Energy generation from grey water in high raised buildings_ The case of India

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Renewable Energy Volume 69, September 2014, Pages 284–289

Energy generation from grey water in high raised buildings: The case of India Prabir Sarkara,

,

,

, Bhaanuj Sharmab, Ural Malikb

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doi:10.1016/j.renene.2014.03.046

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Highlights • Harnessing energy from grey water, while it flows down through high-raised buildings. • We propose micro/pico hydro turbine at the ground floor to generate electricity. • Scaled prototype has been developed and tested. • The proposed design is checked for feasibility in Indian markets.

Abstract Energy consumption in developed as well as developing countries is high, especially in the residential and commercial building sectors. Researchers have been working on several technologies for the reduction of energy consumption in buildings; among them, energy-harvesting techniques are quite promising. In this paper, we explore a possibility of harnessing energy from grey water, while it flows down through high-raised buildings. We propose the usage of a micro/pico hydro turbine installed at the ground floor of a high rise building that utilizes the energy of grey water falling from floors above, to generate electricity. The electrical energy generated from the turbine can be utilized further in numerous ways. Scaled prototype of the same has been developed and tested. The proposed design of a gravity-energized wastewater system in high-rise buildings for generation of hydroelectricity is being checked for its feasibility in Indian markets. Calculation shows that the proposed system is commercially promising for most of the major cities in India. We also discuss cost benefits analysis of the proposed system to support our claims for possible commercialization of this technology.

Keywords Energy generation; High raised building; Sustainability; Grey water; Recycle

Nomenclature Symbols

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Biblioteka Politechniki Lodzkiej

H

height from the ground floor to the top of the collection tank (m)

R

radius of collection tank (m)

X

height of tank (m)

V

volume of tank (m3 )

v

volume of pipe (m3 )

h

head of water (m)

Q

volume flow rate (m3 /s)

t

time to empty tank(s)

A

area of tank (m2 )

a

area of orifice pipe (m2 )

C

coefficient of discharge (−)

g

acceleration due to gravity (m/s2 )

PT

power at turbine (kW)

Ρ

density of water (kg/m3 )

ηo

overall efficiency of turbine (−)

D

diameter of Pelton wheel (m)

Nt

turbine RPM (rpm)

Ng

rotor RPM (rpm)

ηg

generator efficiency (−)

ηg.b

gearbox efficiency (−)

Po

power output from the generator (kVA)

E

total energy generated (kWh)

1. Introduction World's energy requirements are increasing due to the increase in population, and demands from numerous energy-intensive equipments. Buildings have large impacts to the environment [1]. In USA, residential and commercial buildings use almost 70% of the total electricity generated [2]. Electricity consumption in the commercial building sector is expected to rise by 50% by 2025 [1] and [2]. Thus, any small or large strategies for energy saving are welcome. There are many major initiatives, which promise great benefits in terms of reducing environmental impact and emissions of buildings, such as, development of zero energy buildings [3], use of roof top solar-energy system [4], [5] and [6], development of green buildings [7] and [8], and LEED certification of energy used by buildings [9]. Several researchers believe that recycling and reusing of grey water in tall buildings holds promising results. However, before recycling or reusing, water could be used as a potential source for energy generation. By tall or high raised buildings, we mean buildings whose average height is more than 22 m above the surface [10]. We propose here that grey water falling from a certain height in high raised buildings could be a potential source for electricity generation. Through suitable design of storage tanks and selecting a suitable size of turbine, building manufacturers could explore the possibility of installing this system commercially to offset their grid electricity requirements, eventually booking profit after the break-even period.

1.1. Power situation of India and the need for pico and micro turbines India has a large shortage of power. In December 2011, over 300 million Indian citizens had no access to electricity [11]. Over one-third of India's rural population lacks electricity, as did 6% of the urban population. Of those who did have access to electricity in India, the supply was intermittent and unreliable. In 2010, blackouts and power shedding interrupted irrigation and manufacturing across the country [12]. During the year 2010–11, demand for electricity in India far outstripped availability, both in terms of base load energy and peak availability. Base load requirement was 861,591 (MU) against availability of 788,355 MU, an 8.5% deficit [13]. During peak loads, the demand was for 122 GW against the available 110 GW, a 9.8% shortfall. The per capita average annual domestic electricity consumption in India in 2009 was 96 kWh in rural areas and 288 kWh in urban areas for those with access to electricity, in contrast to the worldwide per capita annual average of 2600 kWh and 6200 kWh in the European Union. India's total domestic, agricultural and industrial per capita energy consumption estimates vary depending upon the source. As of January 2012, one report found the per capita total consumption in India to be 778 kWh [11]. Hydropower is an eco-friendly clean power generation method. India is endowed with economically exploitable and viable hydro potential assessed to be about 84,000 MW at 60% load factor. In addition, 6780 MW in terms of installed capacity from Small, Mini, and Micro Hydel schemes have been assessed [14]. In addition, 56 sites for pumped storage schemes with an aggregate installed capacity of 94,000 MW have been identified. It is the most widely used form of renewable energy. India is blessed with an immense amount of hydro-electric potential and ranks 5th in terms of exploitable hydro-potential on the global scenario

[15]. Taking into account the current scenario, extensive research is going on in micro hydel projects. Our study is designed to determine the practicality of the potential hydropower generation from the reuse of wastewater in high-rise buildings that can be used in-situ. A structure is automatically listed as a high-rise when it has a minimum of 12 floors, whether or not the height is known. Pico hydro [16] is a term used for hydroelectric power generation close to 5 kW. It is useful in small, remote communities that require only a small amount of electricity. Even smaller turbines of 200–300 W may power a single home in a developing country with a drop of only 1 m. Pico-hydro setups typically are run-of-stream, meaning that dams are not used, but rather pipes divert some of the flow, drop this down a gradient, and through the turbine before being exhausted back to the stream. We propose that, Pico hydroelectric power generation can be used in high-rise buildings that utilizes the potential head of water being used that drops down through a piping into the turbine rotating its wheel. This turbine is then coupled to a generator. The electrical energy produced is usually stored in a battery, which can then power electrical objects in the building, such as appliances and lights or connects directly to the main supply grid of the building. According to the estimated water consumption, about 76% of wastewater can be utilized for the production of hydroelectricity after some preliminary purification processes (Fig. 1). This purified water, which is also free from solid particles, can be directly fed to the turbine. On an average, in urban environment (population >10,00,000 individuals) water consumption by an individual is between 150 and 200 L [17].

Fig. 1. Water utilization in a typical house by percentage. Figure options

1.2. Emerging scope for installation of micro and pico turbines in high raised buildings Over the years, many high-rise buildings have come up in India. More than 2200 high-rise buildings are already constructed in Mumbai Metropolitan Region (MMR) [18] and there are more than 118 skyscrapers in the same city [19]. In addition to these, more than one thousand mid-rises buildings exist already in the city of Mumbai. Mumbai is undergoing a massive construction boom, with thousands of high-rises and more than 15super-tall under construction. Pune and its surrounding regions are witnessing large construction activities with 1500 already constructed high-rise buildings. Kolkata is also emerging as India's next skyscraper city with 600 existing high-rises and many more under construction [18]. Hyderabad and Bangalore are also having many skyscrapers under construction and could well match up with a city like Mumbai in the near future in terms of the number of high-raise buildings. High-rises are also becoming common in Chennai in recent times after the removal of height restrictions on constructions in the relatively active seismic zone. More than 250 high-rise buildings have been constructed in Kochi [18].

2. Aim and methodology In this paper, we would like to propose a commercially viable system to harness energy from used grey water flowing from different floors of a high raised building. In high-rise buildings, an enormous amount of water is consumed on a daily basis. The water after consumption is normally discharged directly into the drainage system. In certain cities, rules have been formulated to create wastewater collection and recycling plant for high-rise building, which have sufficient space [20] falling from some height carries potential head and thus stored energy. This work aims at harnessing this energy from grey water. After collecting requisite volume of water, it is made to flow through the pipe over the turbine blades, thus rotating the turbine shaft. This shaft is then coupled to a generator converting the mechanical power to electrical power. This electrical power can then be directly used or stored in the batteries for later usage (Fig. 2). We present detailed calculation, feasibility analysis and cost-benefit break-even analysis for practical usage of the proposed system. We also describe a working prototype of the same system.

Fig. 2. Proposed process of harnessing energy from grey w ater. Figure options

Steps followed in developing the proposed system: To solve the optimization problem for the system, a methodology is developed. Generalization of the problem is done for the system, and stepwise calculations are performed with the variables. Further, to check the feasibility of the model, a case study is performed by using constants in place of variables. The steps involved are shown below. 1.

A water collection tank of height X, radius R; is placed at a height (H−X) from the base. Water after being used is discharged into the conduit pipes. This system of conduits discharges the water collected from all the floors above the level of the collection tank (i.e. above height H) into the collection tank.

2.

The collection tank uses a water level sensor. After the required volume of water is collected in the tank, sensor actuates a valve at the bottom of the pipe connected to the tank.

3.

The valve opens up allowing the collected water to flow down. A hydro turbine is attached to the base of the pipe. Water having an initial head h strikes the turbine blades thus rotating the turbine shaft.

4.

Selection of turbine depends on the head and volume flow rate of water and availability. A Pelton wheel turbine is selected in this case because of low flow and reasonable water head [21]. However, other turbine designs can also be explored based upon use.

5.

The turbine is further coupled to an AC generator via a gearbox. Use of a gearbox is justified to achieve the desired RPM for the generator to obtain a steady output frequency.

6.

This generated electric power can either be directly used or stored as charge in the batteries for later use.

Fig. 3, shows the schematic diagram of the proposed system. Detailed of the theoretical calculation of the proposed system and calculation of the Pelton turbine that could be used with a pico/micro generator in the ground level of a high-raise building is described in Appendix 1.

Fig. 3. Schematic diagram of the proposed system. Figure options

3. Expected outcomes The expected energy outputs in a high raised building, based upon the assumption that the total number person on each floor is 100, is shown in Table 1. Table 1. Expected energy output in high-raised buildings w ith different number of floors. No of floors

Head

Energy

20

28.34 m 6.85 kWh (see, Appendix 2 for calculations)

30

43.3 m

15 kWh

40

58.17

28 kWh Table options

From Table 1, for instance, if we take Mumbai (Fig. 4) as a case, with an average number of floors of 40, the estimated energy outcome is 64 MWh (number of Buildings in entire city = 2,299, estimated Energy Output per building = 28 kWh. Thus, total estimated energy output = 2299 × 28 = 64 MWh). This is about 9% of the entire power consumption per day of the same city, which is 733 MWh in 2011 [14].

Fig. 4. Mumbai city show ing high raised buildings. Figure options

Let us take Hong Kong as another instance. For Hong Kong, the yearly power consumption is about 43,140,000 MWh. The expected power generation by the proposed method is 80,665 MWh per year, which could account to about 2% of the total requirement of power. Thus, there is an enormous potential for energy saving if the proposed system is implemented in different cities. Table 2 shows the estimated energy generation that is possible in different cities with most high raised buildings [18]. Table 2. Estimated energy output in cities w ith most high-rise buildings. City

No. of high rise buildings

Energy generation per day (in MWh)

Yearly (in MWh)

1

Hong Kong

7896

221

80,665

2

New York

6504

182

66,430

3

Sao Paulo

6467

181

66,065

4

Singapore

4764

133

48,545

5

Caracas

3864

108

39,420

6

Moscow

3754

105

38,325

7

Seoul

2955

82

29,930

8

Rio de Janeiro

2947

82

29,930

9

Tokyo

2779

77

28,105

10

Toronto

2511

70

25,550

11

Istanbul

2439

68

24,820

12

Mumbai

2299

64

23,360

13

Delhi

1805

50

18,250

…

…

…

…

35

Chennai

689

19

6935

44

Kolkata

619

17

6205

50

Pune

511

14

5110 Table options

4. Case study for feasibility analysis As a case study, we consider a twenty-stored building with on an average of 100 occupants in each floor. The assumed water consumption per individual per day is 171 L [17]. As discussed earlier, the water that can be utilized for production of electricity is 76% of the total water consumption. The collection tank is placed at the height of 10th floor and collects water from the floors above it. The total energy generated in such a system is about 6.8525 kWh (Fig. 5). This is the amount of energy that can light about 98, 70 W tube lights for about an hour, or can run a 6 kW water pump for an hour. Detailed calculations are shown in Appendix 2.

Fig. 5. Proposed system in high raise building. Figure options

4.1. Prototype development The scaled-down version (1:30 approximately) of the proposed system is installed on the campus to validate the concept. In the experimental setup, water flows through the supply tube rotating the turbine wheel coupled to a motor and exits through the draft tube. The minimum height of collection tank for getting a reasonable amount of power is about 30 m or 10 floors. The prototype (Fig. 6) is tested from its performance.

Fig. 6. Working scaled model. Figure options

5. Cost benefit analysis For a twenty storied building with an average of 100 occupants per floor, the initial investment involved, is close to Rs. 1,10,000 (about 1790 USD), based on our market survey. This cost includes the cost of purchasing a Pelton turbine of given specification along with an AC generator. Miscellaneous charges would include the cost of conduits supplying the grey water along with some minor regular maintenance costs. To calculate the break-even point [22], we calculate the equivalent revenue generated by the energy produced, thus offsetting the power requirement from the grid. The point where expenses and revenue are equal gives the break-even point as shown on the graph. No of units generated = Total Energy generated (1 unit = 1 kWh). No of Units = 6.8525 units, Cost per unit of electricity in Mumbai = Rs. 5.75 (about 0.1 USD). Revenue generated = 5.75 × 6.825 = Rs. 39.24, Break-even point = 7.68 years (Investment/(Revenue generated per day*365)) years = (1,10,000/(39.24*365)) = 7.68 years (Fig. 7). However, if the turbine is mass manufactured specifically for this application, the initial investment may reduce and thus the return on investment would come down. Additionally, if government provides tax benefit as incentive for reusing grey water, as provided for the usage of solar powered water heater, the price would further reduce.

Fig. 7. Breakeven point. Figure options

Since, on an average the life of a building is often taken as 40–60 years, the initial investment is very nominal for the monetary return that the system would provide for many years.

6. Discussion and conclusion There has been an enormous increase in the global demand for energy in recent years because of industrial development and population growth. Supply of energy is, therefore, far less than the actual demand. India is a developing country and has its share of energy related problems too. The prevailing scenario is generating growing calls for effective and thorough energy governance in India. India suffered from the largest power outage ever in late July 2012, affecting nearly half of the population. While this incident highlights the importance of modern and smart energy systems, it indicates that the country is increasingly unable to deliver a secure supply of energy to its population, a third of which still lacks access to electricity. The need for alternate ways of energy generation to support the current energy status is evident. This current research work proposes a novel approach that was validated by the theoretical formulations, considering the major associated losses accordingly as well. Installing a Pico hydro turbine at the ground floor of high-rise building has never been practiced in India. The same proposed system could be used in cities of different countries. Moreover, it is an eco-friendly, renewable, clean power generation method. The feasibility of the system is analyzed. Proper implementation of this proposed system can play an imperative role in the solution to the power problem in city life. The proposed system tries to approach zero net energy consumption principle as a means to reduce carbon emissions and reduce dependence on fossil fuels by reducing the overall use of energy. The Zero-Energy building goal is becoming more practical as the costs of alternative energy technologies decrease and the costs of traditional fossil fuels increase [3] and the proposed system supports this effort. Additionally, the proposed system aims at efficiently using water to generate electricity, which is the major objective of a green building [23]. Green Building (or sustainable building) as certified by LEED (Leadership in Energy and Environment Design), refers to a structure and using a process that is environmentally responsible and resource efficient throughout a building's life cycle.

Appendix 1. Theoretical details of the system Let H be the height from the ground floor to the top of the collection tank. Water, from all the floors above, is collected in the cylindrical tank, which has a radius R and has height X. Accordingly, the volume of the tank V is V = π × R × R × X. A water level sensor is used that operates a valve at the bottom, which opens up when this volume of water is filled up in the tank. The tank is further connected with a pipe of radius r carries water of volume v = π × r × r × (H−X). Thus, the head (h) of water can be obtained by calculating center of mass of the system, h = (V × (H−X/2) + v × X/2)/(V + v). Now, in order to calculate the volume flow rate of water (Q) through the pipe, we need the time required to empty the tank (neglecting the effect of volume in the pipe compared to volume in tank, however, considering the variable height of water in the tank as decreasing), thus, we need to integrate the above equation for an interval, t

= A × √ 2× X / g / ( a × C ) , and Q = V/t.

Additionally, power at the turbine shaft is obtained due to the rotational energy gained by the turbine wheel because of the water striking its blades and this is obtained by, PT = ρ × Q × g × h × ηo × 10−3 . Thus, number of times turbine works = total utilizable water/volume of tank. d is the diameter of pipe at the point of water outlet, where it strikes the turbine blades, then, d = 2 × r. A Pelton turbine is selected for the system [21]. The Pelton wheel is a water impulse turbine. Pelton's paddle geometry was designed so that when the rim runs at half the speed of the water jet, the water leaves the wheel with very little speed, extracting almost all of its energy, and allowing for a very efficient turbine. Our selection is Pelton turbine because we have low volume flow rate and reasonably high head. Moreover, it is easily available in Indian markets and the manufacturing costs are low. The other two commonly known turbines, viz. Kaplan turbine and Francis turbine were not considered as, Kaplan turbine requires much higher flow rate and Francis turbine require both axial flow and higher flow rate.

Design of a Pelton turbine Bucket width (B) between the rims of bowls varies from 3.0d to 3.5d ( Fig. 8). Let, L, the length or height of the bowls inside the rim, 0.8B–0.9B. Cd is the depth of the bowls 0.27B–0.3B. Let, l, is the distance from jet center line down to the rim of the bowls, 0.5L–0.6L. M, width of the mouth or outlet is from 1.1d to 1.5d. Then,

diameter of Pelton turbine is given by, D = 0.005 × h × d + 8 × d. And, velocity at inlet of turbine Vi is given by the drop in height, Vi = √ ( 2× g × h ) . Circumferential speed of turbine Ui = Vi /2. Thus, turbine RPM (Nt) is given Download by = UPDF i × 60/π × DExport Search ScienceDirect

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Fig. 8. Pelton turbine. Figure options

Generator calculations For an AC Generator rotor RPM is given as Ng = 120 × f/P, where, f = number of cycles per second, and P = number of poles (such as, 2, 4, 6…). To achieve the required RPM of the generator, a gearbox is used between the turbine and the generator. Also, gearbox ratio G = Ng /Nt ... (14). And, power output from the generator is given as, Po = ηg.b × ηg × Pt. Where, ηg is the generator efficiency, ηg.b is the gearbox efficiency. Thus,

Total

energy

generated

can

be

calculated

by

... (16).

Appendix 2. Model of the system installed in a 20-floor building Calculation for total utilizable water in the building = 171×100(no. of person) × 10(floors) × 0.76 = 130 m3 . Thus, volume of collection tank, V = π × R × R × X = 32.4308 m3 (Taking R = 1.855 m; X = 3 m) and volume of pipe, v = π × r × r × (H−X) = 0.0855 m3 (Taking r = 0.03175 m; H = 30 m(≡10 floors)). The head (h) of water, h = (V × (H−X/2) + v × X/2)/(V + v) = 28.344 m, and t

=

= 2725.441599 s, taking C = 0.98 (for rounded orifice). Thus, Q = V/t = 0.011893286 m3 /s. PT = ρ × Q × g × h × η × 10−3 = 2.477 kW (Taking Pelton turbine efficiency (η = 0.75)) and number of times turbine works = 130/32.43 = 4.

Design of turbine B = 3.3d = 0.209 m, L = 0.8B = 0.167 m, Cd = 0.27B = 0.056 m, l = 0.5L = 0.083 m, M = 1.1d = 0.069 m, D = 0.005 × h × d + 8 × d = 0.516 m. Thus, Vi = √ ( 2× g × h ) = 23.569 m/s, Ui = Vi /2 = 11.784 m/s, Nt = Ui × 60/π × D = 435.5 rpm. Generator calculation for f = 50 Hz supply and 4 pole generator: For, Ng = 1500 rpm, and gearbox ratio G = Ng /Nt = 3.4, using set of Spur gears assuming efficiency ηg.b = 0.94, and generator efficiency ηg = 0.97. Thus,

Po

=

ηg.b.

×

ηg

×

Pt

=

2.258

kVA,

and

total

energy

generated

E = 2.258 × 2725.44 × 4/3600 = 6.8525 kWh.

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