A Comprehensive Review of the Maisotsenko-cycle Based Air Conditioning Systems

Introduction

Although conventional air-conditioning systems are widely accepted to be of high energy consumption, they cover a significant role of needs for air-conditioning. Scientific research focus on improved refrigerants (the global warming potential of which is lower than that of restricted R-12 or R-22) or more than effective compressors; yet, the high operational cost of these units as well every bit its role in atmospheric pollution cannot significantly be limited. As the unsafe environmental effects of chlorofluorocarbons and greenhouse gases (not only as directly emissions, but as well every bit indirect emissions) have been reduced, the interest is focused on environment-friendly cooling technologies. The energy consumed for heating and cooling of domestic premises accounts for 7% of the national total energy need; nevertheless, it is responsible for 29% of the CO₂ emissions. Particularly in Greece, ambient temperatures have a direct touch on the blueprint of the nation'south power demand, while buildings are by and large cooled by conventional vapor compression systems.¹

Evaporative air-conditioning is a really promising technology. Whereas conventional systems utilize chlorofluorocarbon based refrigerants (CFCs), evaporative coolers (ECs) use h2o. Evaporation technology is elementary and functional and has both residential and industrial applications, achieving significant efficiencies in suitable climates (hot and dry).

ECs are based on water evaporation and latent rut utilization. When water evaporates and becomes vapor, the heat is removed from the air, resulting in a cooler air temperature. Equally they do non use any compressor or pump, apart from some fans, their electricity demand is very low, while they can provide the cooling areas with air of satisfactory temperature (almost xix°C–21°C). Substantial free energy, no cfc usage, reduced peak demand, reduced CO₂ and power plant emissions, improved indoor air quality, lifecycle, price effectiveness, easily integrated into congenital-upwards systems, and piece of cake to employ with direct digital control are the principal advantages of ECs. On the reverse, some types of ECs produce an air stream of extremely high humidity (sometimes, the stream is almost saturated) and eat a meaning amount of water.

There are two basic categories of EC: direct and indirect.

• According to the first configuration, the water evaporates into the air to exist cooled; as a result, the production air is cold and wet. A typical direct EC (Dec) consists of a box with voluminous humidification blocks, a water pump, and a water distribution organization. The fan draws in warm and dry out ambient air through the wet blocks, cooling it. The latent heat of the air is used to evaporate the water. Evaporation cools the air while increasing its moisture content or relative humidity. No oestrus is added or taken out of the air; thus, it is an adiabatic process of constant enthalpy.
• On the other hand, indirect ECs (IECs) are based on 2 different streams (working [wor] and production [pro]), in guild to get a relatively drier production stream, only its temperature is not as low equally it would be past a DEC. A heat substitution layer is used betwixt the working airstream and the supply airstream, because the ambience wet-bulb (wb) temperature is theoretically the minimum achievable temperature of a conventional evaporative system. A typical IEC system can achieve an efficiency of about 55%.

An platonic EC would produce air as absurd as the moisture-bulb temperature, while a real cooler cannot attain such a low temperature. Thus, the efficiency of the ECs is divers every bit the ratio of current to maximum possible temperature drop:

DECs achieve loftier values of efficiency (lxxx%–95%); however, IECs cannot reach a value higher than 55%.

Maisotsenko bicycle (G-bicycle) applies an improved design of indirect evaporative cooling. Keeping the humidity ratio of product air constant, information technology succeeds in decreasing the air temperature down to ambient wet-bulb temperature and close to ambient dew-point (dp) temperature, past a smart heat and mass transfer process. Paper sheets of a special type, for optimum wetting and mass transfer between them and the air, are used equally exchange layers, while the product air (which is to cool the air-conditioning spaces) is totally protected by moisture of supplying h2o.²

M-cycle has been designed to optimize the effectiveness of both stages of evaporation (direct evaporation of working stream and heat exchange betwixt streams). Instead of i-stage evaporating (which is practical in conventional evaporative systems), Chiliad-cycle is based on a multi-evaporating approach, which allows to it to achieve high values of effectiveness, college than 105%.³

ECs based on Thou-bike have been already installed to supply cool air to various applications (domestic cooling, commercial and industrial buildings, etc). The advantages of this engineering science in comparison to conventional refrigeration systems or to typical IECs are the lower energy demand (reaches 85% in terms of electricity) and the lower product air temperature. Theoretically, if all conventional air conditioning systems, installed in buildings, would be substituted by evaporative ones, based on Grand-wheel, the estimated reduction of CO₂ emission would be equal to about 24%!

This paper aims at describing in a uncomplicated way the M-cycle functioning and utilization and at presenting some useful experimental data, to testify the high efficiency of One thousand-cycle, nether Mediterranean climate conditions. Although ECs cannot achieve as low temperature as their users want (due to the dew-point temperature restriction), M-cycle is the most effective IEC, the product air of which tends to the outlet air temperature of conventional edifice air-conditioning systems. And, equally it is a quite new engineering (near 8 years), its improvement potential in terms of electricity consumption is not negligible.

G-cycle principles and technical overview

The start step of M-wheel construction is to create the "paths" of dry channels. Both working (pink lines) and production (red lines) streams employ dry out channels (Figure 1). The working stream passes through the perforations and is driven to the wet channels (blueish lines, Figure 2).

Figure 1 Dry out channels configuration.

Figure 2 Moisture channels configuration.

Figure two helps in understanding the M-cycle: the working stream, which volition evaporate the water, is precooled under constant relative humidity (as no mass exchanging takes place along the dry channels). It enters the wet channels under lower temperature than ambient temperature, and the wet-bulb temperature, which is eliminated at each working channel, is related to the inlet temperature. As the working stream passes through the wet channels, the water is evaporated and the required latent heat is absorbed by the dry aqueduct, which becomes cooler and cooler (Effigy 3). In reality, 1 layer of heat and mass exchanger (HMX) is testify on Figure 4.

Figure 3 Rut and mass exchanging.

Effigy four Heat and mass exchanger layer configuration.

An M-cycle–based cooled is structured by 40 heat and mass exchanging layers, creating the following apparatus (Figure 5). Some auxiliary devices (fans and pump) are needed to drive the air and the water into the cooler.

Figure 5 Maisotsenko-cycle cooler configuration.

• The basic principle of the One thousand-bike is that the temperature divergence betwixt the stream is higher than in a typical IEC.^four
• The production stream, of mass catamenia m, enters the cooler at level "upwards" and is led through parallel channels, which are maintained dry and smooth, then no humidity is added or removed from the stream. As it always happens in IECs, the humidity ratio of the product stream is kept constant along the cooler.
• The working stream, of mass flow Yard, likewise enters the cooler at the upper level and is driven through dry channels at this level. As it meets the perforations, an amount of this is led to the lower level, which is e'er wet. Due to the contact of the working air with the moisture surface of the channels at the lower level, the evaporation of the water takes place and the working stream is cooled. This cooling absorbs rut from the working stream while information technology is at the upper level (which is consequently precooled, so beingness libation than the ambient air, it is driven to the rut exchanging zone) and from the product stream, while they are both in the zone of heat exchanging. At this zone the product air is cooled and the working stream is exhausted, most saturated, and cooler than the ambient air. Theoretically, the minimum possible temperature of state is the ambient air dew point; however, generally this state is between the ambient air wet bulb and the dew point.

Precooling of the working stream improves the libation efficiency comparatively to simple indirect evaporative systems; in terms of efficiency, the M-cycle achieves an efficiency of 90%–125%. Its efficiency is significantly affected past menses rates and ambient atmospheric condition and is expressed in wet-seedling terms, in order to indicate the ameliorate performance of a Maisotsenko cooler instead of a typical EC.

M-cycle cooler operation

Evaporation in an IEC is caused 1) by the sensible heat of the working stream and 2) by the sensible estrus of the product stream. It is articulate that, because the ii currents do non interact, any water add-on volition non affect the product stream and its contribution to the increase of the latent heat, which causes evaporation, is linked to the temperature difference of the two streams.^five

The scope of this section is the estimation of the cooling capacity, specific h2o consumption (swc) and efficiency of the M-cycle–based libation, under mutual Mediterranean ambient conditions during the summer period (betwixt 33°C and 36.5°C). To evaluate the operation of an G-cycle–based device, a HMX of a nominal cooling capacity of 0.35 RT was used (it is equal to nigh 4,200 BTU/h or 1.23 kW_c) (Figure 6).

Figure 6 Experimental rig. Notes: A, master suction duct; B, fan; C, secondary resistor; D, splitter; E, air catamenia regulators; F, primary resistors; K, stream ducts; H, exhaust stream duct. Abbreviations: t, temperature; φ, relative humidity; amb, ambient conditions; HMX, heat and mass exchanger; pro, product stream; wor, working stream.

The experimental rig consists of the post-obit devices and parts:

1. Main suction duct: a flexible, isolated φ200 duct is used. The unabridged air passes through this duct.
2. Fan: an centric two-speed fan is used, 80 Westward/120 W of ability and 800 m³/h at ii,000 rpm or 1,100 m³/h at 2,500 rpm of nominal air catamenia.
3. Secondary resistor: a resistor of ane,000 Westward ability is used for assistance in air preheating. Due to its position, this resistor heats the whole air, before splitting the streams.
4. Splitter: for splitting the 2 streams (working and product), a heat isolated "inverted Y" splitter is used. Each stream is channeled to φ150 ducts.
5. Air flow regulators: the air period of each stream is independently controlled. Contrary to the fan, whose speed command refers to the entire quantity of air, here in that location are ii regulators (one for each stream).
6. Main resistors: each of the ii main resistors (of one,000 W) was placed in the interior of the duct of each stream. The halt of electricity has been secured in example the fan is disabled, and so equally to avoid any superheating of the resistors.
7. Stream ducts: these two isolated φ150 air ducts pb each current to the HMX after it has been preheated.
8. Frazzle stream duct: in society to stop the wet air suction past the master suction duct (and exchanger malfunction), the highly humidified working stream is rejected in the temper fairly away from the master air duct entrance.

For measurements, the following instruments were used:

➢ Air flow: velocity probes, of accuracy ±0.ii m/s. The hotwire was placed in the center of each air duct, so as to measure the maximum velocity.

➢ Temperature: thermometers, of accuracy ±0.two°C.

➢ Relative humidity: probes of accuracy ±1%.

The measurement procedure results are shown in Tabular array 1.

Table 1 Ambient (1), cooled air (2), and exhaust air (2) conditions Abbreviations: t, temperature; φ, relative humidity; pro, product stream; wor, working stream.

During the measurement procedure, the mass flow of both streams was m _pro = one thousand _wor = 0.024 kg_da/south (it is reminded that the cooled air is heavier than the frazzle air, due to its depression temperature).

As for the cooling capacity, information technology tends to increment as the ambience temperature increases because the temperature drib tends to increase under college ambient temperature (at "purlieus" atmospheric condition the temperature drops were Δt = 12.0°C at 33.1°C and Δt = 14.2°C at 36.5°C). The correlation of the ambient temperature with the oestrus bachelor for evaporation is besides clear: below 33.1°C, the exhaust stream had a humidity ratio Due west _ii,_wor = 0.221 kg_{due west}/kg_da, while beneath the maximum temperature it had W ₂,_wor = 0.244 kg_westward/kg_da (ie, the evaporation effectiveness increased).^{half dozen}

Unremarkably, the evaporating cooler manufacturers give a typical value of hourly water consumption; even so, this value does not have into account the cooler efficiency. For this reason, the specific water consumption was defined, which is equal to the amount of water the evaporation of which can produce ane kWh_c. This parameter can be easily calculated and applied more often than not in each EC, without whatsoever additional information about the cooler size, mass menstruation, and capacity, so the EC becomes "dimensionless". Using the experimental information, it is concluded that the specific water consumption tends to reduce as the ambience temperature increases due to a higher increment of the cooling chapters, varying betwixt 2.5 kg_w/kWh_c and three.0 kg_w/kWh_c.

The increased amount of heat inserted in the cooler, when the ambience air is hotter, reinforces the evaporation phenomenon, as already mentioned, resulting in higher temperature drops through the cooler. The efficiency of the cooler is directly affected, as the college the temperature, the more than effective the cooler. The results show that an efficiency of well-nigh 90% is easily achieved, as the libation efficiency (additionally, under lower capacity than the nominal) is always greater than 97%, while at a loftier ambient temperature, the efficiency reaches 102%.⁷

Thou-Wheel cooler operation optimization

The most constructive way to optimize the functioning of an EC based on Thousand-cycle is to accommodate the ratio λ = grand _pro/m _wor. In comparison to the nominal setting of λ = ane:

• if λ<1, the water evaporation is stronger (in terms of specific water consumption) and the produced air temperature is lower
• if λ>1, the h2o evaporation is weaker and the produced air temperature is higher.

Ii cases of limited mass flow were examined. Specifically, the ratio λ was adjusted to ane:1, one:ii, and 2:ane; the measurements taken are presented in Tables 2–four, respectively.

Table 2 Nominal air supply weather condition (λ = 1:ane) Abbreviations: t, temperature; φ, relative humidity; pro, production stream; wor, working stream.

Tabular array 3 Efficiency improving past λ adjustment (λ = 1:2) Abbreviations: t, temperature; φ, relative humidity; pro, product stream; wor, working stream.

Table four Consumption improving by λ adjustment (λ = 2:1) Abbreviations: t, temperature; φ, relative humidity; pro, product stream; wor, working stream.

Taking as a fact that whatsoever configuration in the cooler operation is a "deviation" to its standard specifications, a decline of the capacity is initially expected. It is clear that a lower product stream flow affects the cooling capacity because the larger enthalpy driblet between cooled and ambience air does not comprehend the lowered flow of 50%. If the working stream flow is limited, the weakening of the evaporation (so the temperature drop in the product stream is lower) works as an obstacle to the cooling capacity, but not as much as a limited product stream flow does. Consequently, if the aim is to attain the highest cooling capacity possible, the libation should work on λ = 1:i (ie, nether its standard specification).

As for the specific h2o consumption, the results are articulate regarding the working stream flow reduction, where the product of i kWh_c needs about 55% less h2o, simply there is no clear tendency when the working stream flow is reduced. So, if nosotros aim to minimize h2o consumption, the lowering of the working stream mass flow is the best solution (the cooler consumes less than one.v kg_west/kWh_c).^viii

There is no uncertainty nigh the effect of the reduction of the product stream flow on the improvement of the libation efficiency. For λ = one:two, the cooler reaches a 115% efficiency (while for λ = ane:ane would lead to 107%), "gaining" 2°C of additional cooling. On the contrary, it is shown how disastrous a reduction of the working stream menstruation tin can be because the poor evaporation makes the cooler inefficient for pregnant temperature drops. Fifty-fifty then, in this case, the efficiency is comparable to that of DECs, fifty-fifty without producing humid air like these and almost double the efficiency of typical indirect evaporative systems.

Energy-saving potential in cooling systems

The replacement of conventional cooling systems by ones based on K-bike leads to a pregnant ecology benefit, as:

• the electricity consumption is much lower (about 80%) and
• unsafe refrigerants are not used, as water is an 100% renewable energy source.

In this affiliate, a commercial libation based on G-wheel is compared to a conventional i of the same cooling capacity:

• evaporative cooler

  

• conventional cooler

  

As the electricity toll is about 0.14 €/kWh and the water cost is about 0.60 €/m^three, the cost of 2 alternatives can be calculated equally a part of operation hours:

Ignoring the rates of return, it is clear that at most half-dozen,000 hours of operation (Effigy 7), the increased cost of installation of an EC balances the increased cost of functioning of an conventional cooler. Thus, the payback period of an EC, compared to a conventional one, is about 2.5 years.

Figure 7 Operational cost of evaporative libation and of a conventional libation.

Conclusion

In this newspaper, a cooler utilizing the M-cycle is analyzed; the aim was the product of dry and cool air with depression electricity consumption (only a simple axial fan of 750 W consumes electricity) and improvements of the libation characteristics (efficiency and water consumption). The measurements took place at a fairly dry climate and, without whatsoever modification, the cooler can achieve more than 100% efficiency. The efficiency does not depend on the ambient weather condition, but the production stream temperature, which is to be driven to the cooled infinite, is strongly affected by the humidity of the region where the cooler is installed. The specific water consumption of the libation under normal style varies (nether common ambient conditions) between 2.5 kg_{due west}/kWh_c and 3.0 kg_{due west}/kWh_c.

An hands configurable style to increase the efficiency of the cooler is to reduce the product to working mass flow ratio. Yet, this method leads to a pregnant increase of specific water consumption. Given that the increase in efficiency and subtract in consumption are both desirable, the product aqueduct mass flow configuration is proposed, as this can increment the efficiency nearly ten% and reduce the specific water consumption about xv%.

It was also important to understand the energy-saving potential of an EC, based on M-wheel. As it consumes near 80% less electricity than a conventional cooler, its high installation cost is quickly counterbalanced by its lower operation price.

As a conclusion, M-bicycle can satisfy the cooling demand of most Greek cities and it is also expected to do at other Mediterranean regions (of similar ambient conditions), without consuming loftier amounts of electricity and water. At humid climates, the cycle could not be recommended, as both product air temperature and hourly consumption are rather high.

Disclosure

The authors study no conflicts of involvement in this work.

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References

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