Abstract

In the field of waterworks in urban areas, renewable energy is being utilized through implementing solar power generation in the upper parts of facilities and hydraulic power generation using gravity flow of water and excess pressure. As a result, we have learned that utilization of renewable energy not only decentralizes and diversifies energy supply to enhance reliability and environmental performance but also recovers its costs sufficiently. This paper proposes several utilization models of renewable energy in metropolitan waterworks by presenting installation, maintenance and management of power generation equipment with renewable energy, power generation volume, reduction of CO2 emissions, future plans and further utilization methods.

INTRODUCTION

IWA President Kroiss questioned in his keynote lecture (Kroiss 2015), ‘Is energy a key issue for urban water management?’, and emphasized the necessity of reliable energy supply and the importance of taking immediate action against climate change. As one of the actions to resolve these key issues, the Bureau of Waterworks, Tokyo Metropolitan Government (hereinafter referred to as ‘Tokyo Waterworks’), promotes the utilization of renewable energy.

Serving water to 13.04 million people and having the facility capacity of 6.86 million m3 per day and 26,774 km-long distribution pipes in total (as of 2015), Tokyo Waterworks has one of the largest scales in the world. The water transmission and distribution pipe network of a city in which population and industries are concentrated in an undulating plain is extremely complex, but we supply tap water with appropriate pressure for end-consumers. By its activity, we use around 1 percent (around 800 million kWh per annum) of the total electricity usage in Tokyo, which affects the global environment. Therefore, we need to make efforts to save energy.

We have worked for energy saving and environmental countermeasures, such as management of water conservation forests (Nakamura 2011), water leakage prevention measures (Toki 2015) and improvement in efficiency of pump equipment (Iwasaki 2012). As a result, the sand accretion rate of our dam and the leakage rate of the water pipe network reach the top level in the world. Therefore, there are limitations to further improvement. Instead, there is more need to create energy by environmentally friendly power generation. In other words, it is important to create renewable energy by solar power generation and hydraulic power generation. These efforts also work for decentralizing and diversifying energy supply and enhancing its reliability.

Our experience of installing, maintaining and managing a lot of power-generating equipment in urban areas, our past records of utilizing renewable energy in various ways and our plans to promote this utilization in the future provide useful information for other waterworks. This paper proposes several utilization models of renewable energy in metropolitan waterworks.

MATERIALS AND METHODS

Solar power generation in the upper parts of facilities

In urban areas, it is difficult to find spaces for installing solar panels or to secure sufficient sunlight. In purification plants, Tokyo Waterworks has installed solar panels together with coverlids to protect filter basins from foreign matter (Figure S1). In recent years, we have installed solar panels on the top of distribution reservoirs and buildings (Figures S2 and S3). It is noted that we give due consideration to neighboring residents by using anti-glare solar panels to reduce reflection of sunlight.

Data on water pressure and flow rate at each location within the water pipe network are collected by telemeters, and these data are used for automatic control of transmission and distribution pumps. Therefore, if the telemeters stop functioning due to electric power failure, it becomes difficult to implement proper water transmission and distribution. To avoid such a consequence, the important telemeters are equipped with solar panels and storage batteries (Figure S4). The power generation capacity of each equipped solar panel is weak as it is less than 100 W, but it is expected to contribute to the stable operation of telemeters at a time of disaster.

We are currently testing a telemeter equipped with solar panels that have 200 W or more of power generation capacity, aiming at enabling longer operation of telemeters at a time of power failure and decreasing dependence on commercial electric power sources at normal times (Figures S5 and S6). As the size of solar panels becomes bigger, this test includes the study of such issues as safety checks, combination with storage battery and control of charge and discharge. (Figures S1–S6 are available with the online version of this paper.)

Hydraulic power generation using gravity flow of water

In hydraulic power generation, we use the gravity flow of raw water from reservoirs down to purification plants (Figure S7). Although it requires large-scale equipment in the case of high effective head or large flow rate, it is ideal from the viewpoint of power generation capacity. Higashimurayama Purification Plant realizes hydraulic power generation of 1,400 kW by 13.5 m of effective head and 13 m3/s of flow rate (Figures S8 and S9).

Himura Purification Plant, one of the small-scale purification plants of Tokyo Waterworks, which is located in a mountain-ringed area, realizes hydraulic power generation of 7 kW by 54 m of effective head and 0.023 m3/s of flow rate (Figures S10 and S11). To keep the cost low, ready-made hydraulic power equipment was installed and two units were connected in series for the adjustment of high effective head and small flow rate. In general, since the volume of water intake is adjusted by the volume of water distribution, generated energy also changes its volume accordingly. Therefore, we made up a system to secure constant water intake and generated energy 24 hours a day by taking an appropriate volume of water for power generation constantly from the upstream and discharging excess flow back to the downstream after generating. This system can lead to an increased capacity factor of the power generation equipment. In addition, there remains 26 m or more of effective head after generating. The head enables membrane filtration without a pressure pump. In other words, the plant runs hydraulic power generation with its utmost capacity factor by gravity flow to a maximum extent, while it also purifies water through membrane filtration without additional energy at the same time. Although the plant is small-scale, it could be an ideal model case of purification plants taking advantage of gravity flow and renewable energy. (Figures S7–S11 are available with the online version of this paper.)

Hydraulic power generation using excess pressure

Tokyo Waterworks promotes minimization of water transmission and distribution energy by making full use of various energy management systems (Okamura 2013; Kaneko 2014), but in the complex water pipe network, it is nonetheless inevitable that excess pressure arises due to the different distances from pumps and altitudes. The excess pressure becomes unused energy due to the passage of pressure reducing valves and the pressure release at the water supply station. Regarding this unused energy as renewable energy, we generate electricity by waterpower (Figure S12). Although it is difficult to find new land in urban areas, we have installed hydraulic power generation equipment in part of a small space at a water supply station by re-arranging pipework in the station (Figure S13).

At Kohoku Water Supply Station, which Tokyo Waterworks is currently constructing, high excess pressure is expected to arise. Therefore, in tandem with hydraulic power generation equipment, it is planned to install a directly connected water distribution pump (Figures S14 and S15). Under this system, excess pressure is used for power generation or as booster by switching in accordance with water demand, and this is called the Hybrid Energy Saving System (Masuko et al. 2011; Taniguchi 2013; Matsuda 2015). (Figures S12–S15 are available with the online version of this paper.)

RESULTS AND DISCUSSION

Effects of solar power generation equipment

We generated 3.88 million kWh and reduced 1,476 t of CO2 emissions by solar power generation equipment (from 1 April 2014 to 31 March 2015) (Table S1, available with the online version of this paper). In calculating the volume of reduction of CO2 emissions, an emission conversion factor of 0.382 t-CO2 per thousand kWh (Bureau of Environment, Tokyo Metropolitan Government 2012) is used.

As for solar power generation equipment, although there were some unfortunate cases that required minor repairs of lighting damage, no other major problems occurred. Thus, maintenance and management are easy. Accumulated total generation capacity of solar power generation equipment is 6,402 kW. As solar power generation is possible only in daytime and dependent on weather, capacity factor remains relatively low as expected. However, as long as spaces for installation are secured, solar power generation has less trouble and can provide generated energy in a stable manner on a long-term basis. In particular, water supply facilities, such as purification plants and water supply stations, can get sunlight easily in urban areas since they have large spaces including the upper parts of facilities. We are planning to increase the accumulated total generation capacity up to 8,000 kW by 2020 and to 10,000 kW by 2025. However, it is noted that an increase in power sources with fluctuating output, such as solar power generation, may require the arrangement of measures against output fluctuation.

The peak time of solar power generation coincides with that of power demand in urban areas, and it works beneficially for peak shaving or peak-cut. Nagasawa Purification Plant shows a good example. It is a medium-scale purification plant with a facility capacity of 200,000 m3 per day, and solar panels installed on the upper part of filter basins provide most, or sometimes more than 100 percent, of total electricity consumed by the plant during hours with good sunlight conditions. This is partly because water intake and distribution are managed by gravity flow, but solar power generation usually takes up 15 percent (annual average) of total electricity consumption.

Although Japan does not suffer electric power failure so much in a global sense, there are still risks of power failure. It is confirmed that when power failure happened several times, telemeters installed with solar panels and storage batteries functioned properly without any trouble and enhanced their reliability. Such installation appears promising as decentralized electric power sources are applicable not only to telemeters but also to other water supply equipment. From now on, we are studying the installation of solar panels and storage batteries for industrial measuring instruments, automatic water quality meters and electric anti-corrosion equipment and verifying their effects.

Effects of hydraulic power generation equipment

We generated 3.12 million kWh and reduced 1,194 t of CO2 emissions by hydraulic power generation equipment (from 1 April 2014 to 31 March 2015) (Table S2, available with the online version of this paper).

As for hydraulic power generation using gravity flow, this is operated by raw water, which contains some amount of foreign matter and particles. Thus, abrasion, erosion and corrosion of the parts tend to occur to a certain extent. For example, hydraulic power generation equipment at Higashimurayama Purification Plant underwent overhaul and repair for the first time after 14 years from its installation. Especially, parts for shaft seals were found to be damaged seriously. As this could be caused by usage of raw water as shaft seal water, alteration was made in order to use purified water. This improvement can be expected to reduce mechanical failure.

With regard to hydraulic power generation using excess pressure, since the equipment uses purified water that does not contain foreign matter or particles, no major troubles occurred and it is easy to maintain and manage. Generation capacity of the hydraulic power generation equipment is 2,232 kW cumulatively and suitable places for hydraulic power generation are limited, while it can operate 24 hours per day in most cases and thus, capacity factor is relatively high. Therefore, like solar power generation equipment, as long as spaces for installation are secured, it has less trouble and can provide generated energy in a stable manner on a long-term basis.

Direct distribution system for energy saving

We have discussed several renewable energy efforts and all the efforts are technically valuable. Since the hybrid energy-saving system is particularly innovative, we discuss the system in detail. Regarding the system, it is important to reduce energy loss due to pressure release at the reservoir. A direct distribution system is helpful for reducing pressure release. Therefore, we define and introduce a formula to find out how much water distribution energy can be reduced by a direct distribution system.

In this simulation, we choose a mono-modal symmetrical demand pattern for simple calculation. Out of the daily volume in the reservoir of a water supply station, the volume of water supplied by the direct distribution pump is expressed by d, the directly distributed water rate, which is defined in the following formula (Figure 1):  
formula
(1)
Figure 1

Outline of directly distributed water.

Figure 1

Outline of directly distributed water.

Here:

  • d = directly distributed water rate

  • q = daily water volume delivered by direct distribution pump (m3/day),

  • Q = daily water volume distributed (m3/day).

Meanwhile, the remaining pressure of water received in a water supply station compared with the water distribution pressure is expressed as the remaining pressure rate z, which is defined in the following formula (Figure 2):  
formula
(2)
Figure 2

Correlation between water pressures received and distributed.

Figure 2

Correlation between water pressures received and distributed.

Here:

  • Hz = remaining pressure of water received at water supply station (m),

  • Hh = pressure of water distribution at water supply station (m).

Compared with the conventional water distribution system where the entire volume is discharged to the reservoir, how much water distribution energy can be reduced by the direct distribution system is expressed as R, which is defined in the following formula (for the rate of water distribution on energy reduction, the focus is on calculation using a simple formula, so the valuation is based on the water kinetic power reduction rate):  
formula
(3)
Here:
  • P = water kinetic power (kW),

  • Pd = water kinetic power when the directly distributed water rate is d (kW),

  • P0 = water kinetic power when the directly distributed water rate is 0 and all volume is filled in the reservoir (kW).

Pd is the sum of the water kinetic power of the direct distribution pump and the water kinetic power of pump distributed water drawn from the reservoir, for which the formula is as follows:  
formula
(4)
Here:
  • Pc = water kinetic power of direct distribution pump (kW),

  • Pi = distributed water kinetic power of the reservoir's distribution pump (kW).

Pc is calculated as:  
formula
(5)
Here:
  • g = gravitational acceleration (9.8 m/s2)

  • Q = flow rate (m3/day)

  • H = total head (m)

  • γ = specific gravity (kg/L)

  • 86400 = 60 · 60 · 24 (s/day)

  • ηP = pump efficiency = 0.8

  • ηM = motor efficiency = 0.85.

Meanwhile, Pi is calculated as:  
formula
(6)
Substitute formulas (5) and (6) into formula (4) to obtain Pd.  
formula
(7)
Reduction rate R is calculated as:  
formula
(8)
The above shows R is the product of d · z. For example, in the case that the directly distributed water rate is 60%, and the remaining pressure rate is also 60%, the product would be 36%, which is the expected reduction of energy. The correlation between the water distribution energy reduction rate, the directly distributed water rate, and the remaining pressure rate is shown in Figures 3 and 4.
Figure 3

Correlation between energy reduction rate and directly distributed water rate.

Figure 3

Correlation between energy reduction rate and directly distributed water rate.

Figure 4

Correlation between energy reduction rate and remaining pressure rate.

Figure 4

Correlation between energy reduction rate and remaining pressure rate.

Upper limit of directly distributed water rate

Although the water distribution energy reduction rate can be increased by raising the directly distributed water rate, the amount of water supplied through reservoirs decreases in exchange. Consequently, problems of water quality including decline of residual chlorine, buildup of disinfection by-products, and degradation of taste come out. Therefore, it is discussed to what level the directly distributed water rate could be raised within the range not affecting the water quality.

The retention time of the water is 0 during periods of time with less demand, such as night, because all water is distributed directly. However, since the water is typically a mixture of that directly delivered and that delivered from the reservoir, the retention time of the water supplied from a water supply station is typically calculated by applying a weighted average of the retention time of each.

Water quality must be managed most carefully when the rate of the water delivered from the reservoir becomes the maximum. For the operation in which the direct distribution pump is working all the time, this is the time when the demand peaks, and the directly distributed water rate is set so that the retention time during that time may not become excessive.

Additionally, for the operation in which the direct distribution pump is not working all the time but is stopped for some periods, reservoir water is delivered directly. Therefore, it is necessary to set the direct distribution rate so that the retention time in the reservoir may not become excessive.

First, let T0, the retention time of the water all of which is delivered through the reservoir, or of which the direct distribution rate is 0, be defined in the following formula:  
formula
(9)
Here:
  • T0 = retention time of the water of which all is delivered through the reservoir (h),

  • V = daily average amount of water stored in the reservoir (m3),

  • Q = daily water-delivery in water supply station (m3/day).

Subsequently, daily inflow to the reservoir is (Qq) when the direct distribution rate is d. The retention time of water drawn from the reservoir, Ti, is as follows.  
formula
(10)
Here:
  • Ti = retention time of the water drawn from the reservoir (h).

Tb, the retention time of the mixture of directly distributed water and water delivered by pump from the reservoir, is expressed in the following formula:  
formula
(11)
Here:
  • Qi = hourly water volume drawn from the reservoir (m3/h),

  • Qh = hourly water volume at water supply station (m3/h).

The maximum hourly water delivery in the water supply station, Qhmax, which differs according to the demand property of the water distribution area, is generally expressed as follows using time coefficient K:  
formula
(12)
Here:
  • Qhmax = maximum hourly water delivery in water supply station (m3/h),

  • K = time coefficient,

  • Q = daily water delivery in water supply station (m3/day).

On the other hand, Qimax, the maximum hourly water volume, is computed by subtracting Qdmax, the hourly water volume directly distributed from the reservoir, from Qhmax, the maximum hourly water volume, as expressed in the following formula:  
formula
(13)
Here:
  • Qimax = maximum hourly water volume directly distributed from the reservoir (m3/h),

  • Qhmax = maximum hourly water volume (m3/h),

  • Qdmax = hourly water volume directly distributed from the reservoir at the time, when the hourly water volume is maximum (m3/h).

We have made it a rule to distribute a certain water volume, except in the midnight period, when water is distributed exclusively directly. Hence Qdmax is about equal to the quotient upon division of q, daily water delivery, by 24 (Figure 5). Although its precise value is slightly larger, the resulting retention time of the mixed water gives a larger value, which gives an error on the safety side. Therefore, Qdmax is approximated by this formula:  
formula
(14)
Figure 5

Retention time of the mixture.

Figure 5

Retention time of the mixture.

Tbmax, the maximum retention time of the mixed water, is expressed as follows:  
formula
(15)
Here, the retention time magnification between T0 and Tbmax is defined as α. Then, formula (15) is arranged as follows:  
formula
(16)
where  
formula
The directly distributed water rate d is expressed using the value α, as follows:  
formula
(17)
With the above formula, the maximum value of d can be obtained with α and K. As a concrete case, design criteria for waterworks facilities define the standard value of T0 as 12 hours. If Tbmax is defined as 24 hours, which is an appropriate value in managing water quality, α becomes 2, and formula (17) is written as follows:  
formula
(18)

In the case of coefficient K = 1.8, d is 0.69 and consequently, d shall be within 0.69 to make the retention time of the water distributed from the water supply station within 24 hours. In general, α is assumed around 2. Here, we indicate the relation between d and K for three cases where α = 1.5, 2.0, 2.5 (Figure 6). Additionally, we show what the directly distributed water rate becomes at peak time for both demand patterns with small and large K (Figure 7).

Figure 6

Retention time of the mixture.

Figure 6

Retention time of the mixture.

Figure 7

Directly-distributed water rates at peak time for demand patterns with small and large K.

Figure 7

Directly-distributed water rates at peak time for demand patterns with small and large K.

The figures above show that the directly distributed water rate can be set high for the flat demand pattern with small K. This is because the ratio of the water delivered from the reservoir is comparatively small enough for the retention time of the mixed water to satisfy the limit even when the retention time in the reservoir is extended because of the raised directly distributed water rate. On the other hand, it is shown that the fraction of water delivered from the reservoir becomes large at peak time for the demand pattern with large K, and that consequently the directly distributed water rate shall be set low to shorten the retention time in the reservoir.

We show the relation between d and α, which is computed by formula (17) (Figure 8). It shows that although d increases sharply as α increases from 1 to 2, the increase of d becomes gentle as α approaches 2, and the increase becomes milder as α exceeds 2. Meanwhile, when α exceeds 2, the retention time of the mixed water exceeds 24 hours if the capacity of the reservoir is for 12 hours' supply. Consequently, problems of water quality including consumption of residual chlorine and others would come out. Additionally, it should also be considered that water with considerably long retention time (computed by formula (10)) is delivered directly without any mixing in the case that the direct distribution pump stops unexpectedly. Therefore, in general it would be appropriate to consider d where α is within 2. Since the reduction ratio of water distribution energy R is the product of d and z as shown in formula (8), the relation between d and retention time is just the same as the one between R and retention time. It means that R becomes larger as the retention time is set longer, but problems of water quality will come out. In contrast, if the retention time is set shorter, problems of water quality will diminish, but R will also become smaller. By balancing the two parameters, which are trading off each other, in the range where water quality is not affected, such as by setting α within 2, and computing d, the optimum solution of R can be obtained. This is expressed in the following formula, leading from formulas (8) and (17):  
formula
(19)
Figure 8

Correlation between directly distributed water rate d and retention time magnification α.

Figure 8

Correlation between directly distributed water rate d and retention time magnification α.

The above formula enables R to be calculated easily from z and K by setting α in the range where the water quality is not affected.

Future plans for energy diversification

According to the standards set by Tokyo Waterworks, solar power generation equipment has 20 years of lifetime, while hydraulic power generation equipment has 22 years. Considering the trial calculation so far and the actual performance of power generation, it is highly possible to recover costs for construction and maintenance within their lifetime by appropriately reduced electricity costs by self-consumption and power-selling profits though the support of public systems. In particular, the payback period of hydraulic power generation equipment is as short as 10 years. As for solar power generation, the payback period varies depending on installation conditions and time. This is because the price of solar panels decreases year after year, while the purchase price of generated electricity under the Feed-in Tariff scheme goes down. It is necessary to observe price change and system trends closely when planning construction of power generation equipment in the future.

From the viewpoint of decentralization and diversification of energy supply for important facilities, Higashimurayama Purification Plant shows an excellent example. The plant is a large-scale purification plant with a facility capacity of 1.265 million m3 per day and the average power generation is approximately 5 million kWh per annum before the overhaul and repair of hydraulic power generation equipment. It is equivalent to 20 percent (annual average) of total electricity usage within the plant and it has recorded even 25 percent (annual average) (Figure S16). In this case, renewable energy consisting of solar power generation and hydraulic power generation covered nearly 30 percent of the energy mix. This shows a well-balanced energy mix combining several sources of electricity supply, which enhances the reliability of the energy supply. However, the ratio of renewable energy out of the total amount of energy consumption of Tokyo Waterworks as a whole is merely about 1 to 2 percent. We are making an effort to increase the share of renewable energy at other facilities by considering the energy mix.

In Tokyo Waterworks, many facilities are facing a time of renewal. Taking the opportunity of the renewal, expansion and new construction of the facilities, we plan to optimize facility placement and take full advantage of gravity flow for water intake, purification, transmission and distribution (Tokyo Waterworks 2016a, 2016b). In addition, from our past research results and experience, we plan to place the solar panels and hydraulic power generation equipment appropriately and promote this as the utilization model of renewable energy (Figure S17) (Figures S16 and S17 are available with the online version of this paper).

Challenges for hydrogen energy

We launched Tokyo Waterworks Innovation Project (Tokyo Waterworks 2016a, 2016b) in 2016. Under this project, in addition to solar power generation and hydraulic power generation, production and utilization of hydrogen are raised as one of its measures, which propose production of hydrogen, by renewable energy (Figure S18, available with the online version of this paper). Technology for production and storage of hydrogen would be potential measures for fluctuation of power generation output by renewable energy. Also, empirical research on utilization of hydrogen generated by exhausted water purification plant facilities, i.e. by-product hydrogen generated in the production process of sodium hypochlorite for disinfection, has been already planned. The chemical reaction formula is as follows:  
formula
(20)
Molar masses of each substance are as follows:  
formula
 
formula

Therefore, in the case that 1 kg of sodium hypochlorite is produced, 26.9 g of hydrogen gas is also produced at the same time. In general, this process produces higher concentrations of hydrogen gas than the process by petroleum. Asaka Purification Plant shows an interesting example. It is a large-scale purification plant with a facility capacity of 170,000 m3 per day. Asaka Purification Plant produced approximately 520,000 kg (annual average) of sodium hypochlorite for disinfection. This means approximately 14,000 kg (annual average) of hydrogen gas was created and such volume is attractive as unutilized energy. However, several challenges remain to utilizing hydrogen. Especially, one of the most important issues is the cost for packing and transporting hydrogen gas. In future, waterworks and energy supply will get closer through production of water and hydrogen.

CONCLUSIONS

In response to the question, ‘Is energy a key issue for urban water management?’, our answer is ‘Yes’ and this paper shows one of the actions to resolve the issue. As for our own question, ‘Is renewable energy a key to solving energy and environmental Issues?’, our answer is also ‘Yes’. We will continue to enhance the reliability, environmental performance and economic efficiency of energy supply with the utilization of renewable energy. We hope that our action will give ideas for energy-saving and environmental measures to many water utilities.

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Supplementary data