In improving the energy efficiency of water transport systems, two critical stages are involved: assessment (to understand the system's operation and identify potential energy savings) and auditing (to locate and break down the energy losses). Both stages are based on energy balances, which can be conducted using either the extended Bernoulli equation or the energy integral equation. Both equations can be applied, but depending on the system, data availability, and the kind of study to be performed, one is preferable over the other. This paper analyses, applies and compares both equations, with a particular focus on the less commonly used energy integral equation in the hydraulic field. This more general equation includes thermal and transient effects and it is more suitable for analyzing complex systems. In contrast, the extended Bernoulli equation, while simpler to apply, can lead to the loss of relevant information, such as the evaluation of the topographic energy. The main objective of this work is to bridge the gap between these two fundamental energy equations and recommend the most appropriate one for hydraulic problems. Real examples are presented to show their differences and validate our recommendations.

• A comparison is made between the energy integral and Bernoulli's equation, in order to know which one is more appropriate when analyzing a system energetically.

• The most suitable energy analysis for multi-topology systems, which are those with different operating layouts, is explained in depth.

• A guide to analyze the energy of pressurized water systems is presented, with the ultimate goal to improve their efficiency.

Ee

electric energy consumed

Ehp

hydraulic energy delivered by pumps (Ehp=Ee.ηe.ηp)

EN

natural energy supplied to the system

ET

total energy supplied to the system (=Ee+EN)

Eou

output energy useful (supplied to users)

Eol

output energy lost (in leaks)

Eo

output energy (=Eou+Eol)

Eoum

minimum energy required by users

EΔE

excess (Δ) of energy supplied to users (=EouEoum)

Hoi,m

minimum piezometric required

hp

Ie

energy intensity

real energy intensity

Pt

power supplied by compensation tanks

PN

power gravitational (natural)

PSl

total power system losses

Ppl

power pumping station losses

PT

total power supplied

Peh

hydraulic power input (entrance) to the system

Pe

total electric power input

Pf

power dissipated by friction

Poh

output hydraulic power

Pohl

power lost in leaks

Pohu

useful delivered power

Pohum

useful (minimum required) delivered power

Pml

power lost (other miscellaneous losses)

PΔP

power in excess (Δ) supplied

pm

minimum pressure

Qt

tank net flow

u

internal energy

z

elevation

Ws

hydraulic energy delivered by pumps (Ws=Ehp)

friction energy variation

ΔEG

gravitational energy variation

thermal energy variation

ΔT

temperature variation

electric motor efficiency

pump efficiency

global efficiency

global efficiency considering the minimum standard pressure

The first question to address is why the extended Bernoulli equation is preferentially used in hydraulic flows whereas the energy integral equation is, surprisingly, rarely applied. The primary reason is that since most hydraulic flows are assumed to be incompressible and one-dimensional, the extended Bernoulli equation fits these problems perfectly. However, the energy integral equation (White 1973), which includes thermal and mechanical terms as well as various inputs and outputs, is more general and therefore not as immediately applicable. In fact, it can be used to analyze any problem because it performs a general energy balance over a control volume (CV) limited by a control surface (CS). Furthermore, as these two equations have historically different origins, they can only be compared after formulating restrictive hypotheses to simplify the energy integral equation to equate it to the extended Bernoulli equation, a generalization of the original equation formulated by Bernoulli in the 18th century (Rouse & Ince 1963).

The energy integral equation was derived by starting with the first law of thermodynamics and then, using the Reynolds transport theorem (White 1973). In the absence of other energies (such as those related to chemical or nuclear reactions), the total energy per unit mass is the sum of the internal energy (u), the kinetic energy (v2/2, where v is the velocity) and the gravitational energy (gz, where g is the acceleration due to gravity and z the elevation). The most general form of the energy integral equation (three-dimensional) is:
(1)

This form was derived by separating the flow work through the CS (between the system and the external medium) and the shaft work, commonly associated with turbomachines. According to the sign convention for work (work done by the system is considered positive), pump work is negative and turbine work is positive. The other left-hand side term of the equation, the heat transfer rate, , represents the thermal energy exchange between the external medium and the system through the CS because of the existence of a temperature gradient (heat transfer to the system is considered positive). Other variables are, ρ, the fluid density; dV, the differential of volume inside the CV; p, the pressure and, the differential flow through the CS. This formulation has few constraints and is widely used in thermal fluid mechanics.

As most common applications involving fluid flow through pipes or ducts can be solved with sufficient accuracy for engineering purposes treating the flow as one-dimensional, the Bernoulli mechanical equation is an important tool for solving energy problems. Its origin is a force balance (Newton's second law) applied on a fluid particle moving along a streamline in a steady inviscid flow. Under the additional assumption of incompressible flow, the resulting force balance is integrated along a streamline and, as the product of force and displacement is energy, the result is a mechanical energy balance between the initial and final points. The Bernoulli equation is usually expressed in energy per unit of weight (m) as follows:
(2)
where γ is the specific weight of the fluid. This expression is conceptually important. It shows the different forms that a fluid has to convert mechanical energy but has limited applicability. Therefore, it needs to be generalized. As a starting point, the Bernoulli equation is applied to a pipe, assuming its axis as the average streamline of the flow, and including the friction between the fluid and the walls of the pipe throughout the Darcy–Weisbach equation. The addition of shaft work, provided by a pump or removed by a turbine, (White 1973) yields the extended Bernoulli equation:
(3)
where f is the friction factor (dimensionless), L the pipe length (m), hp the pumping head (m) and ht the turbine head (m). A more complete formulation, including inertial terms, is used to model rigid transients, but in the following energy analyses, it is not of interest because of the very different scale of time of these problems. Henceforth, we simply refer to Equation (3) as the Bernoulli equation.

Since the Bernoulli equation (Equation (3)) involves only mechanical energy, whereas the energy integral equation (Equation (1)) includes both mechanical and thermal energy, a crucial point in comparing the equations is related to the coupled nature of the physical problem, that is, with the possible conversion between thermal and mechanical energy, which is the clear boundary between thermal fluid mechanics and hydraulics.

In hydraulics, the special case of incompressible fluids, with both density and viscosity constants, is of considerable importance (Batchelor 1967; Panton 2013). The continuity and the momentum equations are simpler and, more importantly, decoupled from the energy equation (the momentum equation does not include the temperature). Consequently, the mass and momentum equations form a set of four scalar equations with four mechanical unknowns ( and p). Later, when necessary, the obtained velocity field can be substituted into the energy equation to determine the temperature field. On the other hand, for an incompressible fluid, thermal and mechanical energies are only tied by friction.

The integral energy equation (Equation (1)) can be applied to both incompressible and compressible flows. One example is the analysis of a water hammer (elastic conduit with slightly compressible fluid) from an energy perspective (Karney 1990), where velocity and pressure changes are related to account for water compressibility (elastic effects), while the density is assumed only a function of the pressure (which is captured via the bulk modulus of elasticity). Thus, the flow is considered essentially isothermal (due to temperature, there is a negligible change in flow properties) and the thermal and mechanical variables are decoupled.

Conditions under which Bernoulli can be obtained from the integral energy equation (Equation (1)) are well-known and restrictive (White 1973; Munson et al. 1990). They are a fixed CV with one inlet and one outlet section (1 and 2, respectively), being the flow normal to control surfaces and uniform properties over plain perpendicular cross-sections. Briefly, the flow must be one-dimensional, a strong constraint that makes the difference (Figure 1).
Figure 1

Single input, single output systems, to apply the Bernoulli equation.

Figure 1

Single input, single output systems, to apply the Bernoulli equation.

Close modal
On the other hand, the energy equation can be applied over a CV without restrictions, that is, any arbitrary CV, through which the fluid flows, with multiple inputs and outputs (Figure 2).
Figure 2

Multiple input/output systems, to apply the energy equation.

Figure 2

Multiple input/output systems, to apply the energy equation.

Close modal
Considering a steady incompressible fluid flow, dividing Equation (1) by (where is the mass flowrate, kg/s) and rearranging terms, it yields the one-dimensional, incompressible, steady flow, energy equation that includes both mechanical and thermal terms (Equation (4)):
(4)
where , is the heat flow per unit of mass.
A thermal energy balance is obtained combining the mechanical energy balance (the extended Bernoulli equation, i.e., Equation (3)) and the simplified energy equation (Equation (4)). It results in:
(5)
where is the head loss, being and , respectively, the friction and thermal energy variations (per unit weight). Any increase in above q is due to the conversion of mechanical energy into thermal energy that happens because of the viscosity of the fluid. Therefore, represents the mechanical energy loss per unit of weight, through a mass friction loss term. So, in an ideal fluid flow (no viscosity and, therefore, no friction losses), , and therefore, . In conclusion, the simplest Bernoulli equation (Equation (2)) applies, while both mechanical and thermal energies are conserved separately.

The above analysis confirms that, for an incompressible fluid flow, friction is the only link between thermal and mechanical energy. From a mechanical perspective, friction takes its toll converting useful mechanical energy into dissipated heat energy. If there is no heat transfer, the temperature of the fluid increases because of the friction, and, in the end, u2 becomes greater than u1. On the other hand, when the flow is not adiabatic (the system is not well insulated), due to the temperature gradient, heat will flow to the surroundings. Last, for an isothermal flow (both temperature and u, constants), the system has a loss of heat, q, at the same rate that friction converts mechanical into thermal energy.

In short, the same problem can be analyzed from different perspectives using both equations. With different origins (one being integral and thermal, the other differential and mechanical) they follow opposite paths: generalization, in the case of the Bernoulli equation, and simplification, for the integral energy equation, where an initial global power balance integrated over time becomes an energy balance. This last versatile equation can be employed for energy diagnosis and auditing but requires more information than the simplest Bernoulli equation. The latter is recommended to diagnose and audit simple systems and, when applied repeatedly, it can solve more complex systems (such as hydraulic networks), albeit in snapshots.

To show the particularities of each equation, this paper is organized as follows. The next section is a short reminder, through the energy intensity concept, of how to apply the Bernoulli equation to simple and complex systems. The rest of the study is devoted to the integral energy equation. After some previous remarks, two real examples are discussed. First, a closed and hydraulic system, that shows the complementarity of both equations. Later, a multi-topology system, with multiple inputs and outputs, proved the versatility of the global energy equation. A final summary collects all these recommendations.

The energy assessment of water transport systems with Bernoulli's equation is easy and straightforward. For this purpose, it is fundamental to a key and well-known parameter, the energy intensity indicator Ie (kWh/m3) that relates water and energy. Since its dimensions (energy per unit of volume) are pressure dimensions as well, for an incompressible fluid, the energy intensity can be expressed in terms of head (H) in m, as p = γ H. This relationship (with γ = 9,810 N/m3, International System units) is a simple unit change conversion, resulting in:
(6)

For simple systems (one input and one output), these concepts have been well developed (Del Teso et al. 2023) and can be summarized as follows. On one side, the real energy intensity, Ier, is known from field data, including the energy consumed (billed kWh) and the measured pumped water volume (m3) at the delivering point. On the other side, the energy intensity objective, Ieo, is determined by setting target efficiency values. The difference IerIeo indicates the margin of improvement.

This estimation has a weak point. Bernoulli is a stationary equation and during the period considered, the system's conditions can change (e.g. pumping groundwater systems, with water table level variations along the period of analysis). Therefore, the difference Ier – Ieo provides a snapshot assessment, that is strictly valid for a given instant. To some extent, this drawback can be mitigated by using average values over the whole considered period.

Bernoulli's equation can also be used to assess complex systems (many output nodes and, sometimes, more than one input node). A crucial aspect is to identify the final node of the equation because the first one is always the source. This final node, named the critical node, is the one among all the systems that need more energy to achieve the required pressure. Applying the equation between the source and the critical node provides the unitary energy that the system needs, and this maximum value is extrapolated to the whole system. But again, additional care must be taken with the Bernoulli equation, assessing systems with variable time conditions along the considered period of time. In fact, the critical node can depend on the scenario and, even being the same node, the energetic requirements to be covered between both nodes can be different. Again, the energy used to assess the system is the average energy required over the period.

Another fact to consider is that, in the rest of the output nodes, the pressure, although higher than required, is not known. This drawback can be addressed by applying successively the Bernoulli equation between the source and all these output nodes, as many times as necessary up to cover the whole network. However, this is most of the time an awkward strategy due to the high number of nodes. Therefore, a complete physical picture of how the system works can be calculated easily with Bernoulli's equation, although at the expense of simplicity (Cabrera et al. 2021).

In conclusion, Bernoulli is the appropriate energy equation to assess and audit simple systems. Being one-dimensional, both stages (assessment and audit) are, in practice, covered simultaneously. With these results, the energetic system's behavior can be easily labeled (Cabrera et al. 2023; del Teso et al. 2023).

The energy integral equation is subject to almost no constraints and it is therefore valid for any regime, static or dynamic, compressible or incompressible, with or without heat transfer. In our case, for the energy analysis of water transport systems, two relevant simplifications apply: the fluid is incompressible (ρ = C) and the flow is assumed to be adiabatic (no thermal exchange between the system and the exterior, dQ/dt = 0). In these conditions, Equation (1) becomes:
(7)

Thus, the energy integral equation differs from the Bernoulli equation in dimensionality (a CV versus a stream tube, Figures 1 and 2) and the capability to analyze transient flows. This general equation has been previously applied to diagnose (Cabrera et al. 2015) and audit (Cabrera et al. 2010) complex systems. In this study, it is extended to multi-topology systems.

The terms of Equation (7) are physically meaningful. Firstly, because the shaft work added (by pumps) or removed (by turbines) from the system balances the change with the time of the energy stored inside the CV, first integral (it is non-zero if there are tanks inside the CV, with level's variation), and second integral which is a flow term that represents a power global balance between the outgoing and incoming flows. Both integrals include the kinetic energy (negligible in our analysis), the internal energy (its change represents the power dissipated by friction, Equation (5)) and the gravitational head. The pressure head is the only difference between both integral terms.

Some remarks concerning this equation follow:

Closed and open systems

Systems can be open or closed (with or without flow throughout the CS, respectively). When dealing with pressurized water transport, most of the systems to analyze are open. In closed systems, because there are no input nor output flows of energy, it does not make sense to define efficiency.

Thermal implications of water transport systems

There is a growing interest in modeling the water temperature evolution along urban water networks (Agudelo-Vera et al. 2020) because its value impacts the water quality. In fact, some countries have set a maximum temperature threshold of 25 °C at the customer's tap (Blokker & Pieterse-Quirijns 2013). In the context of climate change, with increasingly hot summers, this issue becomes more challenging. Obviously, for other uses (as irrigation), this question has no interest.

In any case, thermal and dynamic problems are decoupled because water is an incompressible fluid. Although the main focus of EPANET (Rossman 2000) is on water quality issues, its solver ignores any potential thermal exchange between soil and water. Even its recent update (Rossman et al. 2020) has ignored this question. Conversely, the models proposed to determine the water temperature evolution (Burch & Christensen 2007; Blokker & Pieterse-Quirijns 2013) are, as expected, decoupled of the hydraulic equations.

In summary, the adiabatic hypothesis assumed in this work may not apply in some cases, but this has no influence on the mechanical energy analysis being performed. The hydraulic variables calculated, and therefore the temperature variation ΔT due to friction, will be the same no matter whether the heat transfer soil–water effect is included or not. The only changed value should be the final water temperature, important from the water quality side, but irrelevant for the purpose of this analysis.

Gravity terms

Gravitational energy, which depends on the potential elevation change of the considered mass, has no absolute value. Therefore, it is a crucial issue to set correctly the system's reference level. This is the lowest active node (consumption or source). Its elevation is set to zero. If real elevations (over the sea level) are kept, the gravitational energy is oversized although the final energy balance will be satisfied because inlet and outlet flows are equal (ρ=C) and the gravitational terms linear. Both inlet and outlet gravitational terms are equally incremented. However, a tank inside the CV breaks the energy balance, because the changes in the energy stored in the tank depend on the square elevation of the level while inlet and outlet flows are no longer equal. Therefore, node elevations must all refer to the lowest node, an invariable reference when it is a consumption node. Nevertheless, if the lowest node is a tank, its elevation varies with the surface level but can be kept as a reference. This will be further analyzed in the following section.

Consider the installation shown in Figure 3. It is a closed system, consisting of two tanks (cross-sections A1 and A2) connected by a pipe (section Ap). The flow begins when the valve is instantaneously opened. The level of reference for the elevation (gravitational energy) is the initial water level of the downstream tank. No heat transfer is considered in the analysis.
Figure 3

Energy analysis of a closed system.

Figure 3

Energy analysis of a closed system.

Close modal
In a closed system without shaft work, the energy equation becomes:
(8)
Inertia and elastic effects can be ignored (Abreu et al. 1999) if:
(9)
The Equation (8), if the kinetic term is disregarded, yields:
(10)
where C is the integration constant.

For the considered period, Equation (10) shows that in the closed system the loss of gravitational energy, ΔEG, equalizes the positive internal energy variation (ΔET= ΔEf). Assuming a quasi-static flow (Rao & Bree 1977), without changes of energy inside the pipe in the period t, water thermal changes came from the absorption of heat generated by friction between pipe and fluid.

Therefore, attention must be placed where the changes happen, in particular in the sub-volume, CV’, equal to the initial CV less the pipe volume. Therefore, Equation (11) can then be written as:
(11)
which confirms the result of Equation (5): the gravitational energy lost (ΔEG < 0) is equal to the gained internal energy. That gravitational energy loss (Figure 3) is:
(12)
The two variables of the integrand are linked by the continuity equation through:
(13)
Combining Equations (12) and (13) and integrating, yields:
(14)

The negative value confirms the loss of gravitational energy. The first term is the energy lost at the first tank, equal to the weight of the discharged volume multiplied by the change of elevation of the center of gravity (assuming a constant reference level). The second term is the energy gained by the lower tank during the filling with respect to the initial reference. Therefore, this term is the correction of the error introduced because of the variation of the reference level.

This result can be better understood by rewriting Equation (12) as:
(15)
The first integral is the first term in Equation (14), while the second, as a function of zf, can be written as
(16)
which is equal to the second term in Equation (14).

In short, for a variable reference elevation, the compensation term of Equation (16) restores the balance and therefore can be kept as a reference level. If it is a reservoir, the lowest level is constant and the correction is null (). In a reverse flow situation (pumping line), the lower tank is emptied while the upper one is filled. In that case, Equation (14) is the same, but the signs of the terms are interchanged.

Finally, the thermal energy variation of the discharged volume,, (White 1973), is:
(17)
where ΔV is the discharged volume, ce the specific heat (1 cal/°C gr), and ΔT the temperature variation. Therefore, in this part of the analysis, the only mechanical variable that influences ΔT is the volume moved, regardless of the time required, which depends on the system's characteristics.

Data of the system depicted in Figure 3 are: D1 = 40 m; D2 = 20 m;1 = 200 m;2 = 100 m; ξ10 = 6 m; ξ20 = 1 m; Dp = 100 mm and L = 5,000 m. The volume transferred between both tanks is 1,256.64 m3. Two scenarios are considered: (a) valve completely open (no local losses; k ≈ 0) and (b) valve almost closed (local losses are relevant; k = 500).

Once the volume has been transferred (1,256.64 m3), the level in Tank 1 has decreased 1 m and the level in Tank 2 has increased by 4 m. Therefore, in the end, both tanks have the same water level (ξ1f = ξ2f = 5 m). Assuming a constant head loss equal to the average between the initial difference in tank levels (105 m) and the final one (100 m), the energy lost by friction (head loss times the discharged volume) results:

This result is equal to the loss of gravitational energy, ΔEG, 350.99 kWh. This value has been obtained from Equation (14) with ho = 105 m and hf = 104 m. From this value, the water temperature variation due to friction is ΔT = 0.24 °C, Equation (17), a minuscule value, because of the high specific heat of water. In fact, the energy intensity to heat water by 1 °C is 1.16 kWh/m3 (Equation (5)), which is equivalent to elevating 1 m3 of water by 426 m. Thus, it is reasonable to ignore thermal effects due to hydraulic friction.

If the valve is open (K = 0), only the pipe dissipates energy but, if partially closed, the energy is lost at both elements (valve and pipe), although the integral energy equation is unable to break down their respective contribution. For this purpose, assuming a quasi-stationary flow (a valid assumption because ), it is necessary to model hydraulically, with EPANET, both elements. Results are:

• (a) Valve open: Time required to transfer 1,256.64 m3, 32 h 40 min; initial velocity, 1.38 m/s; final velocity, 1.34 m/s; pipe average losses 102.5 m.

• (b) Valve partially closed: Time required to transfer 1,256.64 m3, 40 h 00 m; initial velocity, 1.13 m/s; final velocity, 1.10 m/s; pipe losses, at t = 0 h, 72.35 m and at t = 40 h 00 m, 68.90 m; average pipe friction losses, 70.63 m. Valve losses, at t = 0 h, 32.67 m and at t = 40 h 00 m, 31.06 m, average valve's losses, 31.87 m. Therefore, the valve is responsible for 31.09% of the total losses, because it is practically closed (k = 500).

From this simple but conceptually strong case study, it can be concluded that the initial level of the lowest tank, even being variable, can be set as the reference elevation node. Furthermore, it evidences that to complete the audit, it is necessary to solve the hydraulic system's behavior, showing that the discussed equations are also complementary to each other.

The term topology refers, in a hydraulic system, to the water connectivity between nodes and water's sources. Therefore, a multi-topology system operates in different ways. In fact, each form of water connectivity gives rise to a different hydraulic system to be analyzed. Figure 4 shows a generic system, similar to Figure 2, with different connections between input sources and output demands nodes. A real case will be presented later.
Figure 4

Generic CS and CV adapted to a multi-topology water transport system.

Figure 4

Generic CS and CV adapted to a multi-topology water transport system.

Close modal
Assuming the kinetic terms negligible, and a quasi-static behavior of the system, the integral energy equation, Equation (7), yields:
(18)
Let Qt denote the net flow of a tank within the CV, its energy variation is,
(19)
From this result, Equation (18) becomes:
(20)
where ne, no, np and nt denote the number of entrances, outlets, pumping stations and tanks within the CV, respectively, while subscripts used are (e), entrance; (o), outlet; (p), pump and (t), tank. Last, the variables are: u, internal energy; H, piezometric hydraulic head; h, pumping head; z, elevation referred to the lowest active node; Q, pipe flow and Qt, tank net flow.
Equation (20) equalizes input and output powers. Friction losses are indirectly accounted for throughout the internal energy variation. The leak term is embedded in the output power Poh. Rearranging Equation (20), and being Pf the power lost by friction, yields:
(21)
In short, the hydraulic power delivered to the system, Peh, is the sum of the energies provided by sources (gravitational or natural power, PN) and pumps minus (or plus) the tank's contribution, which is equal to the hydraulic output power, Poh, plus the power dissipated by friction, Pf. Finally, the hydraulic output power is the sum of the useful delivered power (Pohu) plus the power lost through leaks, Pohl:
(22)
Previously, with a water balance, the total output flow has been broken down into useful and leaked flows, and, respectively (with Qt the net flow of the internal tanks). Therefore:
(23)
Equation (22) is general and must be customized for each operating mode. For topology Lj, the hydraulic useful output power is:
(24)
For this topology, j, the total system's losses, PSl,j are:
(25)

A final concept must be considered: the minimum energy to be supplied (Hom,i), defined through the minimum pressure, because if the pressure exceeds the standard (minimum) value, pm, then Ho,i>Hom,i and the system will deliver more energy than needed. That is, to some extent, an inefficiency that should be avoided.

Assessment of multi-topology systems

The assessment of a specific topology within a multi-topology system without energy storage (no tanks inside the CV) can be conducted by comparing output with input powers, disregarding what occurs inside the CV. Within this approach for the diagnosis phase, the system can be assimilated to a black box, defined as ‘a fictitious system representing a set of concrete systems onto which stimuli S impinge and out of which reactions R emerge. The constitution and structure of the box are altogether irrelevant to the approach under consideration, which is purely external or phenomenological’ (Bunge 1963).

If the CV includes internal tanks, the flow Qt, calculated from a differential quasi-static continuity relationship (Equation (20)), has to be known in order to perform the assessment. The additional inputs/outputs of these tanks (as water levels, z), although internal to the CV, are similar to the external inputs/outputs of the system.

Therefore, at the diagnostic stage, the system can be associated with a black box, with or without energy storage. Nevertheless, at the next stage, the audit, the whole inefficiency, , must be decoupled in (Equation (25), second term). To do that, each system's element must be hydraulically characterized to assess individually its work, complementing the integral energy equation. So, each possible topology must be diagnosed and audited. Nevertheless, it is convenient to carry out the analysis of each scenario to identify the most inefficient ones and to improve them.

Assessment of each of the topologies

When realizing the diagnostic of each of the topologies in a multi-topology system, if the pumping station losses are included, through the electrical motor and pump efficiencies , the total power input, PT,j can be obtained:
(26)
Being the final efficiency of this topology Lj, :
(27)
A more refined measure of the efficiency, is based on the minimum output power required (), corresponding to the minimum pressure, pm. It is calculated by replacing the hydraulic head outputs, Ho,i by Hom,i in Equation (24). The difference is the excess of delivered power, PΔP:
(28)
Therefore, efficiency can be obtained as:
(29)

Assessment of the whole system

Equations (27) and (29) provide, in power terms, the instantaneous efficiency (snapshots) for different topologies Lj. The whole system assessment over a period t (month or a year) to calculate the average efficiency, η, can be obtained by integrating Equation (27) (or Equation (29)) over time along all the scenarios that the system evolves through. However, it is easier and more direct to calculate η through an energy balance, from:

• Average piezometric heads (at the inlets/outlets of the system along t), based on the recorded pressures along t, and the volumes measured at the inlets/outlets of the system for the same period t.

• The electric energy supplied to the system, Eei, the sum of the electricity bills during period t.

• Over a long period of time, the contribution of the internal tanks is negligible (therefore, the third addend of the denominator, is zero).

All in all, the result is,
(30)
And, if the final efficiency is expressed through the minimum hydraulic head:
(31)
Finally, the excess energy delivered, EΔE, leads to:
(32)

Audit of multi-topology systems

Once the system's energy losses have been determined with the assessment, it proceeds, when necessary (a poor performance result) to audit the system. This involves breaking down the inefficiencies. The audit will quantify all the power lost through friction,, power embedded in leaks,, excess of delivered power due to overpressure, and power inefficiency at the pumping station, . This breakdown is essential to identify what must be improved in the system.

Audit of each topology

The hydraulic characterization of the system's elements is the basis of any audit. Friction and leakage depend on the flow rates through pipes, qk, and on the pipe's physical characteristics (diameters Dk and lengths Lk). Let nl denote the total number of lines (with nl,j active for topology Lj) and Δhk the head loss of each active line. The power lost due to friction, Pf,j, is
(33)
On the other hand, losses from leaks (modeled as a nodal demand) depend on the system's characteristics. Therefore, in addition to the external nodes, noj, on the SC, internal nodes inside the CV, ni, must be considered because, although there is no demand in these multi-topology systems, they can include leaks. Therefore, ni,j is the active internal nodes of the topology Lj (with qol,i leak at node i, modeled as a pressure-demand), the power results:
(34)
The other powers to be considered for Lj are the excess power:
(35)
the hydraulic output useful (minimum) power:
(36)
the useful output power:
(37)
And the power lost at the pumping stations:
(38)
Combining Equations (22), (37) and (38), the final balance for Lj is:
(39)

This balance shows that the total power supplied, PT,,j, is equal to the hydraulic input power , plus the pumping station power losses, , and equalizes the sum of the minimum useful output power, , the excess of the delivered power, , and the powers required to overcome internal inefficiencies (friction, and leaks, ), plus losses at the pumping station, . After the audit, the terms of Equation (39) are known.

Audit of the whole system

Formulating a global audit in terms of power in multi-topology systems is neither useful nor physically meaningful, but it is in terms of global energy as proposed in the assessment. It is better to analyze each topology with its inefficiencies, examine their causes, and amend them. The feasible solution is to improve, one by one, the efficiency of each system's topology, paying more attention to the most energy-consuming one.

The case study comprises the analysis of a multi-topology system with two wells supplying simultaneously water to an industrial area and a small town (Figure 5). Three pumps operate during off-peak hours (0–8 h), filling the two interior tanks, and starting again when the highest tank (Tank 2) is empty. The volume leaked represents 5% of the injected water.
Figure 5

Multi-topology system (Castellón, Spain).

Figure 5

Multi-topology system (Castellón, Spain).

Close modal
The system operates with six different topologies (Table 1), where the fourth, gravitational, spans the longest period (≈13 h/day). Figure 6 details these topologies, with their inlets and outlets (ne and no), tanks (nt) and pumps (np). The topology 2.2 (Figure 6) is not included in the analysis. as it is very simple. It consists of a decoupled simple system filling Tank 1, with no pumps working.
Table 1

Pump status in each of the six operating modes shown in Figure 6

Topology 1Topology 2Topology 3Topology 4Topology 5Topology 6
Pump 1 ON OFF OFF OFF OFF ON
Pump 2 ON ON ON OFF OFF OFF
Pump 3 ON OFF ON OFF ON ON
Topology 1Topology 2Topology 3Topology 4Topology 5Topology 6
Pump 1 ON OFF OFF OFF OFF ON
Pump 2 ON ON ON OFF OFF OFF
Pump 3 ON OFF ON OFF ON ON
Figure 6

Six operating topologies of the system.

Figure 6

Six operating topologies of the system.

Close modal
Each topology is analyzed with a mathematical model (necessary to perform the audit), supported by EPANET (Rossman 2000). The reference node (the lowest active one) changes with the scenario. Leaks are included as a pressure-dependent demand. Figure 7 represents the variations in the tank's water levels, mainly filled during the early hours while Figure 8 shows the status of the pumps for each topology (Table 1).
Figure 7

Daily evolution of the tank's water levels, linked to the topology's evolution.

Figure 7

Daily evolution of the tank's water levels, linked to the topology's evolution.

Close modal
Figure 8

Topologies generated by the actual regulation mode.

Figure 8

Topologies generated by the actual regulation mode.

Close modal

There are different energy sources (pumps or tanks) and different critical nodes for each topology. To assess each system with the Bernoulli equation is impractical because the initial and final nodes change permanently. However, the use of the energy integral equation is straightforward. Table 2 details the results of the diagnostic calculated with Equation (28). These results have been obtained from the inlet/outlet shown in Figure 6.

Table 2

Assessment of the different topologies (ordered by decreasing PTe, values)

Topology 1Topology 6Topology 3Topology 5Topology 4Topology 2.1
PN (from wells) [kW] 1.49
PTe (electric) [kW] 282.53 201.64 154.66 68.58
Pt (tanks) [kW] 154.57 115.35 60.19 33.22 −8.53 −8.42
PT = PN+PTePt [kW] 129.44 86.29 94.47 35.36 8.53 8.42
Pohu [kW] 28.99 52.27 20.14 10.43 7.02 7.77
PSI [kW] 100.45 34.02 74.34 24.93 1.51 0.65
Efficiency (ηj=Pohu/PT0.22 0.61 0.22 0.30 0.82 0.92
Topology 1Topology 6Topology 3Topology 5Topology 4Topology 2.1
PN (from wells) [kW] 1.49
PTe (electric) [kW] 282.53 201.64 154.66 68.58
Pt (tanks) [kW] 154.57 115.35 60.19 33.22 −8.53 −8.42
PT = PN+PTePt [kW] 129.44 86.29 94.47 35.36 8.53 8.42
Pohu [kW] 28.99 52.27 20.14 10.43 7.02 7.77
PSI [kW] 100.45 34.02 74.34 24.93 1.51 0.65
Efficiency (ηj=Pohu/PT0.22 0.61 0.22 0.30 0.82 0.92

The water comes from the wells, which are the lowest nodes of the system. Therefore, only when both good pumps are ON (topology 1), is there a gravitational (or natural) input power contribution from well 1 (well 2 is the lowest). The total input power is the sum of the pump's power minus the power invested to fill the tanks, which only supply power to the system when they are emptying. This occurs in topologies 4 and 5. Summing up, for all topologies (except for L4 and L2.1), the pumping power exceeds the total input power because the internal tanks are being filled.

Efficiency is the ratio between the power output and power input (Pohu/PT) while their difference (PT – Pohu) gives the power lost, PTl, for each topology. The two most inefficient ones are L1 and L3 when Pumps 2 and 3 are simultaneously working.

The mathematical model is used to perform the audit (Equations (33) to (39)), calculating in which parts of the system the power is lost. Table 3 shows the results for topology (T1).

Table 3

Power audit (topology 1)

Breakdown Power Lost (PTl)[kW]
Losses at the pumping station (Ppl74.57
Friction losses (Pf13.58
Power lost by leaks (Pohl2.76
Power lost by filling the tanks from top (Pml9.52
Total Power Lost (PTl) 100.43
Breakdown Power Lost (PTl)[kW]
Losses at the pumping station (Ppl74.57
Friction losses (Pf13.58
Power lost by leaks (Pohl2.76
Power lost by filling the tanks from top (Pml9.52
Total Power Lost (PTl) 100.43

Similar tables for the other topologies will show their respective weak points. In this topology 1, major inefficiencies are located at the pumping stations.

A possible solution to enhance efficiency could be to remove tank 1 and pump 3. This would entail a direct pumping from well 2 to tank 2. Consequently, Pml would disappear and less head would be needed in pump 2. Additionally, feeding the tanks from the bottom should be considered as an improvement, as it requires less head.

A final global assessment (in energy terms) provides relevant information. This value can be obtained from the different topologies' efficiencies, with an appropriated weighted average. However, it is often simpler to determine this value from annual inputs and outputs data. Table 4 presents volumes (elevated and delivered to users), annual energy consumption by the pumps, average pressure at the system outlets, elevation nodes, and minimum standard pressure.

Table 4

Global system assessment

Input
Output
Well 1Well 2 + Pump 3IndustrialTown
Volume [m3641,879 524,774 9,853 1,100,473
Electric energy consumed (billed) [kWh] 403,085 338,885 – –
Average output pressure [m] – – 50 38
Minimum output pressure [m] – – 30 30
Electric energy consumed, Ee 741,970
Natural energy supplied, EN 3,498
Total energy injected, ET 745,468
Energy supplied to users, Eou  357,401
Minimum energy required by users, Eoum  332,874
Average efficiency,  0.48
Average efficiency (minimum pressure),  0.45
Input
Output
Well 1Well 2 + Pump 3IndustrialTown
Volume [m3641,879 524,774 9,853 1,100,473
Electric energy consumed (billed) [kWh] 403,085 338,885 – –
Average output pressure [m] – – 50 38
Minimum output pressure [m] – – 30 30
Electric energy consumed, Ee 741,970
Natural energy supplied, EN 3,498
Total energy injected, ET 745,468
Energy supplied to users, Eou  357,401
Minimum energy required by users, Eoum  332,874
Average efficiency,  0.48
Average efficiency (minimum pressure),  0.45

As expected, the average efficiency calculated over a one-year period (0.45), is an intermediate value between the maximum efficiency (0.92 in T2.1) and the minimum value (0.22 in T1 and T3).

Pressurized water transport systems can be divided into simple, complex (current networks) and multi-topology systems. All of them must be assessed in terms of their energy efficiency and, when it proceeds, audited. Previous papers have presented assessments and audits for simple (Cabrera et al. 2021; Cabrera et al. 2023) and complex systems (Cabrera et al. 2010; Cabrera et al. 2015) using either the Bernoulli equation or the integral energy equation.

The main objective of this study is to discuss which equation is most convenient to apply to a particular system for analyzing it from an energy perspective. Particular attention has been paid to multi-topology systems because they have not been analyzed previously. Each topology creates a different system that requires, at the assessment stage, an independent analysis.

Table 5 summarizes the recommended equation to be used in each analysis. Integral Energy (P) refers to the equation expressed in power terms, its original form, while an (E) indicates that the equation has been integrated over the analysis period to calculate the energy required or delivered.

Table 5

Recommended equation to analyze energetically different systems

Simple systemsComplex systems (networks)Multi-topologic system
One topologyGlobal
Assessment Bernoulli Integral energy (P or EIntegral energy (PIntegral energy (E
Audit Bernoulli Integral energy (EIntegral energy (P–
Simple systemsComplex systems (networks)Multi-topologic system
One topologyGlobal
Assessment Bernoulli Integral energy (P or EIntegral energy (PIntegral energy (E
Audit Bernoulli Integral energy (EIntegral energy (P–

Regardless of the energy equation used, the hydraulic behavior of the system (currently through its mathematic model) is needed to perform the audit. Otherwise, it is impossible to break down the losses.

The transport of pressurized water consumes a large amount of energy that should be reduced within the current context of climate change and growing energy costs. Therefore, the first step is to diagnose the system understanding its operational mechanisms and potential areas for energy savings. If there is an important margin for improvement the system should be audited to identify where and how much energy is lost. Both steps can be performed using either the energy integral equation or the Bernoulli equation. While these equations differ in their origins, once adapted, both are suitable to perform these analyses. However, as each equation has its advantages and disadvantages (spatiality and simplicity, respectively), it is advisable to discern the cases for which each equation is best suited. Additionally, any water audit must complement the energy equation with the hydraulic behavior of the system, currently its mathematical model.

This study has placed particular emphasis on the integral energy equation, rarely used to solve hydraulic problems. In particular, it has been applied to assess and audit multi-topologic water transport systems. In addition, the application to a closed simple system has evidenced that the reference elevation level must be the lowest active node (source of consumption) even when the elevation of the lowest node is a tank with a variable level.

The type of system dictates the applicability of the equations. For simple and stationary systems, the Bernoulli equation is accurate and easy to use, making it the recommended equation. Both equations are useful for complex systems, as long as the topology does not change over time. The energy integral equation requires more data but provides more information than the Bernoulli equation. The use of the Bernoulli equation corresponds to a more direct and intuitive approach, but only provides a snapshot of the system, at the moment the equation is applied. This study has established recommendations on when to apply each equation depending on the system under analysis and the phase of the analysis.

The authors acknowledge the staff of the water company FACSA for providing financial assistance, helpful advice, and the real case study presented in this work.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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