Energy Cost Saving Strategies via Time-of-Use Energy Analysis of Wastewater Treatment and Reclamation I. Aymerich1, Ll. Corominas1, R. Sobhani2, M. Garrido2, L. Rieger3, D. Rosso2 1ICRA,
The monthly Maximum Peak Consumed (MPC) remains similar for the different tariff periods.
The TOU energy tariff structure is mainly described by the Tariff schedule and the different rates applied.
The BSM2 layout of the IWA BSM Task Group was used. The plant layout follows an MLE configuration with 2 anoxic (3,000 m3 total) and 3 aerobic tanks (9,000 m3 total).
The average ammonia in the effluent is around 0.37 mg N/L and always below the effluent limit (5 mg N/L). The total monthly energy consumed is relative constant (~ 150 MWh) (Figure 4b).
The On-peak periods (12-6pm in summer months) have the highest impact on the total costs.
30 15000
Figure 2: Screen-shot of the plant layout
Operating conditions and Control strategy The operating conditions of the WWRF are described in Table 2.
20 5000 0
Analyze different energy tariffs and develop general structure to model energy tariffs. Develop a dynamic energy cost model including Time-ofUse and peak demand charge terms. Demonstrate the importance of using more realistic equipment and energy cost models by applying it to a case study based on California’s TOU energy tariff. Evaluation of different cost saving strategies. TEMPLATE DESIGN © 2008
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Figure 1: Tariff schedule applied in the Southern California TOU-8 energy tariff. Three types of charges and rates are evaluated in this casestudy and described in Table 1. Table 1. Description of the different charges and rates applied in the Southern California TOU-8 energy tariff. Customer Charge (USD/Meter/Month) Energy Charge (USD/kWh/Month) Summer - On-peak Summer - Mid-peak Summer - Off-peak Winter - Mid-peak Winter - Off-peak Peak Demand Charge (USD/kW/Meter/Month) Facilities Related Time Related Summer - On-peak Summer - Mid-peak Summer - Off-peak Winter - Mid-peak Winter - Off-peak
312.31 Delivery Service Generation 0.024 0.024 0.024 0.024 0.024
0.124 0.064 0.038 0.064 0.038
Average influent flow-rate
Unit
20,163
m3·d-1 m3·d-1
RAS flow-rate
18,445
m3·d-1
IR flow-rate
55,338
m3·d-1
15.5
d
SRT
Effluent Ammonia
Effluent TN
20
b) Energy consumption and effluent quality: 150
15
100
10
50
5
0
0 JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN
Energy Cost Results
A tapered diffuser system was modeled with a resulting airflow split of 50% to the 1st aerobic reactor, 30% to 2nd, 20% to 3rd.
300
20000
250 200
15000 150 10000 100 5000
50
0
0 JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN
Figure 6: Maximum Peak Consumed (MPC) and the related Peak Demand Charges (PDC) distributed by tariff periods.
Scenario analysis Different maximum air flow-rates are evaluated. The results show that an appropriate adjustment of the maximum air flow-rate can lead to significant savings in terms of effluent quality, energy and costs.
The Peak demand charges play an important role, giving a significant variability and contribution to the total energy costs.
Max. air flow-rate (m3/h)
Energy usage charges Peak demand charges -1 Average energy price (8.96 cUSD·kWh )
Description
Av. effluent ammonia (mg N·l-1) Av. effluent TN ( mg N·l-1) Energy consumption (MWh·year-1) Energy costs (USD·year-1)
35000 30000
Reference
S2
S3
10,416.6
7,291.7
6,250.0
0.37
0.49
0.45
13.6
13.5
13.1
1,587.4
1,479.0
1,793.9 294,149.4
246,947.8 223,804.5
Take home message
25000
Energy tariffs have a significant impact on operation costs.
20000
Time-of-Use and peak demand charges should be applied when comparing operation strategies and technologies.
15000 10000
The case study showed an overestimation of energy costs when compared with a fixed price.
5000 0 JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN
Figure 3: DO – PI controller
Off-peak MPC Off-peak PDC
Mid-peak MPC Mid-peak PDC
Table 3: Results obtained from evaluating different maximum air flow-rates.
Aeration system DO – PI: the base case is a PI DO controller measuring the DO concentration in the 2nd aerobic reactor and manipulating the total airflow to maintain a set-point of 2.5 mg DO/L.
On-peak MPC On-peak PDC 25000
Using a fixed energy price (average of energy usage rates) and ignoring TOU and peak demand charges may lead to wrong conclusions when evaluating energy costs.
The energy usage charges are about constant throughout the year.
14.32 26.19 7.22 0 0 0
Value
400
Wastage flow-rate
200
Energy consumed
Figure 4: Influent characteristics and plant performance of the control strategy evaluated.
Table 2: Operating conditions
Total energy cost (USD·month-1)
Objectives
25
10000
Total energy consumed (MWh)
Winter season (From Oct to May)
Description
Water Resource Recovery Facilities (WRRF, formerly wastewater treatment plants) are large energy consumers. The consideration of energy tariff structures in equipment or process selection or when comparing control strategies allows a more realistic evaluation and better informed decisions. Energy costs can be optimized in parallel with other operating costs, water quality, and GHG emissions.
a) Influent characteristics:
JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN
Peak Demand Charges Peak demand charges is another incentive to control energy demand during the peak power demand hours by charging additional fees if the short-term energy consumption is above a defined limit.
WWRFs
Potential savings could be achieved by limiting the maximum air supply rates during certain times of the day.
35
20000
Time-of-Use (TOU) Tariffs Common demand-side management mechanisms are Timeof-Use (TOU) tariffs, where consumers are charged different rates depending on the time of the day, week or season to favor energy use at off-peak times.
Temperature
-1
25000
Flow-rate
kW·month
Summer season (From June to Sept)
30000
Impact of the Peak Demand Charges
-1
The Tariff schedule applied (see Figure 1) consists of two types of season tariffs (Summer and Winter) and three types of tariff periods (On/Mid/Off peak periods).
Influent conditions & Plant performance
USD·month
Energy demand response programs are mechanisms to incentivize the reduction of peak demands, avoiding investments in additional infrastructure by balancing energy use and consequently reduce the GHG emissions related to energy generation.
The average influent flow-rate and temperature are presented in Figure 4a.
Temperature (ºC)
Energy Demand Response Programs
Plant under study The WWRF has been modeled using the SIMBA# simulation software.
Effluent conc. (mg N·L-1)
Electrical grids are sensitive to energy consumption spikes and therefore measures have to be taken to adapt production or better to reduce demand peaks.
TOU energy tariff The Southern California TOU energy tariff (TOU-8, 2014) was implemented in the dynamic energy cost model.
Influent Flow-rate (m3·d-1)
Motivation Management of Electrical Grids
Catalan Institute for Water Research, Technological Park of the UdG, Girona, Spain. Email:
[email protected] 2Urban Water Research Center, University of California, Irvine, CA 92697-2175, USA. Email:
[email protected] 3inCTRL Solutions Inc., 470 Anthony Drive, Oakville, ON, L6J 2K5, Canada. Email:
[email protected]
Figure 5: Energy cost model versus fixed energy price.
Using fixed energy prices and neglecting energy tariff structures may lead to biased conclusions.
