Battery-Powered Systems: Efficiency, Control, Economics ECEN 2060
12

# Battery-Powered Systems: Efficiency, Control, Economicsecee.colorado.edu/~ecen2060/materials/lecture_notes/Battery3.pdf · If we discharge the battery more slowly, say at a current

Feb 08, 2018

## Documents

lamkiet
Welcome message from author
Transcript
• Battery-Powered Systems:Efficiency, Control, Economics

ECEN 2060

• 2ECEN2060

Battery capacity

The quantity C is defined as the current that discharges the battery in 1 hour,so that the battery capacity can be said to be C Ampere-hours (units confusion)

If we discharge the battery more slowly, say at a current of C/10, then we mightexpect that the battery would run longer (10 hours) before becomingdischarged. In practice, the relationship between battery capacity anddischarge current is not linear, and less energy is recovered at faster dischargerates.

Peukerts Law relates battery capacity to discharge rate:

Cp = Ik t

where Cp is the amp-hour capacity at a 1 A discharge rate

I is the discharge current in Amperes

t is the discharge time, in hours

k is the Peukert coefficient, typically 1.1 to 1.3

• 3ECEN2060

Example

Our lab batteries

k = 1.15

C = 36 A

Cp = 63 A-hr

Prediction of Peukertequation is plotted atleft

Nominal capacity: A-hrs @ 25C to 1.75 V/cell

36 A-hr

1 hr

56 A-hr49 A-hr46 A-hr45 A-hr

24 hr8 hr4 hr2 hr

What the manufacturersdata sheet specified:

• 4ECEN2060

Energy efficiency

Efficiency = ED/EC

EC = Total energy during charging = vbatt (-ibatt) dt VCICTC

ED = Total energy during discharging = vbatt ibatt dt VDIDTD

Energy efficiency =VDVC

IDTDICTC

= voltage efficiency coulomb efficiency

+V(SOC) Ideal diodes

Rcharge(SOC)

Rdischarge(SOC)

+

Vbatt

Ibatt

Coulomb efficiency = (discharge A-hrs)/(charge A-hrs)

Voltage efficiency = (discharge voltage)/(charge voltage)

• 5ECEN2060

Energy efficiency

Energy is lost during charging when reactions other than reversal of sulfation occur

At beginning of charge cycle, coulomb efficiency is near 100%

Near end of charge cycle, electrolysis of water reduces coulomb efficiency. Canimprove this efficiency by reducing charge rate (taper charging)

Typical net coulomb efficiency: 90%

Approximate voltage efficiency: (2V)/(2.3V) = 87%

Energy efficiency = (87%)(90%) = 78%

Commonly quoted estimate: 75%

• 6ECEN2060

Charge profile

A typical good charge profile:

1. Bulk charging at maximum power

Terminate when battery is 80%charged (when a voltage set pointis reached)

2. Charging at constant voltage

The current will decrease

This reduces gassing and improvescharge efficiency

3. Trickle charging / float mode

Equalizes the charge on series-connected cells without significantgassing

Prevents discharging of battery byleakage currents

Occasional pulsing helps reversesulfation of electrodes

The three-step charge profile usedby the chargers in our power lab

• 7ECEN2060

Battery charge controller

PVarray

Chargecontroller

Prevent sulfation of battery

Low SOC disconnect

Float or trickle charge mode

Control charge profile

Multi-mode charging, set points

Nightime disconnect of PV panel

Direct energy transfer

Charge battery by direct connectionto PV array

MPPT

Connect dc-dc converter betweenPV array and battery; control thisconverter with a maximum powerpoint tracker

• 8ECEN2060

Direct energy transfer

Vpv

Ipv

PVcharacteristic

Batterycharacteristic

PVarray

Charge controller may simply beseries switches

Bulk charge: connect battery directlyto PV array

Other charge modes: pulse current onand off to reduce average current

Nighttime disconnect of PV frombattery

Disconnect inverter when state ofcharge is low

• 9ECEN2060

Maximum power point tracking

PVarray

converter

Insert buck converter into charge controller, and perform maximumpower point tracking in bulk charging mode

Battery reaches full charge earlier in the day

Batterystate ofcharge100%

0%time of daySunrise Sunset

DET

MPPT

In a closed system, the input energy must always equal the loadconsumption. Excess generated energy must be dumped.

• 10ECEN2060

Economics of battery storage

Example: the deep dischargebatteries in our lab

Retail cost: \$150

Assumptions:50% depth of discharge

20 hour uniform discharge

Average voltage 12.4 V

1000 cycles

Energy of each dischargecycle:

I = (Cp/t)1/k = (63/20)1/1.15 = 2.7A

ED = (2.7A)(12.4V)(20hrs) = 0.67 kWh

Battery capital cost per kWh:

(\$150)/[(0.67 kWh)(1000 cycles)] = \$0.22/kWhTypically \$0.10/kWh for large optimized installations

Battery costs more than the energy it stores!

• 11ECEN2060

Backup gas generation

Cost of gasoline

Estimated 5 kWh/gallon, \$3/gal

\$0.60/kWh

Capital cost

\$0.50 to \$1.00 per Watt

Adds another \$0.05 to \$0.10 per Watt if amortized over 19 years

• 12ECEN2060

The value of grid energy

Power supplied by the utility is available

Available on demand, very high reliability

To reproduce this in a standalone PV system:

1. Must generate the power with PV; est. cost \$0.21/kWh

2. Must store in batteries, est \$0.10 to \$0.4 / kWh

3. May additionally need other backup power sources, with additionalcosts but substantially improves reliability

Utility bill has a charge for kWh consumed only

Reliability is worth at least as much per kWh as theenergy itself, but is not included in current pricingschemes approved by PUC

Related Documents
##### Hydraulic machinery lecture_notes
Category: Education
##### Plug-in Electric Vehicles...
Category: Documents
Category: Technology
##### HEV, PHEV and EV Batteries - University of Colorado...
Category: Documents
##### BATTERY MAINTENANCE PRODUCTS - asiarepmining.com · battery...
Category: Documents
##### Neurological exam lecture_notes
Category: Education
##### Global Lead-Acid Battery Trends And Component...
Category: Documents
##### 자바프로그래밍2cs.kangwon.ac.kr/~swson/19Fall_Java2/l...
Category: Documents
##### Acer Laptop Battery, Acer Aspire Battery, Acer TravelMate...
Category: Documents
##### Battery & Charger Stands Battery oer Stands Battery Roller.....
Category: Documents
##### Introduction to Energy Efficient...
Category: Documents