Rooftop Turbine Ventilator : A Review and Update

As an alternative to air-conditioning systems, the rooftop turbine ventilator now is not only widely accepted as industrial ventilation, but also has become a common ventilation feature used in other types of buildings including institutional, commercial and residential. Although it is often thought to be very effective even in the lightest wind conditions, but many scientific studies found that its actual performance in the real building is not very promising due to some outdoor climatic constraints and the weaknesses of the device’s configuration itself. Therefore, in order to describe its full characteristics, reliability, limitations and possible improvement of the device, this paper discusses the current development and future prospects of the turbine ventilator by elaborating the results of some experimental and analytical work that have been done in various aspects of its application and performance. The analysis and summarized findings presented in this paper also recommend some criteria that should be considered in designing future turbine ventilator which is not only ventilate well in high-wind speed region, but also could be an effective multifunction device even in low wind velocity region.


Introduction
Due to the simple operating mechanism and relatively affordable price, a turbine ventilator which is driven by natural wind force is always a preferred choice to ventilate the building without depending on air conditioning system which is usually associated with global warming, finite fossil energy depletion and sick building syndrome (SBS) (Seppanen & Fisk, 2002;Liping & Hien, 2007).This device, which is also known as a rotary ventilator is a wind-driven air extractor which was originally patented by Meadows since 1929.It is considered as one type of roof ventilator which has been defined by American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) (1999) as 'a heat escape port located high in a building and properly enclosed for weather tightness with the primary motive forces being stack effect and wind induction'.Due to these characteristics, the rooftop turbine ventilator can be categorized as both active and passive ventilation strategy.Dominantly, turbine ventilator is classified as an active ventilation strategy although it does not use any electricity to rotate.This is due to the fact that the constantly moving spinning blades driven by outdoor wind force is found to be the major factor that creates centrifugal forces in the turbine, which in turn extract hot air from the building (Khan et al., 2008).On the other hand, the turbine ventilator could also be considered as a passive ventilation strategy, especially in the absence of wind.This is due to the existence of net free vent area i.e. openings between vertical blades which can induce stack effect and extract hot and stale air out from the building if there is sufficient indoor-outdoor temperature differential (Rudd & Lstiburek, 1998).Because of these rationales, the turbine ventilator can be classified as wind-assisted stack ventilation or wind-stack driven ventilation strategy as termed by Allard (1998) since it can maximize both wind-induced and stack effect to extract hot and stale air from the building.
In general, the device composes a waterproof frame which is mounted by a number of vertical vanes in a spherical or cylindrical array and a base duct which is connected by a shaft and bearings.Today, after eight decades since it was first extensively commercialized by Edmonds of Australia in 1931 for the purpose of industrial ventilation, the device now is very well accepted and has become a common ventilation feature used in various types of buildings including institutional, commercial and residential (Ahmed, 2010).Although it is often thought to be effective even in the lightest wind conditions as low as 1.5m/s (Edmonds, 2007), many academic studies found that its actual performance in the real building is not very promising due to some outdoor climatic constraints and the weaknesses of the device's configuration itself.Therefore, in order to describe its full characteristics, reliability and limitations of its current development as well as possible enhancement of the device, the following section will discuss the experimental and analytical work on the performance of turbine ventilator which has been done before.

Experimental and Analytical Work on the Performance of Turbine Ventilator
Among the earliest discussions on the turbine ventilator principle and performance were contained in the reports by Savonius (1932) and Back (1932) who published experimental results comparing cowls and the S-rotor wind driven fan (Revel & Huynh, 2004).Apart from these very early works, there are only a few published work on the device that can be found before the new millennia.These include a few pattern reports and a study by Dale and Ackerman (1993) on the performance of conventional attic turbine ventilators.However, in the last decade, turbine ventilator applications have attracted many attentions in various types of investigation, which can be categorized into three main aspects i.e. its application strategy & performance parameters, effect of configuration variables & modifications and different research tools & methods.

Turbine Ventilator Application and Performance Parameters
Studies on rooftop turbine ventilator applications and functions have been expressed in terms of conventional use of turbine ventilator in different types of building and a combination of turbine ventilator with extractor fan system.The outcome results of its performance were discussed in terms of (i) ventilation rate (in l/s, m³/s, ACH and CMH) (ii) air velocity (m/s) (iii) air temperature (reduction) (°C).Dale andAckerman (1993), Mihailov, Dinchev, Vlaseva, Petrov, andTenev (2003), Lai (2003) and Cox et al. (2011) studied the effectiveness of conventional type of rooftop turbine ventilators applied in different types of building and under different climatic conditions.On the other hand, Porfirio (2004) and Kuo and Lai (2005) investigated the effect of combining the turbine ventilator with electrical extractor fan ventilation system.
In one of the earliest academic studies on the real application of turbine ventilator, Dale and Ackerman (1993) investigated the effectiveness of the 12" (305mm) diameter turbine ventilator in improving attic ventilation of a house already installed with two soffit vents of 0.08m² free vent areas.The study showed that in the US temperate climate and windy condition, the turbine ventilator could reduce the attic temperature by only 0.56°C and increased ventilation rates by 15% from 5.3 ACH to 6.1 ACH on average as compared to the existing condition of the test house.The study also concluded that the ventilation enhancements by the device were a function of wind direction with the best being wind with more upwind shelter.Lai (2003) studied the effectiveness of conventional wind-driven turbine ventilator application in factories and residences under the windy condition of Taiwan.The study indicated that the use of the device is helpful to increase ventilation rate (CMH), thus enhance natural stack ventilation.However, for the zero to low-wind velocity condition, the study revealed that the turbine itself blocked the airflow in the connecting pipeline/duct, thus creating a lower ventilation rate even compared to the condition when there was no ventilator at all.
In temperate climate of South Africa, Cox et al. (2011) studied the possibility of using turbine ventilator to increase ventilation performance that in turn could reduce Tuberculosis (TB) transmission risk in four primary care clinic rooms.Using CO 2 gas tracer technique to assess the efficacy of the wind-driven roof turbines and air-intake grates combination to achieve recommended ventilation rates, the study showed that such installation succeeded to induce higher median ACH compared with natural cross ventilation strategy.It also achieved to exceed the level of 60 l/s per patient as recommended by World Health Organization (WHO) for 95% or more of measurements in 3 out of 4 rooms, while the measurement in another one room indicated the result of 47% complying with that standard due to the relatively low wind speed and smaller size of turbine ventilator installed as compared to the previous three rooms.
On the other hand, Porfirio (2004) conducted a study to investigate the performance of a combined turbine ventilator and electric fan in inducing air change rate (ACH) on a greenhouse under the temperate climate of Brazil.The study revealed that turbine ventilator assisted by inner fan is capable to increase the ventilation rate from 9 ACH to 11 ACH as compared to only 0 to 5 ACH if turbine ventilators were forced only by natural wind.Kuo and Lai (2005) investigated the ventilation performance of the conventional turbine ventilator combined with bathroom ventilation system equipped with extractor fan under the tropical climate of Taiwan.The results showed that such combination was successful to achieve sufficient air change rate in bathroom, thus improved its indoor air quality.The study also concluded that the induced negative pressure resulted from the combining system helped to reduce leakage of odors and moisture to other interior spaces of the building.Vol. 5, No. 5;May 2012 Published by Canadian Center of Science and Education 123

Turbine Ventilator Configurations and Modifications
Studies on impact of configuration variables on turbine ventilator performance have been expressed in terms of turbine diameter (size), blade height, blade configuration, openings and duct/throat diameter.On the other hand, some experimental studies on the modifications of turbine ventilator have been done by combining it with inner extractor fan and with lightpipe for lighting purpose.

i)
Size -Several studies were concerned with the impact of diameter (size) variables of turbine ventilators on its ventilation performance.Based on the wind tunnel and field study results under Taiwan climate conditions, Lai (2003) confirmed the consumer beliefs that the bigger size of turbine ventilator would induce higher ventilation rate.However, it was found that the ventilation rate induced by more commonly used ventilators of 14" (360mm) and 20" (500mm) are more or less the same, meaning that the rate can be considered equal when it comes to the application in engineering.

ii)
Blade Height -Study on the effect of vane or blade height on turbine ventilator performance was carried out by West (2001).By conducting an experimental study on Long Volume Turbines (LVTs) as shown in Figure 1, the study found that 13.5% improvement in flow rate could be achieved if the vane/blade height is increased by 50%.This is based on the results which showed that when turbine ventilators of different vane heights of 170, 250 and 340 mm were tested at a fixed wind speed of 12 km/h, it would induce different airflow rates of 65, 70 and 75 l /s, respectively.iii) Blade Design (curved or straight vane) -In terms of vertical vane or blade design, Khan, Su, Riffat and Biggs (2008) found that for the same size of 300mm (12") turbine ventilator, the curved vane type of ventilator had about 25% larger flow rate than the straight vane ventilator at the same wind speeds.Besides, the study on the different types and forms of turbines ventilator as shown in Figure 2 also revealed that the fabrication material of the device also affect the ventilator's capacity to induce airflow.

v)
With Inner Vane -The experimental study by Lai (2003) on the impact of installing inner vane inside the frame of turbine ventilator has confirmed that the added element can help the device to exhaust better.However, the study concluded that the availability of inner vanes inside the turbine ventilator sized 20" (500mm) and 14" (360mm) do not mark significant differences in the ventilation rate induced.According to Lai (2003), structural factors in the flow and the imperfect shapes of the inner rotating vanes could be the major factors limiting its performance, which should be taken into consideration when designing a new model of the turbine ventilator.The wind tunnel study to investigate the aerodynamic forces and complex airflow associated with this turbine ventilator has also been carried out by Rashid and Ahmed (2003).Conducting an experiment in the aerodynamic laboratory with the use of a simple physical model of both spinning and stationary cylinders, the study succeeded to provide qualitative information on aerodynamic force acting on the device and also on the airflow nature and its relation to drag force components.Regarding the turbine ventilator's blades, the results revealed that in high wind velocity condition, more partial flow separation was induced on that component, thus urged for significant improvement in the blade's future design to ensure its efficiency over a larger velocity range (Rashid & Ahmed, 2003;Lien & Ahmed, 2010).
Another wind tunnel study on the performance of the device was performed by Kuo and Lai (2005) to evaluate the overall ventilation performance of installing a turbine ventilator to an existing duct ventilation system, which serves fourteen bathrooms in Taiwan.For this study, the multi-functional ventilation detector, which is equipped with two sensing connectors, was used to measure and detect a variety of environmental factors such as wind speed, temperature and humidity.On the other hand, the study also involved a full-scale field measurement study.The ventilation rates of the roof ventilator and bathroom ceiling fans were measured using the multi-point measurement method for the HVAC (heating, ventilating, and air-conditioning) duct.Measurements for each controlled different condition lasted three days and the sampling rate of the airflow velocity was set at 10 s intervals.Evaluating the results based on Hygienic Risk Indicator, the study found that there was hygienic risk only when the turbine ventilator was out of order and the bathroom ceiling fans were off.In contrast, the condition of when turbine ventilator is freely rotating and ceiling fans is on showed the best ventilation performance.
Following the study, Lai (2006) and Shun and Ahmed (2007) performed another wind tunnel studies to investigate the ventilation performance of the prototype ventilator that combined PV-powered extractor fan with wind-driven turbine ventilator.By using the dual-loop experimental procedure of the low-speed wind tunnel test, Lai (2006) investigated several parameters like flow structures, induced ventilation rate, pressure difference and operation technique during the prototype development of the device.On the other hand, based on the results of the study done in aerodynamics laboratory, Shun and Ahmed (2007) stressed that the installed fan did not improve the ventilation rate of the device when the outdoor wind velocity is more than 5.0m/s, since the ventilation mechanism of the turbine ventilator was absolutely dominated by the outdoor airflow.
Apart from the field study conducted by Kuo and Lai (2005) as mentioned earlier, Dale and Ackerman (1991), Porfirio (2004) and Ismail and Abdul Rahman (2010) also used a full scale field measurement study to investigate the actual performance of the device under the real climate conditions which the results are already discussed in the previous sub-section.
In contrast with previously mentioned physical experimental methods, Havens (2004) used a numerical study by developing simple mathematical method to model the attic turbine ventilator (ATV) simultaneously as a fan and as wind turbine so that the wind speed can be related to fan airflow for varying rotational speeds.The study found that the correlation is dependent on knowing the co-efficiency of power and torque for a given attic turbine ventilator.With these coefficients, wind speeds and the system characteristic curve for an attic, attic ventilation rate changes can be accurately predicted based on attic turbine ventilator performance independent of other mechanical or natural ventilation forces (Havens, 2004).1, it can be concluded that for the conventional type of turbine ventilator, the resulted ventilation rate, air change rate (ACH) and air velocity induced by the device are found to be very dependent on outdoor wind condition.To overcome this problem, the strategy of combining it with solar-powered extractor fan as have been developed by Lai (2006), Shun and Ahmed (2007), Zain-Ahmed et al. (2007), Ismail and Abdul Rahman (2010) and Shieh et al. (2010) seems to be the most appropriate strategy to ensure its consistency and reliability in the low-wind velocity region.Furthermore, it is expected that the performance of this strategy could significantly be enhanced if the entire optimum configurations listed in Table 1 are taken into consideration in developing a new turbine ventilator model.

Conclusion
Based on the previous experimental and analytical work that have been done in various aspects of rooftop turbine ventilator application and performance, this paper has elaborated the full characteristics, current status and also the possible development of the device for the coming future.The previous studies which involve physical measurement and CFD numerical simulation methods have discussed many important aspects of the device, including the clarification of the actual airflow patterns inside and outside the device, its typical ventilation performance in relation with different configurations as well as the actual applicability of the turbine ventilator in the real building and under real climate conditions.Some of the recent studies also confirmed that the hybrid turbine ventilator (HTV) which can maximize both free energy from the sun and outdoor wind could be a more potent and reliable alternative compared to conventional ones in ensuring higher rotation rate and its consistency in low-wind velocity region.Based on the analyses and summarized findings, this paper has also suggested some criteria, current limitations and future prospects of the device which need to be considered in developing future design of rooftop turbine ventilator which is not only ventilate well even in low wind speed region, but also could be a simple yet effective multifunction device.

Figure 4 .
Figure 4. Typical configuration of turbine ventilator (a) with inner vane (b) without inner vane

Table 1 .
turbine, length and diameter of ducting and the effect of its integration with other devices like axial fan and lightpipe.The summarized results are shown in Table1.Summary of previous research on optimum configurations of turbine ventilator