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Innovative technologies in HVAC&R systems for energy saving and environment preservation
Get PDFEnergy savings and the environmental impacts of energy systems are distinctive issues which have driven scientific research in heating, ventilation and air conditioning systems over the last couple of decades.
The main reasons are increasing energy costs and concerns related to negative environmental impacts, in particular anthropic greenhouse effect. There are two contributors to the greenhouse effect for refrigerating systems. The first is due to the primary input energy, in large part from fossil energy, that is, organic substances which produce carbon dioxide upon burning. Therefore, if the energy efficiency increases, this contribution will decrease.
The second contribution is directly due to the operating fluid or refrigerant; the principal halogenated refrigerant fluids utilized are strong greenhouse gases when they leak to the atmosphere.
In order to change these fluids there are two primary options being pursued. The first is the use of natural fluids: carbon dioxide, hydrocarbon, ammonia; the second is the use of new synthetic fluids such as HFOs (hydrofluoroolefins). Both of these solutions have lower (or zero) greenhouse impact relative to halogenated refrigerants. Unfortunately both approaches involve several efficiency and technological problems.
Another way to reduce the working fluid’s contribution is to decrease the system refrigerant charge, accounting for standard charge restrictions.
After an introductory chapter, this work presents an air-water commercial reversible heat pump with an aluminium louver-fin flat-tube minichannel heat exchanger, with a small internal volume, allowing for high charge reduction.
Further a user friendly code to simulate the minichannel heat exchanger working as an evaporator or condenser is described. The model allows the user to calculate the heat exchanger performance and to optimize the circuitry. The model has been used to validate and simulate an automotive air conditioning condenser working with the new HFO-1234yf fluid, including the effects of the presence of oil.
Later, three applications with natural fluids are investigated: the first is a low charge ammonia chiller for an air conditioning system; the second is a simple reversible water/water carbon dioxide heat pump working for a residential building winter heating, summer cooling and tap water production; the third is a carbon dioxide reversible heat pump working with an ejector as the throttling device for increasing the system efficiency.
Finally, a method to optimize circuit length in a finned coil evaporator and a comparison of fluid evaporating performance, in particular for the new fluids HFO-1234yf and HFO-1234ze(E), is described. The new HFO refrigerants benefit more from circuit optimization than do the halogenated refrigerants they are designed to replace.
In summary the goal is to show some solutions for chiller and heat pump systems in reference to the energy efficiency, charge reduction, and halogenated fluid substitution problems, taking into account that there is no univocal solution to these problems, but each application needs to be studied and optimized considering its own features.I temi dell’efficienza energetica e dell’impatto ambientale sono requisiti che sempre più negli ultimi decenni hanno guidato la ricerca scientifica riguardante i sistemi di refrigerazione, condizionamento dell’aria e riscaldamento.
Le principali ragioni sono: l’aumento del prezzo dell’energia con una conseguente maggiore attenzione ai costi di esercizio e la necessità di ridurre i contributi indiretto e diretto all’effetto serra antropico. Il contributo indiretto è legato all’energia primaria utilizzata, nella maggior parte dei casi energia fossile quindi combustione di sostanze organiche con produzione di anidride carbonica, attualmente il principale gas serra; per ridurre questo contributo è necessario incrementare l’efficienza energetica del sistema.
Il contributo diretto è dovuto ai refrigeranti, i fluidi alogenati attualmente utilizzati sono fortemente gas serra se immessi in atmosfera.
Si è indirizzati da un lato a sviluppare sistemi utilizzanti fluidi naturali: l’anidride carbonica (il cui contrito all’effetto serra, nettamente inferiore agli HFC, è 0 se si utilizza quella esistente), gli idrocarburi, l’ammoniaca; dall’altro lato si sono introdotti nuovi fluidi sintetici HFO a ridotto effetto serra. Si cerca inoltre di ridurre la carica di refrigerante del sistema, tenendo conto che in alcuni casi vi sono vincoli normativi proprio sulla carica.
Nella prima parte di questo lavoro si presenta l’utilizzo di uno scambiatore di calore a minicanali in una PDC aria-acqua commerciale; questo tipo di scambiatori a basso volume interno consente di ridurre considerevolmente la carica di refrigerante del sistema.
In seguito si presenta un modello di simulazione per il calcolo delle prestazioni e l’ottimizzazione di evaporatori e condensatori a minicanali ed una sua validazione/applicazione per calcolare un condensatore funzionante con fluido R1234yf tenendo conto anche della presenza dell’olio.
Si presentano poi soluzioni utilizzanti i fluidi naturali: un chiller ad ammoniaca a bassa carica, una PDC ad anidride carbonica per un edificio residenziale, ed uno studio sperimentale sull’uso dell’eiettore come organo di laminazione per incrementarne l’efficienza di queste PDC.
Si presenta infine un metodo per l’ottimizzazione della lunghezza dei circuiti negli evaporatori a batteria alettata ed un confronto delle prestazioni di diversi refrigeranti in evaporazione con particolare riferimento ai nuovi HFO R1234yf e R1234ze(E).
In sintesi l’obbiettivo di questo lavoro è quello di offrire diverse soluzioni che vadano nella direzione dell’incremento dell’efficienza, della riduzione della carica e dell’impatto ambientale, tenendo conto che non esiste una soluzione univoca a tali problemi, ma che ciascuna applicazione va studiata ed ottimizzata nelle proprie peculiarità
Experimental analysis of an air–water heat pump with micro-channel heat exchanger
No full textA multi-port extruded (MPE) aluminium flat tube air heat exchanger was compared to a round tube finned coil (FC). The MPE heat exchanger has parallel flow vertical tube configuration with headers in horizontal position and conventional folded louvred fins. The two heat exchangers were mounted on a 10 kW cooling capacity R410A packaged air heat pump. They were sized to approximately obtain the same cooling and heating capacities in chiller and heating mode, respectively. Climatic room steady state tests without frosting phenomena occurring during heat pump operation, demonstrated that the round tube and the flat tube heat exchanger performance are comparable. The MPE heat exchanger was tested with different refrigerant inlet distributor/outlet tubes configurations to investigate the effect of liquid refrigerant distribution. Cycling frosting/defrosting operations were tested with two equivalent machines placed in parallel outdoor and working at full load condition, one of the units was equipped with the MPE heat exchanger while the other mounted a standard finned coil. Penalization factors were analytically introduced to evaluate frosting associated heating energy and energy efficiency degradation. Test results indicate that both the heat pumps are penalized by frost formation but both the penalization factors are higher for the MPE-unit than the FC-unit one in the -6 to 4 °C air dry bulb temperature range. For the two units, a roughly linear dependence of the heating energy penalization factor and of the energy efficiency factor from the difference between outdoor air and saturated air at the evaporation temperature humidity ratio can be pointed out
Energy efficiency of a reversible refrigeration unit using R410A or R32
No full textThe aim of this article is twofold: first, to compare the performance of R410A with that of R32, which is considered a possible HFC substitute with lower global warming potential (675 instead of 2088); second, to exploit the effect of the circuit length in the finned coil of a packaged air-to-water reversible unit with given water plate heat exchanger and scroll compressor. Both scopes are pursued through the analysis of a case study, a system with nominal cooling capacity of about 74 kW at 35◦C dry-bulb outdoor air temperature and a nominal heating capacity of about 70 kW at 2◦C dry-bulb and 1◦C wet-bulb ambient temperature. The performance of the two refrigerants inside a “real” machine is simulated by means of an advanced numerical model of a packaged reversible refrigeration unit. The system consists of a single refrigerating circuit with two identical scroll compressors. The compressor was characterized by its experimental performance curve according to Standard EN 12900 (EN 2013a) for both R410A and R32. Off-the-shelf copper tubes and louvered aluminum fins were considered for the condenser and typical brazed-plate heat exchanger for the evaporator of the chiller configuration. The finned coil heat exchanger was first thermodynamically optimized for R410A and R32 for both condenser and evaporator operation with regard to the number of internal circuits according to the total temperature penalization performance evaluation criteria (Cavallini et al. 2010; Brown et al. 2013), without changing the overall heat exchanger dimensions. The effect of finned coil circuit length on the performance of the investigated reversible unit with the two refrigerants was then analyzed at nominal design conditions and under a seasonal perspective. Based on the modeling work, it is possible to conclude that R32 system efficiency performance is acceptable as alternative to R410A for packaged air-to-water reversible unit
Thermophysical Properties and Heat Transfer and Pressure Drop Performance Potentials of Hydrofluoro-olefins, Hydrochlorofluoro-olefins and their blends
No full textThis paper considers the heat transfer and pressure drop performance potentials of halogenated propene isomers during in-tube condensation and in-tube flow boiling using the penalty factor and total temperature penalization concepts, respectively. In particular, five isomers are considered: R-1233xf, R-1233zd(E), R-1234yf, R-1234ze(E), and R-1243zf. In addition, to these five pure fluids, the heat transfer and pressure drop performance potentials are investigated for five R-32/R-1234yf blends and for twenty-seven blends being considered as part of the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) Low-GWP Alternative Refrigerants Evaluation Program. The paper also presents thermophysical property estimations for the five pure fluids relative to R-134a or R-123, and the five R-32/R-1234yf blends relative to R-134a. The thermophysical properties considered are the ones that influence the heat transfer and pressure drop performance potentials, and include the thermodynamic properties temperature, pressure, density, latent heat of vaporization, and specific heat, and the transport properties thermal conductivity and viscosity. The paper also presents a literature review of relevant articles for condensation and boiling heat transfer and pressure drop of fluorinated propene isomers
Performance assessment of an off-the-shelf R744 heat pump equipped with an ejector
No full textThis paper presents the experimental analysis of a prototype R744 water/water heat pump (5 kW heating power at 0 C evaporation temperature, 100 bar gas cooler pressure, 35 C gas cooler outlet temperature) which was equipped with a two-phase ejector. According to the traditional lay-out, the energy that was recovered from the expansion process inside the ejector improved the circulation of refrigerant through the evaporator. The same heat pump was also provided with a back-pressure valve as the expansion device, i.e. a direct comparison of the performance was possible between the heat pump systems, either equipped with the ejector or the expansion valve. Tests were performed to evaluate the performance when the heat pump produced hot tap water, i.e. heating water from the main waterworks temperature to storage temperature (∼60 C). Additionally, an investigation took place to understand how the system performed when space heating was required, i.e. providing hot water with a limited temperature lift, however at relatively high inlet temperatures. Experiments demonstrated that the ejector is helpful to improve the heat pump performance, thus promoting the diffusion of R744 units also for domestic space heating and air conditioning. Technological issues related to lubricant recovery were faced
Heat transfer and pressure drop penalization terms (exergy losses) during flow boiling of refrigerants
No full textHeat transfer coefficient and pressure drop correlations are used to analyse the boiling heat transfer performance potentials in plain evaporator tubes of several conventional refrigerants and two newer fluorinated propene isomers possessing low global warming potentials. These correlations are used to calculate two penalization quantities expressed in terms of the refrigerant saturation temperature drop due to pressure drop and the driving temperature difference. These penalization terms are combined into a single Performance Evaluation Criterion dubbed Total Temperature Penalization (TTP). Using the two penalization terms and the TTP, several refrigerants, including the newer alternatives R1234yf (CF3CF=CH2) and R1234ze(E) (CF3CH=CHF), are evaluated for their boiling heat transfer performance potentials in plain evaporator tubes. Furthermore, the usefulness of the technique is illustrated through several examples of the optimization of evaporator tube length
A data-driven approach for fault diagnosis in HVAC chiller systems
No full textFaulty operations of Heating, Ventilation and Air Conditioning (HVAC) chiller systems can lead to discomfort for the user, energy wastage, system unreliability and shorter equipment life. Faults need to be diagnosed early to prevent further deterioration of the system behaviour and energy losses. Since it is not a common practice to collect historical data regarding unforeseen phenomena and abnormal behaviours for HVAC installations, in this paper a semi-supervised, data-driven approach is employed for fault detection and isolation that makes no use of a priori knowledge. The proposed method exploits Principal Component Analysis to distinguish between anomalies and normal operation variability and a reconstruction-based contribution approach to isolate variables related to faults. The diagnosis task is then tackled by means of a decision table. The fault diagnosis algorithm performance is assessed by exploiting an experimental dataset from a frictionless centrifugal chiller system
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