Wastewater treatment plant in the Vaucluse

A small agricultural commune in the Vaucluse Department of southern France built a wastewater treatment plant below the village far from any electricity lines. They planned to dig a trench to make a connection to the grid, but the owner of the field that had to be crossed refused permission. The plant was finished and only needed a three-phase supply of 3 x 400 V. The commune then decided to supply it using PV panels.

The technical specification was to drive a three-phase pump using 2.2 kW for 3-4 h everyday to transfer the effluent over a height of around 2 m.

Table 5.26 shows all the parameters of this stand-alone system. The first eight lines summarise the system characteristics (voltage, power, current, etc.). All these parameters are variable and will modify the balance of the system that must

image280

Wastewater

Pump

2200

W

Standby

12

W

treatment

command:

plant – Vaucluse

I pumping

108

A

0.50

A

V system

24

V

System

0.85

Ah

efficiency

I panel

7.23

A STC

Battery

900

Ah/100 h

capacity

No. panels series

2

No. panels parallel

4

I panel total

28.92

A STC

Consum./day

93

Ah

Power 1 panel

125

W

Installed

1000

W

power of panels

Solar irradiance

T day

Duration /

Duration/

Consum.

Consum.

Power

Production

Capacity

(kWh/m2/m)

ave. (°С)

day (h)

month (h)

standby

Total

panel

(Ah/month)

(Ah)

tilt 60° S

(Ah/m)

(Ah/m)

(at 29 V)

Jan.

102.6

6.4

0.75

23.3

372

2879

7.70

3160.4

900

Feb.

103.6

7.9

0.75

21.0

336

2601

7.70

3190.9

900

Mar.

152.8

11.5

0.75

23.3

372

2879

7.65

4676.6

900

Apr.

142.2

13.9

0.75

22.5

360

2786

7.62

4334.3

900

May

147.3

18.5

0.75

23.3

372

2879

7.55

4447.0

900

Jun.

155.4

22.8

0.75

22.5

360

2786

7.42

4612.3

900

Jul.

169.0

25.1

0.75

23.3

372

2879

7.00

4730.6

900

Aug.

173.0

24.6

0.75

23.3

372

2879

7.00

4843.4

900

Sept.

160.2

20.3

0.75

22.5

360

2786

7.42

4754.7

900

Oct.

127.1

16.7

0.75

23.3

372

2879

7.55

3838.4

900

Nov.

102.6

10.4

0.75

22.5

360

2786

7.65

3139.6

900

Dec.

87.1

7.0

0.75

23.3

372

2879

7.70

2683.0

704

produce enough energy to cover the pumping requirements. The calculations used an Excel spreadsheet in which each cell can be individually programmed. For example, by increasing the number of panels in parallel, the total charge current of the system is proportionately modified.

The following lines on the chart cover the sizing of the system: for each month, the average energy production and remaining battery charge are calculated. The solar data are taken from the Photovoltaic Geographical Information System (PVGIS) website[56] established by the solar laboratory of the European Commis­sion, which gives solar irradiance data for locations in Europe and Africa based on statistics from 1981 to 1990. This website also gives average daytime temperatures, which enables the nominal charge account of the panels to be calculated for each month and temperature. We will take as a reference the current at 29 V that cor­responds to an end of charge. We will not take the internal resistance of the battery into account, as this current is low (approximately 30 A) compared to the large capacity of the battery (900 Ah). The drops in voltage across the wiring and reg­ulators are estimated at 1 V. Two 30 A regulators were installed in parallel to have some reserve in case of overload due to clouding and to be able also to add some extra panels if necessary.

All the energy calculations are carried out in Ah so as to get away from the charge and discharge voltages, which would be too complicated to model in this example. Alternatively, this calculation can be made in PVsyst, which will, in a more general manner, take account of the battery voltage. The charge/discharge efficiency of 85% in Ah is a conservative value that allows for some ageing of the system.

This simple calculation assumes that the daily solar production is uniform over the month: the monthly irradiance is divided by the number of days in the month. This procedure is practical, because with experience, we finish by knowing for a given region, what a panel can supply each day. In the example of the wastewater treatment plant, the solar charge increases from 2.8 kWh/m2/day in December to 5.6 kWh/m2/day in August when the panels are tilted at 60°.

The system could therefore consume twice as much energy at the peak of summer compared to the low point in winter. However, it should not be forgotten that a panel connected to a battery without an MPPT regulator remains at the battery voltage (plus the reduction in voltage due to the wiring and the working voltage of the regulator) and therefore charges on average at the corresponding current. In the case in point, it is often simpler to calculate the whole system in charge and discharge amperes and assimilate the energy to the Ah at the ‘battery voltage’. To determine the nominal charge current of the panel, we take a max­imum battery charge voltage (here 29 V) and find on the I/V curves of the panel the corresponding current for the daily temperatures during each month of the year. In this example, we have assumed that in July-August the panel is at its NOCT
temperature (daily temperature of around 25 °C), and for the less hot months, we have estimated the fall of NOCT to a minimum of 20 °C in winter.

The estimation of the efficiency of the system takes account of the charge/ discharge efficiency in Ah of the battery and of the loss due to the inverters. As the system also functions by day, the efficiency is slightly higher than if all the energy had to pass through the battery (in the case of lighting, for example). We will take a value of 0.85, or 2% higher than the nocturnal value.

The calculation of power consumption is therefore expressed as

Подпись:Cons = conp + stby

where

conp = consumption of the pump, stby = command standby.

Подпись: Prod Подпись: /rr X /pan Подпись: ■Np: Подпись: Eta Подпись: (5.37)

For the solar production, we will calculate:

where

/rr = monthly irradiance,

/pan = current ‘for the month’, function of temperature; Npan = number of panels in parallel;

Eta = efficiency of system = 0.85.

The charge state of the battery (capacity) is thus:

Подпись: (5.38)Cap = MIN[Ca; (Prod – Cons) + Cap (те – 1)]

where

Ca = battery nominal capacity,

Cap (те – 1) = final capacity of previous month.

As this calculation is circular throughout the year, we are assuming that the battery is fully charged during the best month of the year (here August); therefore, for this month we enter the nominal capacity Ca instead of the state of charge formula.

By varying the number of panels in parallel, we can quickly find the minimum power to be installed to meet the technical requirements.

Figures 5.62-5.64 show the electrical diagrams of this installation and review of the PV array.

I

s

 

Solar panels

8 x Mitsubishi
PVMF125TE4N

 

Lightning protection varistors

 

image288

The sizing of the battery needs to allow for 10 days of autonomy in case of bad weather. This period is not more because of the climatic conditions of the region where winter is often dry and sunny.

image289

Figure 5.64 Wastewater treatment plant solar array

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