key: cord-0757578-qtxukjdk
authors: Lo Basso, Gianluigi; de Santoli, Livio; Paiolo, Romano; Losi, Claudio
title: The potential role of trans-critical CO(2) heat pumps within a solar cooling system for building services: The hybridised system energy analysis by a dynamic simulation model
date: 2020-09-21
journal: Renew Energy
DOI: 10.1016/j.renene.2020.09.098
sha: 6ae16cc979535c1f6788a31dbe87e5d6ad9de558
doc_id: 757578
cord_uid: qtxukjdk

The rotary desiccant wheels application in the air conditioning systems are used for the air dehumidification by means of hygroscopic layers for water vapor adsorption. Nevertheless, external heat sources are required for water desorption to close the air treatment cycle. This paper investigates on the possibility to integrate in that cycle a new component, such as the trans-critical CO(2) heat pump, to reduce the contribution of external thermal sources. In so doing, the high temperature waste heat discharged by the heat pump hot sink can be fruitfully exploited. Additionally, a PV array has been added to the typical layout based on the solar collectors, in order to assure the heat pump electrical driving. The energy analysis is carried out by calculating the energy performance indicators of the whole cooling system, simulating it by a dynamic model built in the MATLAB SIMULINK environment. Specifically, an air handling unit has been properly sized to supply cooling load to a reference conference hall of 1200 m(3), with changes in boundary conditions (i.e. solar radiation, daily temperature and relative humidity variations). Indeed, three different cities representing the most typical Italian climatic zones, have been considered for assessing the proposed technical option suitability.

It is well known how the building sector is responsible for approximately 21.6% of the global primary 28 energy consumption over the European Countries [1] . For that reason, interventions on the building envelope 29 allow reducing the energy need of existing buildings by enhancing their energy performance. In this 30 framework, increasing the energy efficiency of existing buildings is a great challenge to cope with, by Sources) have to be applied as much as possible in the next-generation buildings, the solar thermal power 42 can be fruitfully exploited to provide the so-called thermal cooling. By that technique, it is possible to have 43 the cooling effect without mechanical work and electricity consumption once alternative materials have been 44 used. Moreover, the low-grade heat hailing from the sun can be useful to drive totally or partially such 45 devices. On the basis of that working principle, various technical solutions and plant layouts have been 46 developed in several international research projects. Reviewing the scientific literature on that topic, it 47 6 dynamic simulation tools on the basis of First and Second Law of thermodynamics. Thus, Almohammadi 137 et al. [34] proposed an innovative SDACS (solar powered adsorption cooling system) with three axial 138 finned tubes heat exchangers connected in parallel. They tested on field a demo version able to 139 accomplish a COP equal to 0.52 after the operating parameters optimization by the multi-objective 140 genetic algorithm application. Finally, other authors investigated on the possibility to integrate within 141 solar cooling closed cycles PCMs (Phase Change Materials) for storing thermal energy [35] . On that 142 topic, a recent work of Loubani et al. [36] proposed an interesting option to figure out the challenge of 143 free cooling for humid climates at night-time. Indeed, they integrated moderate melting point PCMs 144 within rooms ceiling, along with the evaporative cooling system and desiccant wheel. The most original 145 aspect of that work lies in the use of Trombe Wall for storing solar energy over the day to regenerate the 146 desiccant wheel. Finally, it is worth of highlighting the solar chimney concept for increasing the closed 147 cycles efficiency. By that technique it is possible to cool down the PV modules for driving an electric 148 water source heat pump, and to guarantee high coefficient of performance of that machine [37] . The 149

working principle consists of spraying water parallel to the downward airflow, within a channel which is 150 created by modifying the PV module back shield. In that channel, part of sprayed water evaporates 151 cooling down the air; the remaining water drops down to the channel bottom side and it is stored in a 152 small vessel. Since the PV module receives solar radiation, its surface temperature increases heating up 153 the air in the back shield, so that the air circulation is favoured. At the end of that process, the PV 154 module is cooled down by the air and the stored water can be delivered to the HP condenser. 155

As regards the open cycle systems, which typically include rotary desiccant wheels, they became 156 increasingly attractive owing to their low electric energy consumptions. However, their efficiency rarely 157 overcome the threshold value of 0.5, inasmuch the energy performance is strictly correlated to the 158 evaporative cooling section efficiency and to the saturator effectiveness. different Italian cities and in low latitude isolated islands. In the first work [42] the authors evaluated the 173 system energy and economic performance by varying the solar collectors technology. In the second one, 174 using the same methodological approach, a plant hybrid version, including PV array, was proposed. In so 175 doing, it was possible to face the big challenge to efficiently provide cooling power in all those remote 176 areas characterised by extremely hot and humid conditions. The plant hybridization generally requires 177 more available surface. As a consequence, several research projects dealt with the potential application 178 in desiccant cooling system of both water-cooled and air-cooled PV/T technologies [44] [45] [46] . 179

Unfortunately, those devices are still expensive, and they are not yet beneficiary of incentive schemes, so 180 that their adoption can be not cost-effective. Finally, it is worth of noticing that other technical options 181 are suitable for thermally regenerating the desiccant materials. 182

In accordance with literature, the Figure 1 Specifically, they proposed a new desiccant wheel system with closed loop air regeneration. The air 211 regeneration process occurred inside a closed loop and a trans-critical CO 2 HP was utilized for the 212 recycled air regeneration including dehumidification, cooling and heating process. Moreover, an air heat 213 exchanger was inserted to pre-cool the recycled regeneration air, which decreased the evaporator cooling 214 load. Having fixed the saturation temperature at the evaporator equal to 10 °C and the discharge pressure 215 of 8 MPa, the COP rose up to 6.5. By adopting that solution solar collectors or other thermal cascades 216 are unnecessary and only PV arrays can electrically drive the HP. 217

After this premise, the main aim of this paper is to assess the suitability of a hybridised version of the 218 solar-assisted desiccant cooling plant. By integrating different well-proven and commercial technologies, 219 the authors investigate on the possibility to substitute some system components so as to produce a fully-an indirect evaporative cooling module and a chiller [42] , the integration of trans-critical CO 2 HPs has 222 been proposed. In so doing, it is possible to lessen, on one hand, the external heat sources contribution, 223 on the other hand, to generate additional electricity by PV modules for the building needs. By the 224 dynamic simulation modelling, the energy performance, referred to three different Italian cities, along 225 with some key performance indicators useful for HVAC designer, have been presented and discussed. 226

Finally, the main operating parameters values of a commercial water-source CO 2 HP have been analysed 227 for that purpose. 228 229

Carbon dioxide is a safe, economic and sustainable refrigerant which can be suitable for heat pumps and 231 cooling systems. Due to its thermophysical properties CO 2 has good heat transfer characteristics, working 232 within a wide operating temperature range. Indeed, starting from supercritical conditions, the CO 2 233 temperature can rapidly pass from 180 ° C to almost 0 ° C allowing to get high energy performance for heat 234 pump systems. The trans-critical CO 2 HPs coefficient of performance can reach also very high levels, owing 235 to the possibility to use small compressors, mitigating the energy losses associated to the working fluid 236 pressurization [55,56]. Yet, there are two main issues to take into account when CO 2 is used as a refrigerant: 237 the first one concerns the high operating pressure, (i.e. beyond the critical pressure of 73.75 bar), and the 238 second one is the low critical temperature, since the carbon dioxide reaches its critical point at a temperature 239 of 304.13 K (31.1 ° C). Figure 2 shows the typical cycle transformations on the P-h plane. Compared to the 240 most common refrigerants, carbon dioxide-based equipment requires a carefully designed system to cope 241 with the peculiar characteristics of operating temperature and pressure. Referring to the thermodynamic cycle shown in Figure 2 , the basic layout of a trans-critical CO 2 HP is 246 composed by an evaporator, a high pressure compressor, a gas cooler, an internal heat exchanger (IHEX) and 247 the expansion valve (see Figure 3 ). It is noteworthy that in the supercritical region, the temperature and 248 pressure values are independent properties, hence the CO 2 temperature downstream the gas cooler it is not 249 correlated to the inner pressure level. Notwithstanding, that temperature is one of the most important 250 parameters affecting the HP efficiency, because it determines the heat release amount. Moreover, to get 251 better energy performance, the optimization process aims at indicating what is the best gas cooler outlet 252 temperature corresponding to the maximum operating pressure. The heat transfer mechanism within that component is also very different from the traditional condensing 257 process associated to the use of typical refrigerants. Indeed, in the supercritical region, the phase-change 258 cannot be distinguished, so that the transformation is not isothermal. Consequently, the heat transfer occurs 259 by sensible cooling and the carbon dioxide undergoes a wide temperature difference. 260

For that reason, the trans-critical CO 2 HPs are very suitable for high temperature applications [58] and they 261 can be also used for boosting low temperature distribution networks, as reported in [59] . Anyway, the best 262 energy performance can be accomplished as the gas cooler outlet temperature is the lowest. That implies a 263 wide temperature difference for the secondary fluid, which is the end-user' energy carrier. Therefore, for 264 building applications the gas cooler is partitioned very often. That technical choice allows to serve separately In the trans-critical CO 2 cycle the pressure drops are generally very high, i.e. from 140 bar to 35 bar, in the 294 major part of applications. That entails very large expansion losses, which could be mitigated by substituting 295 throttle valves with turbo expanders. In such a way, the mechanical work recovery is more feasible and 296 beneficial. Indeed, the expander integration leads to a reduced optimum gas cooler pressure as well as to an 297 improved COP value of 33% more [64] . 298 expander for mechanical work recovery. For instance, rotary, reciprocating, vane, screw and scroll are 300 suitable option in addition to turbines. However, each design solution is characterised by its own advantages, 301 drawbacks and limitations. Figure 5 shows three different layouts integrating turbo expanders able to drive 302 the compressors. highest COP (i.e. 3.52 instead of 3.49 for a) and 3.16 for c)) and the optimised maximum pressure equal to 306 101.3 bar. Additionally, they demonstrated that an internal heat exchanger could be incorporated but it is not 307 so effective when work recovery is implemented. 308

In the end, it is possible to state that recent researches on those machines are showing the trans-critical CO 2 309 cycles suitability, since they can perform well for water and air heating applications. For that reason, the 310 authors decided to integrate this technology within the solar cooling plants in order to evaluate the potential 311 benefits hailing from the energy system hybridization. 312 313

The solar cooling HVAC systems can be considered as a special application of the DEC systems (Desiccant 315

Evaporative Cooling), in which thermal renewables are involved to heat the air stream for water desorption 316 from the desiccant layer. The operating principle of such plants has been depicted in Figure 6 , where EH 317

indicates the electric heater, ES is the thermal energy storage, ECs are the evaporative cooler and REG 318

represents the regeneration coil. The dehumidification process does not occur by water condensation, due to 319 the air stream flowing through a cold heat exchanger. Moreover, the overall cooling effect can be 320 accomplished combining the IEC (indirect evaporative cooling) system with the sensible cooling provided by 321 an electric chiller. Given that the desiccant wheel has to be regenerated, the most common solar cooling 322 The unconventional AHU has been designed to meet the energy need of a reference room located in three 335 different Italian cities. 336

According to previous works, that approach is useful to identify criticalities and suitability of these plants 337 when they are built in different climatic zones. In this way, it is possible to account for performance 338 variations caused by low and high relative humidity as well as outdoor environmental temperature. 339

A wide room, characterized by only two dispersing surfaces, has been assumed as reference case for cooling 340 load calculations. Specifically, it is a 300 m 2 conference hall of floor surface, with a height equal to 4 m, 341 which is designed for 200 people inside. 342

On the basis of Italian standards, such as UNI 10339, and building data analysis [66] the air flow rate for 343 ventilation has been set equal to 8000 m 3 /h and no recirculation is required. Consequently, the ventilation 344 rate is equal to 6.66 vol/h. Since for that end-use the cooling load is mainly due to inner heat generation and 345 partly to the solar irradiation, it has been considered almost constant over the simulation time. Table 1 . accordance with the stored water maximum temperature range, which have been assumed as external 397

As regards the PV array, starting from the electricity need for driving the backup electric heater and HPs, the 399 installed capacity has been deduced in a first approximation by the equivalent hours' values. Specifically, 400 only the equivalent hours referred to the summertime have been considered to compute the PV array peak 401

Then, by the dedicated simulation model described in in Supplementary Material section, the more detailed 403 been reported. Since the trans-critical CO 2 HP has been integrated within the solar cooling system, the first 408 analysis has been carried out in order to optimise the HP energy performance. To do so, for each case study 409 the COP values have been evaluated by varying the CO 2 pressure within the gas cooler component, having 410

fixed the minimum CO 2 outlet temperature. It is important to highlight how that temperature is strictly 411 related to the maximum inlet air temperature into the desiccant wheel regenerator. From an energy point of 412 view, it would be possible to shrink the HP rated cooling power output by increasing the recovery heat 413 exchanger effectiveness. In such a way, the indirect evaporative cooling system contribution can grow. 414

However, that choice penalises the heat pump COP values, affecting also the optimal discharge pressure as 415 well as the CO 2 outlet temperature from the gas cooler, as shown in Figure 8 . It is worth of noticing that all of data depicted in Figure 8 refer to the HP performance analysis associated to 420

Rome climatic conditions. Additionally, since the HP cooling power decreases as the effectiveness values 421 enhances, a lower heat amount can be discharged to the hot heat sink. Therefore, the required heat for 422 regenerating the desiccant wheel must be provided by more solar collectors or by the electric heater 423

By a qualitative energy analysis, it is possible to discuss the main thermodynamic effects on the whole solar 425 cooling cycle deriving from the trans-critical CO 2 heat pumps application. To do so, the overall system COP 426

can be calculated combining Equation 1 and Equation 2, and it reads as: 427

Additionally, the Heat Recovery Fraction (HRF) has been introduced in order to account for the HP 430 contribution to the regeneration heat by the Equation 6. When T air,HP is equal to T air,reg the heat recovery fraction is 100%, therefore the solar cooling system 458 efficiency is dependent only on the selected HP technology, since other external sources are unnecessary.

the Equation 10, it can be noticed how reducing ΔT r-a and increasing T ev some additional energy gain can be 462 attained. In that case, heat transfer takes place directly between the refrigerant and air, and the temperature 463 difference between the heat sinks will be even smaller. However, the main features to address by designers 464 are the heat exchangers size, constructive technology and materials, especially for trans-critical HPs, seeing 465 that in the gas cooler the phase change does not occur. 466

Having said this, some authors proposed the air precooling upstream the desiccant wheel in order to improve 467 the moisture removal capacity, dehumidification efficiency, refrigeration capacity and enthalpy efficiency 468 concluded that, under those climatic conditions, the best COP value was achieved when the indirect 471 evaporative cooler was used for the air precooling instead of using a chiller, even if the water consumption 472 was increased. However, from those studies it emerged how such an option can be effectively adopted in all 473 those areas characterised by high-temperature and high-humidity environments. On the basis of that concept, 474 the selected layout for this work could be modified by moving backward the HP evaporator, before the 475 desiccant wheel. In that case, since the inlet air temperature is lower, the HP could get better performance in 476 terms of COP. Yet, the most common water-source CO 2 HPs commercial versions are able to produce chilled 477 water at 7 °C, entailing a typical saturation temperature equal to 0 °C. Therefore, for that purpose, HPs 478 characterised by saturation temperature at the evaporator equal to 8-10° are required. That interesting option 479 has not been addressed here by the authors, but it will be reasonably explored in further developments of this 480

Referring to Figure 9 , it is possible to note that the best performance can be achieved when the gas cooler 482 pressure is equal to 140 bar. 483 484 Figure 9 -Trans-critical CO 2 HP Coefficient of Performance vs. Gas cooler pressure with changes in geographic area 485 and optimised CO 2 outlet temperature from the gas cooler minimise the electrical input power which is provided by PV arrays. 490

In such a way, it is possible to save roof surface for installing the electrical renewable source. In order to size 491

properly the PV array, the net metering option has been accounted for. In detail, only the equivalent hours 492 related to the cooling season have been used in calculations for determining the PV receiving surface. 493

As a consequence, a renewable electricity excess occurs over the building heating season, which could be 494 exploited to partially meet the overall building electricity need. 495

The reference outdoor environmental conditions, provided by the current standards, lead to an 496 underestimation of the required heat for desiccant wheel regeneration. That is due to the lower value of 497 external actual temperature over the plant operational hours, compared to the reference one. 498

For that reason, to reach the required regeneration temperature more thermal power has to be supplied. 499

Consequently, to analyse the actual hybrid plant behaviour, dynamic simulations has to be performed. 500

Referring to Milan environmental conditions registered in May and August (see Fig. 10 and Fig. 11 ) it is 501 possible to note that the thermal power for regeneration is higher in May, even if the cooling power is almost 502 absent. In that case, the required thermal power is just above 100 kW, while in August it ranges between 87 503 kW and 98 kW. From data it emerges that the desiccant wheel operates to dehumidify, and the heating 504 ventilation system runs basically in free cooling mode. Furthermore, the evaporative cooling systems are 505 enough to keep under control the supply air temperature as well as the absolute humidity to the design 506

Conversely, during the hot season the higher outdoor temperature affects positively the heat recovery 508 exchanger effectiveness, reducing the exhaust air temperature difference in the regenerator. Thus, cooling 509 power is generally high, and it is partially provided by the trans-critical CO 2 HP. Fig. 12 ). The required cooling power is almost null, and 517 the ventilation systems runs in free cooling mode similarly to the Milan case study. Simulating the August 518 time period, it can be noticed how the maximum cooling power is higher (i.e. 28 kW), while the regeneration 519 power goes down up to 78 kW approximately, as reported in Figure. 

When the dynamic simulations have been performed for Palermo, which is a city located in the southern part 526 of Italy, the peak regeneration power in May is quite similar to the thermal power related to the Rome case 527 study, i.e. 100 kW, but the physical dehumidification process occurs more often. Obviously, since Palermo 528 belongs to the hot climatic zone the cooling power increases over the whole simulation time period, up to 30 529 kW. It is worth of noticing that to simulate properly the solar cooling operation over the middle season the 530 control strategy for load following (in terms of temperature and absolute humidity) must be implemented. 531

Indeed, if an adjusting system was not implemented, simulations will run from a mathematical point of view, 532 but results will be not reliable, and they could not have any physical sense. Depending on the inner space end use, the building cooling load is strongly related to the solar irradiation 539 and to the internal sensible and latent heat generation. According to literature [72], buildings may be chilled 540 even if the outdoor environmental temperature is lower than the indoor one. Having said this, three different 541 conditions can occur: the first one consists of an outdoor temperature lower than the supply one; the second 542 case is related to the external absolute humidity value lower than the supply absolute humidity; the last one is the contemporary occurrence of both previous cases. In Figure 16 , the air transformations during the control 544 application have been reported, while bypasses and recirculation for hot and cold air have been depicted in 545 Table 2 . Having said, the dynamic simulations have been performed for both the traditional and hybrid solar cooling 570 systems over the whole time period, i.e. starting from May to September. Figure 18 reports the thermal and 571 electrical energy balance associated to the traditional solar cooling layout when it operates in three different 572 climatic zones. 573

From data it emerges that the cooling load for the air treatment in Palermo is almost 80% higher than in 574

Milan, while the enhancement in the required thermal power for the desiccant wheel regeneration is limited 575 to 19.3%. 576

It is important to point out that the regeneration heat is independent of systems layout, but it is correlated to 577 the outdoor environmental conditions. Furthermore, that heat, in the reference system, is provided only by 578 solar collectors and by an electrical heater which is PV driven. 579

Referring to the energy balance, the sum of those amounts is higher than the regeneration heat due to the fact 580 that the storage device energy losses has been taken into account. The same approach has been used to 581 analyse the hybrid solar cooling system performance. Figure 19 depicts clearly how the heat recovery hailing 582 from the trans-critical CO 2 HP can contribute to increase the solar cooling thermal COP value, allowing to 583 save the roof surface. The HP heat fraction for Milan, Rome and Palermo are equal to 15.9%, 23.52% and 584 24.57%, respectively. That entails a reduced number of solar collectors and consequently a lower thermal 585 energy to provide by external source from the cycle. On the basis of those outcomes, it is possible to state 586 that the hybridization option by integrating the trans-critical CO 2 HP is more suitable where high cooling 587 loads for the air treatment are needed. Specifically, when the evaporative cooling is not enough to keep under control both supply absolute 595 humidity and temperature, higher sensible heat has to be subtracted from the air stream by means of the HP. 596 Therefore, the larger the cooling load, the larger the recovered heat from the HP is. In such a way, owing to 597 the leverage effect of the heat pump COP, even if more electricity is required, the solar cooling thermal COP 598 increases. Since the heat pump is fed by the PV array the hybrid solar cooling system is still completely renewable. In Table 3 Finally, Table 5 summarises the renewable electricity production occurring during the winter season which 625 hails from the PV arrays. It is notable how Milan and Palermo are characterised by very close values owing 626 to comparable electricity off-takes of heat pump and electrical heater. Conversely, only 11.06 MWh/y can be 627 used in Rome in order to increase the building renewable electrical fraction. Nevertheless, since Rome case study shows the higher roof surface saving, it should be possible to oversize the PV array to increase its 629 yearly capability for that purpose. 

In this paper a preliminary energy analysis on the possibility to hybridise a solar cooling HVAC system has 636 been presented. A MATLAB SIMULINK model has been built and dynamic simulations when outdoor 637 environmental conditions change have been performed. The main findings can be outlined as follows: 638

• The trans-critical CO 2 HP maximum operating pressure has to be 140 bar approximately in order to 639 maximize the COP value, i.e. 2.4. In that way it is possible to transfer heat from hot carbon dioxide 640 to the exhaust air stream effectively; 641

• Dynamic simulations are the best solution to properly size the solar cooling system since the 642 absolute humidity plays a key role. Indeed, the system design based on the reference outdoor 643 environmental conditions leads to the energy needs underestimation and the HVAC should not be 644 able to keep under control the inner space comfort values; 645

• The solar cooling hybridization by the trans-critical CO 2 HP and PV arrays shows the best results 646 when it is applied in hot climatic zone, accomplishing energy gain up to 38% more and saving roof 647 surface due to receiving surfaces reduction of 21% less, such as in the Rome case study. 648

• The control strategy implementation leads to significant energy saving for the regeneration process, 649 reducing also, the water consumption related to the DEC and IEC units integrated in the power plant. 650

In detail, for Rome and Milan case studies the achieved energy benefits are equal to 66.6% and 64% 651 respectively, while the associated water demands can be reduced up to 54.1% and 55.6%. Palermo 652 case study results less sensitive to the adjusting system operation compared to the other cases. 653 hybrid system time response. In the end, a graphical tool for a fast sizing process should have to be 657 developed so as to help HVAC designers during the technical feasibility analysis. Additionally, the hybrid 658 solar cooling plant seems to be a promising alternative solution for heading towards the NZEB qualification 659 for buildings and to totally produce green cooling. 

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