A new version of the large pressure jump (T-LPJ) method for dynamic study of pressure-initiated adsorptive cycles for heat storage and transformation
Introduction
Exhaustion of fossil fuels and increased demand for heating and cooling makes adsorptive heat storage and transformation (AHST) driven by low-temperature heat as an energy and environment saving alternative to the common absorption and compression systems [1,2]. Two types of the cycles suggested for AHST essentially differ by the way of adsorbent regeneration:
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by adsorbent heating up to the temperature sufficient for adsorbate removal;
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by reducing the adsorptive pressure over adsorbent at a constant temperature.
The first, temperature-initiated (TI), adsorptive cycles are very common and widely used to realize cooling and heating modes [3,4]. The second, pressure-initiated (PI), adsorptive cycles are much less spread and were suggested mainly for the temperature upgrading mode [5,6]. The first dynamic comparison of TI and PI cycles has recently been reported in Refs. [[7], [8], [9]], however, more experimental and numerical studies are still necessary.
A typical PI cycle consists of two isosteres and two isotherms (Fig. 1). The initial adsorbent state (point 1 in Fig. 1a) corresponds to temperature ТM and pressure of the adsorptive vapour РL = Р0 (ТL), where Р0 (ТL) is the saturation vapour pressure at temperature ТL. A weak isostere is characterized by the equilibrium adsorbate content w1 = w (ТM, РL). Then, the adsorbent is heated up to temperature ТH (stage 1–2) at constant uptake w1. At point 2, an adsorber is connected with an evaporator maintained at ТM that is the temperature of an external heat source which temperature level to be upgraded. The evaporator generates the constant pressure РM = Р0 (ТM) of adsorptive. This pressure jump Р2 → РM causes the vapour adsorption that leads to an increase in the equilibrium adsorbate content up to w2 = w (ТH, РM) (point 3 in Fig. 1a). The heat of adsobate evaporation Qev is absorbed in the evaporator at T = TM and the useful adsorption heat Qads is released at constant temperature ТH (isotherm 2–3) in the heating circuit of a consumer.
Then, the adsorber is disconnected from the evaporator and cooled down to the intermediate temperature ТM (isostere 3–4) at constant uptake w2. At point 4, the adsorber is connected with a condenser maintained at temperature ТL and pressure РL = Р0 (ТL). This pressure drop Р4 → РL results in the adsorbate desorption to restore the initial uptake w1 = w (ТM, РL) (point 1). The heat Qdes needed for adsobate desorption is supplied to the adsorbent at temperature ТM from the external heat source (isotherm 4-1). The excess of adsorbate is collected in the condenser releasing the heat Qcon to the ambient and the cycle is closed. This cycle was considered in Refs. [5,10] for upgrading industrial waste heat and suggested in Ref. [6] for increasing the temperature potential of the ambient heat to the level sufficient to heat dwellings (the so-called HeCol cycle).
The useful heat can also be released at variable temperature T > ТH (route 2–3′-3): after connecting the adsorber with the evaporator at point 2, the heat of adsorption is released and the adsorbent temperature increases up to Т(3*), if all thermal masses are equal to zero. If the thermal masses are non-zero, the adsorbent temperature increases up to Т(3′) < Т(3*), because a part of the adsorption heat is spent for heating the adsorbent and the heat exchanger. After that, the rest of adsorption process (3*-3) or (3′-3) is isobaric (at P = PM) and proceeds until the equilibrium is set at point 3 corresponding to (ТH, РM). This non-isothermal cycle allows supplying a consumer with a smaller quantity of heat, however, of higher quality – at T > ТH (Fig. 1a). This cycle was realized and studied in a first prototype of the HeCol heat transformer [[11], [12], [13], [14]].
Sorption process in a PI AHST unit is initiated by a fast variation of the sorptive pressure. Therefore, the Large Pressure Jump (LPJ) [15] method was proposed for experimental studying adsorption dynamics in AHTS, initially being suggested as a volumetric (V) version. The V-LPJ is based on recording a small change of vapour pressure over the adsorbent, which is further used for calculating the instantaneous water uptake/release [15]. The main intrinsic limitation of the V-LTJ method is a small mass of adsorbent (0.5–1.0 g) that can be tested, so that only the simplest, flat layer, configuration of the adsorbent bed was studied. It is quite far from realistic configurations applied in modern well-designed AHST units [[16], [17], [18], [19], [20], [21]].
To overcome this obstacle, a new version of the LPJ method (called thermal LPJ or T-LPJ) is proposed here for lab-scale evaluation of the AHST dynamics. It allows enormous reduction of time and material resources necessary for testing and evaluation of real AHST units. It repeats the standard methodology of measuring their specific power but is applied to much smaller, yet representative, pieces of real “adsorber - heat exchanger” (AdHEx) units. The T-LPJ is the simplest and ultimate version of the LPJ method. It is also very economic because it does not need any expensive and sophisticated equipment, like a high precision pressure sensor for V-LPJ. Validation of the new approach is made by comparison with the literature data on the methanol adsorption heat [22] as well as on testing a prototype of the HeCol unit with the same adsorbent under the same boundary conditions [12].
Section snippets
Experimental
The adsorbent. A commercial silica gel Grace Davison 8926.02 was used as a host matrix for the synthesis of a composite sorbent LiCl/silica tested in this paper. Lithium chloride was purchased from Aldrich and used as delivered. The silica gel was dried at 473 K for 2 h, impregnated with an appropriate amount of aqueous LiCl solution to complete filling of the pores and subsequently dried again at 423 K until the sample weight became constant. The salt content was 21 mass. %. Textural
Results and discussion
We consider below a temporal evolution of the HCF temperature Tout at the adsorber outlet and corresponding temperature difference ΔT = Tout - Tin for the adsorption and desorption stages of the cycle described above (Fig. 1b). These conditions are typical for HeCol cycles and were used for testing the first HeCol prototype in Ref. [12].
Conclusions
Adsorptive heat storage and transformation (AHST) driven by waste or renewable heat is a promising technology to decrease primary energy consumption. In this paper, a new version of the Large Pressure Jump (LPJ) method for studying the dynamics of ad/desorption initiated by jump/drop of adsorptive pressure over the adsorbent is suggested and verified. On the contrary to the common volumetric LPJ version (V-LPJ), the new approach (T-LPJ) can be applied to investigate the dynamic behaviour of
Acknowledgements
The authors thank the Russian Science Foundation for financial support of this study (grant N 16-19-10259).
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