Personalized evaporative cooler to reduce energy consumption and improve thermal comfort in free-running spaces

sustainability Article Personalized Evaporative Cooler to Reduce Energy Consumption and Improve Thermal Comfort in Free-Running Spaces Ana Tejero-González * and Paula M. Esquivias Research Group in Thermal Engineering, Department of Energy and Fluidmechanics, School of Engineering, Universidad de Valladolid, Paseo del Cauce No.59, 47011 Valladolid, Spain; [email protected] *Correspondence: [email protected] Received: 18 October 2019; Accepted: 12 November 2019; Published: 16 November 2019   Abstract: The need to reduce energy consumption in buildings is imperative, but we must maintain individual thermal comfort of the occupants to ensure their well-being and productivity. Personal conditioning systems (PCS) have been suggested as a strategy to achieve both energy efficiency and thermal comfort, as they are considered to be low-energy consumers, allow increasing set-point temperatures, and give occupants the chance to control their own personal environment. While most warm-environment PCS are based on air-movement devices, the potential of using desk direct evaporative coolers (dDEC) has been scarcely explored. This work presents the results of the characterization of a dDEC and its potential for improving the indoor temperature and thermal comfort in a free-running office space. The study proposes adapted corrective power (CPa) and cooling fan efficiency for evaporative systems (CFEe) indexes. Results show that the dDEC achieves thermal comfort with a local effect, and it is recommended to be positioned directly on the desk surface, orientated to the occupant, and at a certain distance. Under these conditions, the CPa reaches − 2.8 ◦ C, involving better CFEe than the desk fans studied in the literature. Speed has little effect on the local air conditions, though it does improve the perception of thermal comfort. The relative humidity of the air does not exceed the recommendable limits, but renovation of the indoor air enables better conditions. Keywords: desk direct evaporative cooler; personalized cooling system; adapted corrective power; predicted mean vote; free-running buildings 1. Introduction 1.1. Energy Context: Toward Net Zero Energy and Carbon Urban populations can spend an average of 80 to 90% of their time in indoor spaces [ 1 ] and use heating, ventilation, and air-conditioning systems (HVAC) to achieve thermal comfort and good air quality. Data regarding European building stock highlight that these buildings are responsible for approximately 40% of energy consumption and 36% of CO 2 emissions, hence, they are largest energy consumer in Europe. Currently, approximately 35% of the building stock of the European Union are over 50 years old and almost 75% of buildings are energy inefficient [ 2 ]. Buildings in Spain account for 31% of all energy use in the country, with 66% of this energy consumption attributed to HVAC [3]. Worldwide, most adults spend one-third of their life at work [ 4 ]. Furthermore, office buildings represent 23% of the total non-residential floor space in Europe and, together with wholesale and retail trade buildings, account for more than 50% of energy use from non-residential buildings [ 5 ], with one third of their energy consumption attributed to HVAC [6]. Sustainability 2019,11, 6451; doi:10.3390/su11226451 www.mdpi.com/journal/sustainability Sustainability 2019,11, 6451 2 of 15 Over the last few years, issues have arisen in Europe that are closely related, including global warming awareness, the energy efficiency of buildings, the risk of overheating, alongside the achievement of human comfort in indoor spaces. Energy efficiency improvement in buildings relies on the reduction of energy demand, an improvement in the performance of the building services (HVAC, lighting, and hot water supply) and the integration of renewable energies [ 7 , 8 ]. These strategies should not hinder the objective to ensure thermal comfort for occupants, although frequently this is not achieved. In order to reach the European goals for 2050, the Directive 2010/31/EU introduced a mandatory implementation of nearly zero energy buildings (nZEB) by 2021 [ 8 ]. The approach toward this ambitious target must be cost-optimal to make nZEB cost-effective and to always maintain the high expectations of the occupants in terms of indoor comfort [9]. 1.2. Occupants’ Thermal Comfort: Personal Control Over the Thermal Environment to Improve Satisfaction In tertiary buildings, especially in medium and large companies, it is common to implement HVAC services to achieve a certain indoor climate for the entire space, despite the energy bills. Moreover, due to gender, age, and other human factors, these systems do not always provide thermal comfort for all occupants. The same HVAC systems can also be a source of indoor contaminants if disregarded, affecting occupant health [10]. In addition to this, under general HVAC systems, occupants are not able to freely adjust the set-point temperature or air movement to their personal thermal requirements, while greater possibilities of personalized control over climate quality can transform into a wider tolerance to indoor climate conditions [ 11 – 14 ]. Personalized thermal control even achieves decreased HVAC energy consumption, as shown by individuals’ feedback conducted by Li et al. [ 15 ]. Moreover, a poor indoor climate can also affect occupant productivity, which personalized control can improve [ 16 – 18 ]. User control is, therefore, essential for individual satisfaction [19]. Consequently, this scenario requires not only an improvement in the efficiency of centralized HVAC services, but also the consideration of new ways to provide control for occupants over their personal thermal environment, which results in a greater tolerance of temperature and decreased HVAC system energy consumption. 1.3. Personal Conditioning Systems: Energy Saving and Increased Comfort In order to improve individual occupant satisfaction while saving a significant amount of energy, several studies highlighted the adequacy of personalized comfort systems, especially for those spaces with sedentary occupants, such as offices [12,20–23]. While conventional services approached the entire volume of the occupied space, personal conditioning systems (PCS) acclimatized the micro-environment of each occupant by providing direct cooling to the individual’s body [ 20 ]. It was found that the implementation of PCS allowed relaxation of the global temperature range without increased thermal comfort dissatisfaction. This provided a decrease in energy consumption due to lower energy requirements compared to general HVAC systems and relaxation of the set-point temperature [22]. As Hoyt, Arens, and Zhai [ 22 ] indicated, by incorporating PCS, the indoor set-point temperature was increased in cooling mode without compromising individual thermal comfort. They found that increasing the indoor set-point temperature by 1 ◦ C saved about 10% of the energy consumed for air-conditioning. Therefore, making use of these complementary devices could influence the design of the HVAC systems, which are commonly oversized in order to reduce the transient discomfort when people enter the conditioned indoor environment from an extreme outdoor climate [ 24 ]. In addition, PCS allow users to control their thermal environment, adapting it to their personal requirements. Thus, PCS enhances both energy saving and perceived comfort [21]. Among the typical technologies to provide individual thermal comfort reviewed in the literature [21,25,26], are: Sustainability 2019,11, 6451 3 of 15 •Ventilation: desk fans, ceiling fans and nozzles; •Radiant cooling and/or heating: footwarmers, legwarmers or radiant panels; •Evaporative coolers, which also provide humidification; •Comfort chairs, possibly incorporating thermo-electric devices and/or fans; •Garments, which usually incorporate phase-change materials and/or fans. Recent studies demonstrated that air movement plays an important role in thermal comfort [ 27 ], so strategies followed for warm environmental conditions are based on personalized ventilation. However, individual ventilation may not be an effective strategy for heating, because high air speeds can create a risk of local discomfort due to draught rate. Nicol and Humpreys [ 28 ] stated that, although EN 15251 allowed an increase in neutral temperature for air speed if the occupant has direct control of air movement, e.g., through a fan, its application in free-running buildings was uncertain. They also concluded that humidity had little effect on the neutral temperature, though this may create discomfort with temperature rises above neutral conditions. Zhai et al. [ 29 ] demonstrated that with the use of personalized ventilation, thermal comfort could be maintained even if room temperatures reached 30 ◦ C for a relative humidity of about 60%. Through their revision of the literature, Vesel ý and Zeiler [ 25 ] determined that, if a maximum of 1.5 m/s air speed was driven toward the upper body region, thermal comfort could be achieved under air temperatures up to 30 ◦C. In order to characterize the efficiency of the air-movement-based personalized devices, Zhang et al. [18] introduced the concept of corrective power (CP), defined as “the difference between two ambient temperatures in which the same occupant thermal sensation is achieved—one with PCS in use, and one without PCS (i.e., uniform environment) as the reference condition”. The advantages of PCS for individual body cooling have motivated various researchers to explore their effectiveness [ 20 , 21 , 30 ]. Schiavon and Melikov [ 30 ] studied the performance of conventional and low power fans and found that their cooling efficiency was good compared to conventional fans. They also studied the cooling effect on different body parts in terms of thermal sensation and thermal comfort. Nonetheless, PCS are not new and they are used unofficially in working environments [ 21 ]. As Bauman et al. [ 12 ] stated, these systems provide individual comfort requirements, hence achieving higher satisfaction than the conditions perceived in uniformly conditioned spaces. 1.4. Personalized Evaporative Coolers The majority of PCS proposed for warm environments are based on personal ventilation. Studies where personal evaporative coolers are implemented are very scarce [ 24 , 31 , 32 ], and they are usually contextualized in a conditioned space [24,32]. Evaporative cooling is a well-known technique in HVAC systems, either by reducing air temperature through an adiabatic evolution of the humid air or by water cooling through cooling towers [33]. The phenomenon of evaporative cooling bases on the evaporation of water within non-saturated air, decreasing the temperature due to the energy that water requires to evaporate. Its application is particularly effective in hot and dry climates [ 32 ], where relative humidity is low, especially in naturally ventilated buildings; furthermore, these occupants accept a significantly wider range of thermal conditions compared to those of buildings with mechanical cooling [ 11 ], so the increase in relative humidity is compensated. Therefore, small or medium-sized direct evaporative coolers aimed toward personal cooling are more common in homes and small businesses in hot and arid regions [31,33]. Yang, Cui, and Lan [ 33 ] highlighted cost-effectiveness and its potential for energy saving among the benefits of this passive cooling technology, but also its “environmental friendliness” because it avoids, or at least limits, the use of some refrigerants which can increase danger to the environment. Sustainability 2019,11, 6451 4 of 15 Compared to other diverse technologies for personal cooling, the effect of evaporative coolers is due not only to air movement but also to decreased air temperature, resulting in a larger cooling capacity. In order to increase their portability and, therefore, their applicability for personal cooling, Day and Sumathy [ 31 ] focused their research on the development of evaporative cooling devices with smaller sizes and weights, but increasing their cooling capacity for personal cooling. The purpose of this research was to study the potential of a desk direct evaporative cooler (dDEC) to improve the indoor temperature and the thermal comfort in an office space under free-running conditions by adapting the concept of corrective power (CP) for evaporative coolers. 2. Materials and Methods In order to study the potential of the selected dDEC, it is firstly characterized in terms of air conditions provided (saturation efficiency, air velocity and flow rate). Then this paper analyses its capability to improve the thermal conditions at an occupied office spot, by testing its effect on a climate chamber simulating an office space. This section describes the methodology followed towards this purpose. It firstly presents the proposed system; then, the testing space and measuring equipment used. Finally, it explains the tests performed and the different operating parameters analysed. 2.1. Desk Direct Evaporative Cooler (dDEC) The target system was a commercial, personalized cooler to be placed at a desk, where the effect was based upon the adiabatic, evaporative cooling phenomenon (Figure 1). Its dimensions were 16.5 cm ×16.5 cm ×17 cm and its weight was 0.86 kg while dry. It was equipped with a small fan with three speed levels and a media that became humid by capillarity from water available at a small side tank. Hence, evaporation in the dDEC took place from a humid surface. It was plugged in via a USB and was said to require only 10 W. The specifications of the product stated a 2 m 2 actuation area and 6–8 ◦ C as the lowest temperature achievable, enabling up to 8 h of operation under the lowest speed level. No further technical specifications were available. Sustainability 2019, 11, x FOR PEER REVIEW 4 of 15 Day and Sumathy [31] focused their research on the development of evaporative cooling devices with smaller sizes and weights, but increasing their cooling capacity for personal cooling. The purpose of this research was to study the potential of a desk direct evaporative cooler (dDEC) to improve the indoor temperature and the thermal comfort in an office space under freerunning conditions by adapting the concept of corrective power (CP) for evaporative coolers. 2. Materials and Methods In order to study the potential of the selected dDEC, it is firstly characterized in terms of air conditions provided (saturation efficiency, air velocity and flow rate). Then this paper analyses its capability to improve the thermal conditions at an occupied office spot, by testing its effect on a climate chamber simulating an office space. This section describes the methodology followed towards this purpose. It firstly presents the proposed system; then, the testing space and measuring equipment used. Finally, it explains the tests performed and the different operating parameters analysed. 2.1. Desk Direct Evaporative Cooler (dDEC) The target system was a commercial, personalized cooler to be placed at a desk, where the effect was based upon the adiabatic, evaporative cooling phenomenon (Figure 1). Its dimensions were 16.5 cm × 16.5 cm × 17 cm and its weight was 0.86 kg while dry. It was equipped with a small fan with three speed levels and a media that became humid by capillarity from water available at a small side tank. Hence, evaporation in the dDEC took place from a humid surface. It was plugged in via a USB and was said to require only 10 W. The specifications of the product stated a 2 m2 actuation area and 6–8 °C as the lowest temperature achievable, enabling up to 8 h of operation under the lowest speed level. No further technical specifications were available. Figure 1. View of the target desk direct evaporative cooler. 2.2. Case Study and Measuring Equipment This work focused on the application of the proposed dDEC to improve the thermal conditions in offices of free-running buildings. The target space for the study was the climate chamber available at the laboratory of the Thermal Engineering Group of the University of Valladolid, which was prepared and equipped to simulate an office space with two desks. The climate chamber had dimensions of 4 m × 4 m × 3 m and was built with 40 mm polystyrene sandwich panels with 0.6 mm steel plating. It had four windows 0.9 m × 1.4 m each, which were double-glazed 4/8/4, and a door with dimensions of 0.82 m × 2.04 m, with a total glazed area of 6.7 m2 (Figure 2). Performing the tests in this isolated space permitted us to disregard transmissible thermal loads. The chamber permitted ventilation to be supplied through four rotational diffusers and had two luminaires equipped with four lamps each. Figure 1. View of the target desk direct evaporative cooler. 2.2. Case Study and Measuring Equipment This work focused on the application of the proposed dDEC to improve the thermal conditions in offices of free-running buildings. The target space for the study was the climate chamber available at the laboratory of the Thermal Engineering Group of the University of Valladolid, which was prepared and equipped to simulate an office space with two desks. Sustainability 2019,11, 6451 5 of 15 The climate chamber had dimensions of 4 m × 4 m × 3 m and was built with 40 mm polystyrene sandwich panels with 0.6 mm steel plating. It had four windows 0.9 m × 1.4 m each, which were double-glazed 4/8/4, and a door with dimensions of 0.82 m × 2.04 m, with a total glazed area of 6.7 m 2 (Figure 2). Performing the tests in this isolated space permitted us to disregard transmissible thermal loads. The chamber permitted ventilation to be supplied through four rotational diffusers and had two luminaires equipped with four lamps each. Sustainability 2019, 11, x FOR PEER REVIEW 5 of 15 (a) (b) (c) Figure 2. (a) Layout of the climate chamber furnished as an office space placed inside the laboratory and location of the measuring points; (b) overview of the climate chamber; (c) relative size and height of the sensors at the O1 measuring point and the desk direct evaporative cooler (dDEC) studied. Lighting yielded a total sensible load of 228 W and the only electric device was a personal computer with an expected sensible load of 250 W. Occupation during the tests ranged from 0 to 3, generating 75 W and 55 W per person as the sensible and latent loads, respectively, considering sedentary activity (1.2 met) [34]. Due to the absence of transmissible thermal loads and because ventilation, when supplied, was of the same conditions as the ambient air of the climate chamber, the only variable thermal loads were due to occupation. The measuring equipment was placed within the space, as shown in Figure 2. The measuring point O1 corresponded to the occupied desk where the dDEC was placed, whereas O2 was on an adjacent desk without a dDEC. Only the former desk was expected to be constantly occupied and was equipped with the thermal comfort measuring equipment Vivo Comfort, obtained from the manufacturer Dantec Dynamics. Both O1 and O2 points also had Testo 173 sensors. The measuring points E1 and E2 were Testo 175 sensors placed in the laboratory out of the climate chamber and aimed to check that air conditions out of the target space did not vary, thus validating the hypothesis that no transmissible thermal loads occurred. Finally, another sensor, Testo 175, was placed at measuring point O3, corresponding to the outlet of one of the diffusers, to ensure that air conditions were also maintained during the tests when ventilation was implemented. The ventilation airflow supplied was measured using an airflow rate meter before the beginning of the corresponding test. O2, O3, E1, and E2 measured the temperature and relative humidity. Table 1 describes the measuring equipment used and their characteristics. Figure 2. ( a ) Layout of the climate chamber furnished as an office space placed inside the laboratory and location of the measuring points; ( b ) overview of the climate chamber; ( c ) relative size and height of the sensors at the O1 measuring point and the desk direct evaporative cooler (dDEC) studied. Lighting yielded a total sensible load of 228 W and the only electric device was a personal computer with an expected sensible load of 250 W. Occupation during the tests ranged from 0 to 3, generating 75 W and 55 W per person as the sensible and latent loads, respectively, considering sedentary activity (1.2 met) [ 34 ]. Due to the absence of transmissible thermal loads and because ventilation, when supplied, was of the same conditions as the ambient air of the climate chamber, the only variable thermal loads were due to occupation. The measuring equipment was placed within the space, as shown in Figure 2. The measuring point O1 corresponded to the occupied desk where the dDEC was placed, whereas O2 was on an adjacent desk without a dDEC. Only the former desk was expected to be constantly occupied and was equipped with the thermal comfort measuring equipment Vivo Comfort, obtained from the manufacturer Dantec Dynamics. Both O1 and O2 points also had Testo 173 sensors. The measuring points E1 and E2 were Testo 175 sensors placed in the laboratory out of the climate chamber and aimed to check that air conditions out of the target space did not vary, thus validating the hypothesis that no transmissible thermal loads occurred. Sustainability 2019,11, 6451 6 of 15 Finally, another sensor, Testo 175, was placed at measuring point O3, corresponding to the outlet of one of the diffusers, to ensure that air conditions were also maintained during the tests when ventilation was implemented. The ventilation airflow supplied was measured using an airflow rate meter before the beginning of the corresponding test. O2, O3, E1, and E2 measured the temperature and relative humidity. Table 1describes the measuring equipment used and their characteristics. Table 1. Measuring equipment. Sensor Parameter Measured Range Accuracy Temperature and relative humidity sensor Testo 175 Dry bulb temperature (◦C) −20/+55 ±0.4 ◦C Relative Humidity, RH (%) 0/100% ±2% RH (from 2 to 98% RH at +25 ◦C) Vivo comfort Dantec Dynamics Air velocity, v (m/s) 0.05/1.0 <1 m/s: ±0.01 m/s, ±0.025·v >1 m/s: ±0.1·v Operative temperature (◦C) 0/45 0 –10 ◦C: ±0.5 K 10 –40 ◦C: ±0.2 K 40 –45 ◦C: ±0.5 K Air flow meter TSI model: 8715 DP-Calc Air volume flow, V (m3/h) 42/4250 ± 3% of the measured value for V >85 m 3 /h ±12 m3/h for V ≤85 m3/h Hot wire anemometer Testo 0635 1535 Air velocity (m/s) 0−20 ±0.03 m/s+4% of the measured value Multimeter FLUKE 75 III Voltage (DC) (V) 3.2; 32; 320 ±0.3% +1 Current (DC) (mA) 32; 320 ±1.5% +2 2.3. Performed Tests First, the air conditions provided by the dDEC were characterized for each of the three speed levels. Airflows were obtained from the average air velocities measured with the hot wire anemometer, as indicated in Table 1, and the measurements were taken on a 3 × 3 grid of a plenum adapted at the dDEC outlet. Temperature and relative humidity at the system inlet and outlet were also measured to determine the saturation efficiency achieved. Actual power requirements for each speed level were measured using the multimeter, as described in Table 1. The obtained values are described in Section 3.1. Next, the system was characterized within the target space and placed on the occupied desk. With the aim of studying the improvement introduced on the thermal conditions, several combinations of influencing parameters were reproduced during three tests. The parameters considered included the system speed level, its position on the desk relative to the occupant (Figure 3), and the possible existence of general ventilation in the target space to replace the indoor air. Table 2decribes these parameters. Table 2. Influencing parameters and conditions studied. Parameter Conditions Studied Air renovation indoors (N) Without air renovation (V) With air renovation (airflow for 2 people for a category 2 of indoor air quality [ 35 ]) Speed level (V1) Lowest speed (V2) Medium speed (V3) Highest speed Relative position to the occupant (O) Oriented toward the occupant, 65 cm away, at the surface of the desk (H) Oriented toward the occupant, 65 cm away, elevated 25 cm from the surface I Oriented toward the occupant, 20 cm away (D) Deflected away from the occupant, 65 cm away The different combinations of parameters studied in each of the three tests are described in Table 3. Sustainability 2019,11, 6451 7 of 15 Sustainability 2019, 11, x FOR PEER REVIEW 7 of 15 Figure 3. Relative position of the dDEC to the occupant. The different combinations of parameters studied in each of the three tests are described in Table 3. Table 3. Operating conditions implemented during the tests. Test Air Renovation Speed Levels Tested Positions Tested Ambient Base Conditions Air dry Bulb Temperature (°C) Relative Humidity (%) 1 N V1, V2, V3 O 27 40 2 V V1, V2, V3 O 27 40 3 N V1, V3 H, C, D 25 40 The total duration of each test varied between approximately 3 to 4 h depending on the requirements of the parameter modifications. To study the evolution of the thermal conditions during the tests, the air dry bulb temperature, the relative humidity, and the velocity were monitored together with the predicted mean vote (PMV) and the draught rate (DR); the latter two corresponded to Fanger’s thermal comfort model [34]. Monitoring of the PMV focused on approaching the acceptable indoor thermal comfort categories, namely I, II, and III representing the most, average, and least restrictive levels, while DR monitoring permitted observation of any risk of thermal discomfort due to draught. Because the dDEC is a personalized system, the study of its cooling potential at the target space was proposed for only one office worker as the baseline occupation condition. The occupant was sedentary and their garments corresponded to 0.5 clo, i.e., the expected amount of clothing in a freerunning space during summer. Occupation was not restricted during the tests, thus enabling the study of the effect of door opening and punctual occupation peaks, thereby reproducing real conditions in offices. 2.4. Adaptation of Efficiency Indexes to Evaporative Personalized Coolers In order to study the efficiency achieved by the target dDEC, this research proposed an adaptation of the corrective power (CP) index to enable its use in this evaporative cooling case. Contrary to the CP used in the literature [18], which quantified the capability of correcting the temperature toward neutral conditions only with the perceived effect of air velocity, the proposed adapted corrective power (CPa) was defined as   = ∆   + ∆   (1) where ΔTev is the temperature modification due to evaporative cooling and ΔTfan is the temperature modification due to the air conditions induced by the device’s fan. In this case, working with comfort measurement equipment, ΔTev is the temperature difference between the occupied zone with the dDEC and the reference case Figure 3. Relative position of the dDEC to the occupant. Table 3. Operating conditions implemented during the tests. Test Air Renovation Speed Levels Tested Positions Tested Ambient Base Conditions Air dry Bulb Temperature (◦C) Relative Humidity (%) 1 N V1, V2, V3 O 27 40 2 V V1, V2, V3 O 27 40 3 N V1, V3 H, C, D 25 40 The total duration of each test varied between approximately 3 to 4 h depending on the requirements of the parameter modifications. To study the evolution of the thermal conditions during the tests, the air dry bulb temperature, the relative humidity, and the velocity were monitored together with the predicted mean vote (PMV) and the draught rate (DR); the latter two corresponded to Fanger’s thermal comfort model [ 34 ]. Monitoring of the PMV focused on approaching the acceptable indoor thermal comfort categories, namely I, II, and III representing the most, average, and least restrictive levels, while DR monitoring permitted observation of any risk of thermal discomfort due to draught. Because the dDEC is a personalized system, the study of its cooling potential at the target space was proposed for only one office worker as the baseline occupation condition. The occupant was sedentary and their garments corresponded to 0.5 clo, i.e., the expected amount of clothing in a free-running space during summer. Occupation was not restricted during the tests, thus enabling the study of the effect of door opening and punctual occupation peaks, thereby reproducing real conditions in offices. 2.4. Adaptation of Efficiency Indexes to Evaporative Personalized Coolers In order to study the efficiency achieved by the target dDEC, this research proposed an adaptation of the corrective power (CP) index to enable its use in this evaporative cooling case. Contrary to the CP used in the literature [ 18 ], which quantified the capability of correcting the temperature toward neutral conditions only with the perceived effect of air velocity, the proposed adapted corrective power (Cpa) was defined as CPa=∆Tev +∆Tfan (1) where ∆ T ev is the temperature modification due to evaporative cooling and ∆ T fan is the temperature modification due to the air conditions induced by the device’s fan. Sustainability 2019,11, 6451 8 of 15 In this case, working with comfort measurement equipment, ∆ T ev is the temperature difference between the occupied zone with the dDEC and the reference case ∆Tev =TO1−TO2(2) and ∆ T fan is the temperature difference for the same predicted mean vote under different conditions of air velocity and relative humidity, hence, ∆Tfan =T(PMV,RHO2,Tr,(v=0)) −TO1(3) Finally, the cooling fan efficiency (CFE) index proposed in the literature for personalized ventilation systems [ 27 ] was also adapted to personalized evaporative coolers by using Cpa instead of the cooling effect of the fan. Thus, the evaporative cooling fan efficiency (CFEe) was described as CFEe =|CPa| . W(4) where . W is the electric power required by the device in W, thus, CFEe is given in ◦ C/W. This index allowed a fair comparison between different personalized devices in terms of the effect generated against power required. 3. Results The results obtained throughout the tests described above are presented herein. 3.1. Air Conditions Provided by dDEC To characterize the air conditions provided by the dDEC, the saturation efficiency was studied [ 36 ]. ε=Tin −Tout Tin −Tsat in (5) where T in is the inlet temperature of the dDEC, T out is the outlet temperature, and T sat in is the saturated temperature at the inlet conditions. Table 4describes the results obtained for the saturation efficiency, air velocity, and air flow rates measured at the dDEC outlet. These were average values obtained from the measurements performed on the 3 × 3 grid, as defined in Section 2.3. Table 4also shows the values for the electric power obtained at each air speed level through the voltage and current values measured with the multimeter. These values corresponded to the dDEC connected to a personal computer, and were much lower than the power given in the technical specifications. Table 4. Results for the dDEC saturation efficiency, air flow, and electric power at each air speed. Parameter V1 V2 V3 ε(%) 29.8 17.3 16.2 Air velocity, v (m/s) 0.5 0.6 0.7 Air volume flow, V (m3/h) 54 71 84 Electric power (W) 0.9 1.9 2.6 3.2. Conditions Achieved in the Office Space Before studying the evolution of comfort conditions in the target space, the hypothesis that no transmissible thermal loads existed required validation. This was checked using the results presented in Table 5, showing that the conditions outside of the target space (measuring points E1 and E2) were maintained. This table also demonstrated that the temperature and the relative humidity of air Sustainability 2019,11, 6451 9 of 15 supplied through the climate chamber diffusers during the second test were maintained at the same baseline conditions. Table 5. Temperature and relative humidity measured in the outdoor environment and in the ventilated air. Test E1 E2 O3 HR (%) T (◦C) HR (%) T (◦C) HR (%) T (◦C) 1Average 43 26.9 43 26.9 - - Standard deviation 0.83 0.08 1.15 0.05 - - 2Average 38 27.3 36 28.3 37 28.1 Standard deviation 1.29 0.25 1.11 0.24 0.95 0.19 3Average 37 24.3 35 24.8 - - Standard deviation 2.01 0.10 1.01 0.21 - - Figures 4–6present the measured variables during tests 1, 2, and 3, respectively. In all tests, the measuring started without the dDEC. The periods when the dDEC operated under the different possible modes (flow rates) are shown. The three comfort levels, i.e., I, II, and III for the predicted mean vote (PMV) and draught rate (DR) corresponded to the limits given in the standard ISO 7730, the latter being those at risk of local discomfort [ 34 ]. The Draught rate for test 2 was not obtained due to an unexpected measurement problem. However, by comparing the evolution of the measured air velocity in tests 1 and 2, it was determined that the strictest comfort range was maintained. Sustainability 2019, 11, x FOR PEER REVIEW 9 of 15 Test E1 E2 O3 HR (%) T (°C) HR (%) T (°C) HR (%) T (°C) 1 Average 43 26.9 43 26.9 - - Standard deviation 0.83 0.08 1.15 0.05 - - 2 Average 38 27.3 36 28.3 37 28.1 Standard deviation 1.29 0.25 1.11 0.24 0.95 0.19 3 Average 37 24.3 35 24.8 - - Standard deviation 2.01 0.10 1.01 0.21 - - Figures 4–6 present the measured variables during tests 1, 2, and 3, respectively. In all tests, the measuring started without the dDEC. The periods when the dDEC operated under the different possible modes (flow rates) are shown. The three comfort levels, i.e., I, II, and III for the predicted mean vote (PMV) and draught rate (DR) corresponded to the limits given in the standard ISO 7730, the latter being those at risk of local discomfort [34]. The Draught rate for test 2 was not obtained due to an unexpected measurement problem . However, by comparing the evolution of the measured air velocity in tests 1 and 2, it was determined that the strictest comfort range was maintained. (a) (b) (c) (d) Figure 4. Evolution during test 1 of (a) dry bulb temperatures, (b) predicted mean vote, (c) relative humidity, and (d) air velocity and draught rate. (a) (b) 0 1 2 3 23 24 25 26 27 28 29 0 50 100 150 200 250 Dry bulb Temperature [C] Time [min] O2 O1 Air speed level 0 1 2 3 30 35 40 45 50 55 60 65 70 0 50 100 150 200 250 Relative Humidity[%] Time [min] O2 O1 Air speed level 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 10 20 30 40 50 60 70 80 90 100 0 50 100 150 200 250 Low Air velocity [m/s] Draught Rate [%] [min] DR DR-I DR-II DR-III v 0 1 2 3 23 24 25 26 27 28 29 0 50 100 150 200 Dry Bulb Temperature [C] Time [min] O2 O1 Air speed level 0 0.2 0.4 0.6 0.8 1 1.2 0 50 100 150 200 PMV [min] PMV I II III Figure 4. Evolution during test 1 of ( a ) dry bulb temperatures, ( b ) predicted mean vote, ( c ) relative humidity, and (d) air velocity and draught rate.