst is further reduced because of

incomplete dissociation whilst high efficiency is maintained.

In neither of the arrangements 2 and 3 are the reaction extents shown for dissociation mandatory. Both arrangements are tolerant of fairly wide variations in this reaction extent. FIG CAPTIONS

Fig 2 and Fig 3. Variations of Fig 1 (exothermic terminal for direct work recovery) which allow low pressure fluid lines.

Typical reaction extents are shown. Other quantities shown indicate the relative mass flows of ammonia (in liquid and vapour form), nitrogen and hydrogen (masses combined) , and the total for all components (indicated by the sum sign) .

Claims

1. A solar energy collection system in which a fluid dissociates endothermically and subsequently recombines exothermically into an undissociated fluid, comprising: a) means for introducing the undissociated fluid, at elevated temperature, into a absorber-reaction vessel, b) means for focussing solar radiation onto the absorber-reaction vessel, c) means operable from a master control for controlling the orientation of the means for focussing solar radiation, d) means for conducting the fluid from the absorber-reaction vessel through a first counterflow heat exchanger to effect cooling, e) means for conducting the fluid to a reaction chamber wherein partial recombining of the dissociated products occurs with release of heat energy, f) a separator for separating undissociated fluid and dissociated fluid, g) means for returning dissociated fluid to the reaction chamber, h) means for returning undissociated fluid to the first heat exchanger so as to heat it prior to entry into the absorber-reaction vessel, i) means for maintaining the fluid at superatmospheric pressure, j) means for recovering heat energy from the reaction chamber.

2. A solar energy collection system in which a fluid dissociates endothermically and subsequently recombines exothermically into an ^"undissociated fluid, comprising: a) means for introducing the undissociated fluid, at elevated temperature, into an absorber-reaction vessel, b) means for focussing solar radiation onto the absorber-reaction vessel, c) means operable from a master control for controlling the orientation of the means for focussing solar radiation, d) means for conducting the fluid from the absorber-reaction vessel through a first counterflow heat exchanger to effect cooling, e) means for conducting the fluid to a reaction chamber wherein partial recombining of the dissociated products occurs with release of heat energy, f) a separator for separating undissociated fluid and dissociated fluid. ll

g) means for returning dissociated fluid to the reaction chamber, h) means for returning undissociated fluid to the first heat exchanger so as to heat it prior to entry into the absorber-reaction vessel, i) means for maintaining the fluid at superatmospheric pressure, j) means for recovering heat energy from the reaction chamber, whereby the work output is obtained directly from the thermochemical fluid stream by the use of turbines and conversion of the entire exothermic terminal is effected into a heat engine the rejected heat of which is the heat dissipated by the isothermal compressor.

3. A system as claimed in either of claims 1 or 2 substantially as hereinbefore described and illustrated.

4. All novel features hereinbefore disclosed either separately or collectively.