Innovative on-Shore System recovering Energy from Tidal Currents

Abstract

An innovative system for the recovering of energy from tidal currents is proposed. The system is composed of a blade submerged in sea waters and connected to a vertical bar which, moving up and down through the tide action, transfers energy to a double effect piston pump. The latter feeds a pressurized reservoir able to provide water flow rate, at a suitable pressure level, to a hydraulic turbine. The basic configuration involves a four-bar linkage connecting the vertical bar and the piston pump. The system can be easily employed in all those sites whose seabed quickly deepens and whose tidal currents are parallel to the coast. The proposed system is a valid alternative to the current tidal energy converters: its big dimensions are necessary to balance the low efficiencies of the overall energy conversion. At any rate, during the working the seabed is not altered, neither is the aquatic fauna damaged.

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Available online at www.sciencedirect.com
Available online at www.sciencedirect.com
ScienceDirect
Energy Procedia 00 (2017) 000–000
www.elsevier.com/locate/procedia
1876-6102 © 2017The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.
The 15th International Symposium on District Heating and Cooling
Assessing the feasibility of using the heat demand-outdoor
temperature function for a long-term district heat demand forecast
I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc
aIN+ Center for Innovation, Technology and Policy Research -Instituto Superior Técnico,Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal
bVeolia Recherche & Innovation,291 Avenue Dreyfous Daniel, 78520 Limay, France
cDépartement Systèmes Énergétiques et Environnement -IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France
Abstract
District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the
greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat
sales. Due to the changed climate conditions and building renovation policies, heat demand in the future could decrease,
prolonging the investment return period.
The main scope of this paper is to assess the feasibility of using the heat demand –outdoor temperature function for heat demand
forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665
buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district
renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were
compared with results from a dynamic heat demand model, previously developed and validated by the authors.
The results showed that when only weather change is considered, the margin of error could be acceptable for some applications
(the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation
scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered).
The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the
decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and
renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the
coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and
improve the accuracy of heat demand estimations.
© 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and
Cooling.
Keywords: Heat demand; Forecast; Climate change
Energy Procedia 142 (2017) 29–36
1876-6102 © 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific committee of the 9th International Conference on Applied Energy.
10.1016/j.egypro.2017.12.006
10.1016/j.egypro.2017.12.006 1876-6102
Available online at www.sciencedirect.com
ScienceDirect
Energy Procedia 00 (2017) 000–000
www.elsevier.com/locate/procedia
1876-6102 © 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific committee of the 9th International Conference on Applied Energy.
9th International Conference on Applied Energy, ICAE2017, 21-24 August 2017, Cardiff, UK
Innovative on-Shore System recovering Energy from Tidal Currents
Silvio Barbarelli*, Mario Amelio, Gaetano Florio, Nino Michele Scornaienchi
Mechanical, Energy and Management Engineering – University of Calabria, Ponte P. Bucci cube 44/C, 87036 Rende (Italy)
Abstract
An innovative system for the recovering of energy from tidal currents is proposed. The system is composed of a blade
submerged in sea waters and connected to a vertical bar which, moving up and down through the tide action, transfers
energy to a double effect piston pump. The latter feeds a pressurized reservoir able to provide water flow rate, at a
suitable pressure level, to a hydraulic turbine. The basic configuration involves a four-bar linkage connecting the
vertical bar and the piston pump. The system can be easily employed in all those sites whose seabed quickly deepens
and whose tidal currents are parallel to the coast. The proposed system is a valid alternative to the current tidal energy
converters: its big dimensions are necessary to balance the low efficiencies of the overall energy conversion. At any
rate, during the working the seabed is not altered, neither is the aquatic fauna damaged.
© 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific committee of the 9th International Conference on Applied Energy.
Keywords: Tidal Currents; Innovative System; On Shore; Immerged Blade; Double Effect Piston Pump; Hydraulic Turbine.
Nomenclature
A swept area [m2]
As piston area [m2]
B blade length [m]
CA friction torque [Nm]
cD drag coefficient [-]
CI torque of inertia [Nm]
* Corresponding author. Tel.: +390984494650; fax: +390984494673.
E-mail address: silvio.barbarelli@unical.it

30 Silvio Barbarelli et al. / Energy Procedia 142 (2017) 29–36
2 Silvio Barbarelli et al./ Energy Procedia 00 (2017) 000–000
CL lift coefficient of the blade [-]
CM blade torque [Nm]
CR resistant torque [Nm]
cR resistant coefficient [-]
CW weight torque [Nm]
D blade drag force [N]
Dp piston diameter [m]
f frequency [Hz]
g gravity [m/s2]
L blade lift force [N]
Ic crank moment of inertia [kg m2]
Is crank-piston moment of inertia [kg m2]
P pressure [Pa]
R blade resistant force [N]
Rc cranks length [m]
R1 decrement of the cranks length [m]
RII increment of the blade distance [m]
Rs connecting crank length [m]
mair air mass [kg]
mp pump flow rate [kg/s]
mres reservoir water mass [kg]
mt turbine flow rate [kg/s]
Mb blade mass [kg]
Mc cranks mass [kg]
Ms connecting crank and piston mass [kg]
Pow power [kW]
pres reservoir pressure [bar]
thalf-cycle half cycle time [s]
Tair air temperature [k]
T tide period [h]
t time [h]
U blade velocity [m/s]
V tidal velocity [m/s]
Vo peak tidal velocity [m/s]
Volp pump volume [m3]
Volres reservoir volume [m3]
W relative current velocity [m/s]
Z vertical bar length [m]
Greek letters
 angle between the lift force and the vertical direction [°]

aspect ratio of the blade [-]

El electrical efficiency [-]

p pump efficiency [-]

tot total efficiency [-]

t turbine efficiency [-]

crank angle [°]

Silvio Barbarelli et al. / Energy Procedia 142 (2017) 29–36 31
Silvio Barbarelli et al./ Energy Procedia 00 (2017) 000–000 3
1. Introduction
In a problematic worldwide energy scenario characterized more and more by the need to replace fossil fuel energy
sources [1], present research lines are addressed toward renewable sources [2]. Among these, tidal currents are
considered a perfectly predictable, but not yet fully exploited energy source of particular interest [3]. Often off-shore
solutions, employing big stand-alone turbines or array turbines, are chosen [4]. This is the case of the Openhydro
turbines [5], being developed in Nova Scotia (Canada) and in the Irish Sea, or of the Seagen turbines [6] being
developed in the UK.
Other alternative systems are, for example:
- the Biostream device [7] which is an oscillating-hydrofoil made up of a foil linked to a mechanical arm
actuating a hydraulic circuit, thanks to the lift force produced by tides;
- the tidal kite turbine [8], which is a turbine connected to a kite anchored to the seabed, potentially very
efficient and operating at currents lower than 1.2 m/s;
- the Archimedes screw tidal turbine (Flumill solution) [9], generating electricity by means of the screw
rotation.
However, the necessity to construct civil infrastructures in the open sea, and the maintenance operations involving
skilled manpower imply high installation costs which discourage potential investors [10]. On the contrary, the systems
conceived with the basement on-shore will be surely cheaper and can be easily installed [11]. The size of the frame to
which to connect the turbines constitutes an important issue of these solutions: the frame has to be thin enough to
avoid high visual impact but at the same time thick enough to support the sea thrusts [12].
For this reason a system whose mobile parts are completely immersed in water has been conceived. The turbine
[13] has been replaced by a simple blade profile moving alternately up and down under the tide effect, able to support
the acting forces. The alternate motion can be converted into pressure energy by a double effect piston pump, which
feeds a pressurized reservoir. Finally, a hydraulic turbine, connected to an electric generator, receives water under
pressure from the reservoir.
In this paper, the authors illustrate the architecture of the proposed system together with its main features.
2. System description
The original purpose was to create a system able to catch energy from the tides flowing parallel to the coast by
means of a simple profile like a pedal submerged in the sea near the coast, moving up and down in alternate motion,
and to convert this alternate motion in the rotating motion of a turbine placed onshore. This system could resolve all
the issues linked to the maintenance operations and cut the installation costs. The main advantage is to remove the
group turbine-generator from the sea, anyway paying the price of lower efficiencies.
The vertical alternate motion can be transferred to ground by a four-bar linkage useful to actuate a piston pump,
which delivers pressurized water to a reservoir filled partly with water and partly with air. A hydraulic turbine receives
the water flow circulating in a closed circuit: in fact, the flow is discharged in a tank but it is successively re-pumped
into the reservoir by the piston pump.
Figure 1 shows the system configuration highlighting the various components: the blade, the four-bar linkage, the
double effect piston pump, the pressurized reservoir, and finally the turbine-generator group.
The pressure level of the reservoir changes according to the tidal current velocities variations, which determine the
entity of the lift thrust acting on the immerged blade and so the consequent action of the piston pump.
The hydraulic blade profile is conceived as moving up and down like a pedal under the lift thrust of the tide itself,
with the possibility of changing its inclination as shown in the detail of Fig. 1. This change helps the profile to move
up and down because the lift force direction changes in accordance. For this purpose, the profile is chosen symmetrical.
3. Mathematical model
With the aim to calculate the equations related to the blade motion, the dynamic equilibrium is considered. The
acting moments with respect to the crank hinge are the following: torque CM generated by the lift force acting on the
blade; resistant torque CR generated by the water pressure on the piston pump; friction torque CA due to the drag force

32 Silvio Barbarelli et al. / Energy Procedia 142 (2017) 29–36
4 Silvio Barbarelli et al./ Energy Procedia 00 (2017) 000–000
acting on the blade in the vertical direction; weight torque CW due to the weight of the elements involved in the vertical
motion of the blade; torque of inertia CI proportional to the inertia of the system.
Fig. 1. System configuration and blade particular.
The motion equation is [14]:
WARMI
CCCCC 
(1)
In order to obtain the time evaluation of the various quantities (angle, velocity, power, etc.), the above described
torques have been written as function of the crank angle (

).
As shown in Fig. 1, the blade profile, invested by the tidal current of velocity V, moves toward the top with velocity
U. The thrusts of lift (L), drag (D) and resistance (R) of the blade profile are given by the following equations [15]:


2
2
2
1B
cWL
L

;


2
2
2
1B
cWD
D

;


2
2
2
1B
cUR
R

(2)
The various torques with respect to the cranks hinge are defined as:





 )cos(
2
cos

cM
R
B
LC
;
ssR
RpAC
;





 )cos(
2

cA
R
B
RC
;
)cos(

ccW
RgMC
(3)
While the total torque of inertia, by considering the inertia of the various components, is given by:
 


RMRRRMRRMIIC
ssmIIcbIccscI
2
))()((2 
(4)
The signs of the torques change according to the direction of the blade: when it goes up the signs are those of eq.
1 except the last part of the vertical path when the blade changes inclination for braking before reaching the top and,
consequently, the torque CM changes sign. The opposite happens when the blade goes down except for the weight
torque CW that is always negative.
All the above described formulas have been implemented in a software developed in Simulink® environment,
taking into account the first sizing of the system considering a squared blade (B) of 8 m, a vertical bar (Z) of 9 m, a
piston pump diameter (Dp) of 0.25 m and maximum theta oscillations of  25°. The masses considered are: blade mass
(Mb) of 1096 kg with a net weight in water of zero, cranks mass (Mc) of 850 kg, connecting crank and piston mass
(Ms) of 355 kg. In the next figures, some outputs of the software are reported. Figure 2 shows the crank angle and the
attack blade angle oscillations by considering a pressure of the reservoir of 4 Bar and a tidal velocity of 1.5 m/s.
Fig. 3 instead shows the trend of the various torques acting on the system in relation to a complete cycle, by
considering a pressure of the reservoir equal to 9 Bar and a tidal velocity of 2 m/s.
pressurized air
water level
water
reservoir
hydraulic turbine +
electrical generator
double effect
piston pump
water
pipes
sea level
blade

Z
R
c
R
s
B
crank angle
L

Silvio Barbarelli et al. / Energy Procedia 142 (2017) 29–36 33
Silvio Barbarelli et al./ Energy Procedia 00 (2017) 000–000 5
Fig. 2 Crank and attack angles oscillations – Fig. 3 Torques trend by changing the crank angle –
reservoir pressure of 4 Bar, tidal velocity = 1.5 m/s. reservoir pressure of 9 Bar, tidal velocity = 2 m/s.
4. Main results
The performances of the system are determined by the tidal current velocity: in fact, when the tide rises the
oscillations frequency of the mobile parts increases and consequently the power and the efficiency of the system
increase. However, with the aim to maintain high efficiencies, the reservoir needs always-higher pressures that imply
lower frequencies. The work done in a cycle is higher as much as the pressure grows, but the power is obviously
linked to the time in which this work is done. There will be an optimal value of the resistant torque, and, for an
assigned geometry of the various components, of the reservoir pressure, for any tidal current velocity value.
This occurrence can be focused by evaluating the overall efficiency of the system starting from the flowing energy
of the current through the vertical area swept by the blade profile, expressed as:
A = 2BRcsin(

max) (5)
Taking into account the energy flowing across the transversal area A above defined and the overall efficiency

tot,
the power of the system is simply given by the following equation:
AρVPow
tot
3
2
1


(6)
Unfortunately, the overall efficiency is not immediately calculable. The efficiency is defined as the energy supplied
by the electrical generator in a half cycle on the energy flowing through the swept area A in the same time (thalf-cycle).
By considering that the energy supplied by the generator is equal to the pressure energy pumped by the piston pump
[16], whose volume is indicated as Vol, and taking into account the pump efficiency

P, the turbine efficiency

T and
finally the generator efficiency

El, the following expression is proposed:
f
AρV
Volpηη
AtρV/
Volpηη
η
resTPEl
cyclehalf
resTPEl
tot 33
4
21






(7)
where f is the frequency of the moving blade and pres the pressure of the reservoir. The frequency f depends on different
factors: the various torques involved as well as the overall inertia of the system. It can be calculated by means of the
software implemented in Simulink whose equations have been illustrated previously.
The first analysis of the system can be done by considering constant the various efficiencies involved in eq. 7 (

El,

P,

T) by assuming values traditionally adopted for them.
So an electrical efficiency of 90%, a pump efficiency of 85%, and finally a turbine efficiency of 85%, are assumed.
Particularly the latter value is true under the hypothesis when considering a Pelton turbine working at variable
rotational speed, so that the efficiency is always at the maximum value.
-30
-20
-10
0
10
20
30
010 20 30 40 50 60
degrees [°]
time[s]
crank angle
attack angle
-600
-400
-200
0
200
400
600
-24 -16 -8 0 8 16 24 32 40 48 56 64 72
torques [kNm]
crank angle[°]
torque
resistant torque
friction torque
weight torque
inertia torque

34 Silvio Barbarelli et al. / Energy Procedia 142 (2017) 29–36
6 Silvio Barbarelli et al./ Energy Procedia 00 (2017) 000–000
Fig. 4 Efficiencies curves of the system by changing the tidal velocities, the system frequency and the pressure of the reservoir.
Figure 4 reports the efficiency curves, and the reservoir pressure of the system by changing the tidal current velocity
from 1.5 m/s to 3 m/s. The maximum values of efficiency follow a trend highlight by the dashed line: they are obtained
for particular values of pressure and velocity. By assigned a piston pump diameter (Dp) of 0.25 m, the following
correlation involving these quantities is found:
pres= 3.6 V2 – 0.16 V – 4.88 (8)
Obviously, for maintaining the pressure to the optimized values, a control strategy is required which is described
as it follows.
4.1. Control strategy
The pressure control is done by changing the flow rate feeding the turbine. The reservoir receives a flow rate at
each cycle provided by the pump equal to the product of its delivered volume and to the system frequency.
fVolm
pp


.
(9)
The quantity of mass accumulated in the reservoir instead can be calculated as it follows
cycle
res
airair
res
cycle
res
t
p
TRm
Vol
t
m









/

(10)
where the pressure pres is provided by eq. 7.
By considering a half tidal cycle of 6 hours with a sinusoidal trend as:






t
T
VV o

2
sin
(11)
with T equal to 12 hours and Vo equal to 3m/s, and considering moreover a reservoir of 16 m3 pressurized with a mass
of air of 50 kg, taking into account equations 7 and 10, it is possible to calculate the water mass accumulated in the
reservoir which results:
2
2
88.4
6
sin486.0
6
sin4.32
6
cos254.0
3
sin96.16






































tt
ttTRm
dt
dm
airair
res



(12)
The flow rate delivered to the turbine is then calculable as the flow rate provided by the pump deducted by the
water mass accumulated in the reservoir, i. e.:
dt
dm
mm
res
pt

..
(13)
0.00%
2.00%
4.00%
6.00%
8.00%
10.00%
12.00%
14.00%
16.00%
18.00%
0 5 10 15 20 25 30 35 40
efficiency
pressure [bar]
V=1.50 m/s
V=1.75 m/s
V=2.00 m/s
V=2.25 m/s
V=2.50 m/s
V=2.75 m/s
V=3.00 m/s
Trend of the
maximum
efficiencies

Silvio Barbarelli et al. / Energy Procedia 142 (2017) 29–36 35
Silvio Barbarelli et al./ Energy Procedia 00 (2017) 000–000 7
Fig. 5 Pressure and flow rate changes of the turbine by varying the tidal velocity
Figure 5 illustrated the optimized pressure and flow rate changes during a half cycle of tide with maximum peak
velocity of 3 m/s: the pressure is expressed in meters of water column and the flow rate in litres per second. The tidal
velocities are overlapped on the bullet points of the graph. When the tidal velocity increases, the pressure of the
reservoir augments until 275 m, while the flow rate augments until 45 l/s. When the tidal velocity decreases, the
pressure of the reservoir decreases until 40 m but in this case the flow rate feeding the turbine is higher because of the
delivering of the accumulated mass (see eqq. 9 and 12).
Figure 6 shows how the efficiency and the power of the system, optimized by a system control pressure, change
with the tidal current velocity. The efficiency increases by augmenting the tidal velocity and the higher values
approach the limit of 20%. The power reaches interesting values, from 70 to 470 kW, in the range of 3÷5 m/s of tidal
currents velocities.
Fig. 6 Efficiency and Power of the system by changing the tidal velocities
5. Discussion
The authors present a new system able to convert energy from tides flowing parallel to the coast by means of simple
components placed directly onshore. This system is inspired by the recent “Oscillo Drive” technology [15], which is
addressed instead to exploiting marine waves. That happens through point absorber buoys connected to a hydraulic
circuit and, finally, to a turbine. In a similar way, the proposed system is equipped with a component, the blade profile,
immersed in water, working efficiently to convert the lift thrust of the tidal current into electric energy. That happens
by means of intermediate devices like four bar linkage, piston pump reservoir and hydraulic turbine.
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3
2.9
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2
1.9
1.8
0
5
10
15
20
25
30
35
40
45
50
0 50 100 150 200 250 300
flow rate [l/s]
Pressure [m]
0%
2%
4%
6%
8%
10%
12%
14%
16%
18%
20%
0
50
100
150
200
250
300
350
400
450
500
1 2 3 4 5
efficiency
Power [kWatt]
V [m/sec]

36 Silvio Barbarelli et al. / Energy Procedia 142 (2017) 29–36
8 Silvio Barbarelli et al./ Energy Procedia 00 (2017) 000–000
The novelty of the present work is to offer a cheap, easily installable onshore solution in all those situations where
the tidal current are parallel to the coast (straits, lagoons, estuaries and so on) and the seabed quickly drops with respect
to the coast. The efficiency of the system is lower than a traditional turbine [18], but at the same time, the system is
structurally more resistant and requires less maintenance and at a lower cost. It could be an interesting alternative to
the present technologies exploiting tidal currents, involving onerous turbines immersed in the open sea, continuously
worn out by the sea.
6. Conclusion
A new system collecting energy from sea currents has been proposed, which can be placed directly on the coast
and which can be advantageously used in sites where the seabed drops rapidly from the coast.
The system is innovative because, operating directly on the coast, it aims to achieve installation costs lower than
those of the systems placed in the depth of the sea. Of course, it does not fit all sites, as it is necessary that the sea
current is significantly important near the coast. It is also particularly suitable for those sites in which the current
changes cyclically since the mechanism that drives the blade adaptable to these changes and keeping the blade always
in perfect alignment with respect to the current.
Simple preliminary estimates indicate that the system is able to supply electric power even with not excessively
large dimensions and in sites where the currents are not significantly great. Moreover, it is designed in compliance
with safeguarding both the environment and aquatic fauna.
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