Home Energy Storage System Project Number: 18

ECE 445 Design Report

May 4, 2011

Project Designers C. Ryan Garner

Ryan Pecora

TA: Jim Kolodziej

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ABSTRACT

The cost of energy is continuously increasing, and the need for residential electric power is also

on the rise. These two increases lead to higher energy bills for the residential sector. With society

becoming more and more technology driven, an economic solution to the power price increase is

needed. Many individual green energy options and high efficient energy usage options are available, but

no major home energy system overhaul has been proposed nor is commercially available at this time.

The Home Energy Storage System is the solution to the higher energy consumption and higher prices

problem presented by the technology driven society of the 21st century. The purpose of this system is to

reduce the overall energy costs of a residential household by utilizing an energy storage system. The

main system function will be to charge a large energy storage system, most likely a battery bank, during

non-peak utility cost hours at night or from any available green energy sources so that the house can be

powered during the day without the higher costs of the grid. This system would essentially allow a home

to use the cheaper energy of the night during the day.

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TABLE OF CONTENTS

1. INTRODUCTION ....................................................................................................................1

1.1 System Benefits ..................................................................................................................1

1.2 System Features ..................................................................................................................1

1.3 Full System Performance Requirements ............................................................................2

1.4 Model System Specifications .............................................................................................2

2. DESIGN PROCEDURE ...........................................................................................................3

2.1 Overall Project Design Block Diagram ..............................................................................3

2.2 Battery Bank .......................................................................................................................3

2.3 System Control ...................................................................................................................3

2.4 Current Limiter ...................................................................................................................4

2.5 Inverter Circuit DC-AC ......................................................................................................5

2.6 Rectifier AC-DC .................................................................................................................6

2.7 Boost Converter ..................................................................................................................7

2.8 Controlled Relays ...............................................................................................................9

2.9 Green Energy Options ........................................................................................................9

2.10 Grid Power ........................................................................................................................9

2.11 Home Energy Needs .........................................................................................................9

3. DESIGN VERIFICATION .....................................................................................................10

3.1 Individual Part Verification ..............................................................................................10

3.1.1 Grid Transformer ...........................................................................................................10

3.1.2 Rectifier .........................................................................................................................10

3.1.3 Battery Bank ..................................................................................................................11

3.1.4 Current Limiter ..............................................................................................................11

3.1.5 System Controller ..........................................................................................................11

3.1.6 Inverter ...........................................................................................................................11

3.1.7 Boost Converter .............................................................................................................13

3.1.8 Tolerance Analysis ........................................................................................................14

3.2 Full Model System Testing ...............................................................................................14

4. COST ANALYSIS .................................................................................................................16

4.1 Parts ..................................................................................................................................16

4.2 Labor .................................................................................................................................16

4.3 Total Cost ..........................................................................................................................16

5. CONCLUSIONS ....................................................................................................................17

5.1 Project Completion ...........................................................................................................17

5.2 Challenges and Successes .................................................................................................17

5.1 Future Work ......................................................................................................................18

5.2 Ethics ................................................................................................................................18

APPENDIX – PROJECT PICTURES ....................................................................................19

REFERENCES .......................................................................................................................21

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1. INTRODUCTION

The home energy storage project consisted of the specifications of a full scale system, but mainly

focused on the proof of concept model that incorporates all the functionality of the full system on a

smaller scale. The model runs with voltages at roughly ten times less than the full system and much

smaller currents but is used to prove that the idea is feasible. The model included all the necessary

power conversions for battery charging and discharging modes as well as the control system to run the

overall circuit.

The idea of the full scale system is to convert the cheaper grid power at night and any available

green power into power for the home during the daytime peak cost hours of the grid. This keeps the

energy costs down for the house as it will not be using grid power when the prices are higher. The

battery storage for the system would be large enough to power the house all day and then be recharged

at night. The system controller would make decisions based on the overall goal to reduce energy costs. It

will use any available green energy to charge the batteries and power the home as a preference, since

this green energy could be considered “free” energy for the home. The controller would then utilize the

real time pricing information of the smart grid to determine the best time for charging the batteries from

the grid when necessary.

The model of the system is a scaled down representation of the full scale system. It takes power

from the grid through a 120:12 transformer and converts it to a DC voltage that gets boosted to the

voltage used to charge the batteries. The system can also take voltage from a simulated green energy

source and boost the voltage to charge the batteries from that. The output side of the model includes

taking the DC power from the batteries during discharge mode and converts it to AC power for the

model load. The whole system is controlled by a PIC that runs charge and discharge modes depending

on the time of day, since real time pricing is not available yet, and automatically switches to green

power to charge or power the load when it is available, meaning the green voltage is detected. The

controller operates the circuit through the use or relays, which connect and disconnect the parts of the

circuit necessary for the desired mode of operation.

1.1 System Benefits

Self sufficient home energy system: When connected to green energy options and the grid, this

system would be able to power the home completely with uninterrupted power.

Green energy options: With the correct green energy power sources such as solar cells, the

home could use green energy and minimum grid power during the night to offer clean power

for the home.

Power during outages: If the grid power goes down due to a storm or power line problems, the

household would still have power from the vast supplies of the energy storage system.

Lower energy bills: With the use of self provided green energy and only use of grid power

during non-peak times when needed, the overall power bills would be reduced from the

reduced dependence on the power grid.

1.2 System Features

Green energy connections: The system will be ready for green energy option connections such

as solar cells.

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Battery management system: The advanced battery management system will keep track of

power levels in the battery storage bank to determine the amount of time needed to charge

from the grid during non-peak hours.

System control: The main control system of the home storage system will determine when non-

peak utility hours are for charging, as well as managing any green options for charging during

the day when stored energy levels are low. The system would be programmed to utilize all

green energy options when possible and use grid power as a last resort in all cases.

1.3 Full System Performance Requirements

Store approximately 30kWh with an option for expansion

Operate at room temperature (between 60 and 80 degrees Fahrenheit)

Interface with two pole, single phase, 120V home electric service

At least 85% efficient as an overall system

Provide up to 200A service for the house

System in a sealed and ventilated area to prevent injury

Provide constant uninterrupted power

1.4 Model System Specifications

Transform 120Vrms to 12Vrms from the wall to simulate the house grid power in the model

Convert the 12Vrms to roughly 13V DC for the batteries to charge from

Charge the batteries completely

Convert any green energy DC voltage not used for charging and the battery DC voltage to

12Vrms for the model house loads

Be able to run house model loads from the batteries alone once charged

Connect the batteries for charging once depleted and reconnect them to house loads once

charged

Change over sources over without power interruption

Use green energy to charge batteries

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2. PROJECT DESIGN

2.1 Overall Project Design Block Diagram

Figure 2.1.1: System Block Diagram

2.2 Battery Bank

The battery bank is the main part of the energy storage system. This battery bank would be

charged by the grid during non-peak hours to provide energy during peak hours to the home. It would

also be charged by any green energy options added to the system. In the case of green energy, the

battery bank would consist of two separate battery sections to allow the charging of one section from

solar generation for example while the other section would be providing power to the house. Five 3.2V

lithium iron phosphate rechargeable batteries will be used to simulate a battery bank in the model. The

bank as a whole will provide about 16V to the inverter circuit to create the house power.

2.3 System Control

The system control is a PIC16F877A, programmed to provide control signals to optocouplers

throughout the circuit depending on different conditions. A MAX232 chip is used to interface with a PC

through serial using PuTTY. Initially the system must be initialized with the unloaded battery voltage

and the current time. The unloaded battery voltage allows the PIC to calculate the starting capacity of

the batteries, which is then displayed in a percentage with two decimal places. The PIC is also an

interrupt-driven real time clock, which takes a 1Hz clock signal to keep track of the time. To monitor

changes in battery capacity, a current monitoring circuit is connected to the ADC. This is simply a 1

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ohm resistor connected between the negative of the battery pack and the ground. The voltage across it is

equal to the current going through it, so the PIC can see what the current is. The resolution is fairly low,

as the PIC can only see steps of .02A with its ADC, but it is good enough to track the capacity. The

ADC is read every second and capacity is adjusted, but the capacity and time is only shown every 15

seconds. This helps to conserve battery power, and also keeps the serial output less cluttered.

The PIC has two modes: charge and discharge. Charging mode takes place between 10pm and

6am, and during this time the relay connecting the grid to the batteries is enabled, along with the grid

from the batteries to the load(so that any loads running can still be powered). Between 6am and 10pm,

the system enters discharge mode and opens the relay connecting the grid to the batteries. In this mode

the batteries are connected to the loads, which are run independently of grid power. If green energy is

available, the PIC favors it in either mode. In charge mode the grid will be disconnected and the

batteries will charge off of green energy, while in discharge mode the green energy will be connected to

the batteries to power loads. This means the batteries serve as a buffer, charging if there is excess green

energy or discharging if there is insufficient green energy. In order to determine if green energy is

available, a high-impedance voltage divider is connected from the green energy circuit to pin 21 of the

PIC. Since it is high-impedance it does not waste any current, but the PIC is able to see when the voltage

of the pin is high and then switch the appropriate relay. Figure 2.3.1 shows the pinout for the PIC. The

relay controls are pins 16, 17, and 19. Pin 21 is the input for the green energy voltage divider circuit, and

pints 25 and 26 are connected to the MAX232 for serial communication.

Figure 2.3.1: PIC Pinout

2.4 Current Limiter

The current limiter is needed to ensure an appropriate amount of current is fed to the batteries. In

order for proper charging, the batteries that are being used in the model require constant voltage and

current. If the current were not limited, the batteries would overheat and could be permanently damaged.

The circuit limits the current to .5A, which is well within the 1C limit of the batteries. A slower charging

rate will improve battery longevity, and help in keeping the batteries in balance. The circuit uses two

BJTs and two resistors in voltage follower setups, and simple circuit analysis will show that the circuit

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limits the current to the desired amount with only a modest 2.4V drop in voltage. To account for this

drop in voltage, the rectifier provides a voltage slightly higher than needed.

The main drawback to the current limiter is its efficiency. It dissipates excess power as heat,

which is extremely wasteful and causes the charging efficiency to be very low. Future revisions of the

system will have a more novel way to limit the current without wasting as much. Figure 2.4.1 shows the

final current limiter circuit used in the model.

Figure 2.4.1: Current Limiter Circuit

2.5 Inverter Circuit DC-AC

The inverter converts the DC power from the battery bank to AC power for the house power

systems. The inverter consists of four MOSFETs, the battery bank as a DC source, and an RL load,

which is a combination of a series inductor and the model house load. The four FETs will be switched

on and off alternating such that 1 and 4 are on and then 2 and 3 are on. The gates will be switched at

60Hz, which is the desired output frequency for home power. This 60Hz clock signal will be provided

by a function generator. The logic NOT gate will send a low signal to FETs 2 and 3 when the clock is

high so that they turn off when 1 and 4 are on.

The initial design of the inductor included a 300uH inductor for the output. The inductor keeps

the output from being a strict square wave which would result from only using the MOSFETs. The

inductor resists the change in current and so it “smoothes” out the square wave into a more sinusoidal

wave. In order to build the correct inductor, equation (2.5.1) was used to determine the number of turns

needed in the inductor given the core size that was obtained.

(2.5.1)

In the equation, N is the number of turns, L is the desired inductance, l is the equivalent length of

the core which is the average of the inside and outside circumferences, μ is the core permeability, and A

is the cross sectional area of the core. In the initial calculations, the permeability was estimated, and

after building the initial design, it was determined that the number of turns was too low. The initial

number of turns was calculated to be about 24 turns and after adjustment, the final turns number was

about 58 turns.

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The initial design called for four IRF520 power MOSFETs. However, they could not be used

since the 5V clock signal was not high enough to switch the FETs. Tests to drive the power FETs with

smaller FETs and BJTs were not successful. Instead the circuit was redesigned to use optocouplers,

which eliminated any high side gate problems and could handle the relatively high power of the inverter

load.

Simulation of the inverter circuit using Matlab produced the waveform shown above. This is

preliminary simulation which is why the AC output looks choppy. This was simulated using a 300uH

inductor in series with a 1mΩ load. The value of inductance can be changed in the lab to smooth out the

AC waveform. However, the fundamental AC voltage is produced from the battery bank as desired. The

simulation produced a waveform that is too sharp for specifications. However, this is a result of the

simulation itself and the differential equations involved. This was further proven as the problem because

increasing the inductance on the output in the simulation did not smooth the curve as expected. The final

build test waveform matched the expected waveform much closer, as seen in the verification section.

Figure 2.5.1 shows the output waveform of the Matlab simulation for the inverter design. Figure

2.5.2 shows the inverter schematic used in the design and in the simulation. The pulse generator for the

model system was the function generator to produce the required 60Hz signal needed for proper

operation.

Figure 2.5.1: Simulated Inverter Waveform

Figure 2.5.2: Inverter Schematic

2.6 Rectifier AC-DC

The rectifier converts AC grid power to DC power in order to charge the batteries. This circuit

consists of four 1N4001 diodes, a 12Vrms AC signal provided by the transformer, and a parallel RC

load of a capacitor to reduce output ripple as well as the model house resistive load. The final load of the

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rectifier is the batteries through the boost converter. The rectifier is only used when the batteries are

being charged from the grid source. The following equation was used while sizing the capacitor for a 1%

output ripple.

(2.6.1)

The first capacitor sizing gave a 70mF capacitor needed for the first estimate of current and a 1%

voltage ripple at the battery voltage. This is a very large capacitor, however the calculation estimations

were generally larger than what will most likely been seen in the model. In the final design, a 12mF

capacitor was used because 70mF capacitors are expensive and a 12mF capacitor was the largest

available.

Simulation of the rectifier circuit using Matlab resulted in the waveform shown in figure 2.6.1.

This is the DC voltage provided by the converter and has a very small ripple as desired. It also shows

successful AC to DC conversion from 12Vrms to 15.36V DC. This simulation was run using the 70mF

capacitor and 100mΩ load. Figure 2.6.2 shows the rectifier schematic used in the final design. In the

figure, the resistive load is shown for simulation purposes and represents the batteries through the boost

converter.

Figure 2.6.1: Simulated Rectifier Waveform

Figure 2.6.2: Rectifier Schematic

2.7 Boost Converter

The boost converter provides the voltage increase from the green energy option or the rectifier

DC voltage to the battery storage system in order to get a steady voltage magnitude that is high enough

for battery charging. The voltage also has to be boosted high enough to overcome the voltage drop from

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the current limiter, which was about 2.4V. This part was added to overcome the source voltage

difference between the green energy option and the grid rectified voltage. The boost converter has a

feedback controller that uses a voltage divider for reference to keep the output voltage constant with a

range of input voltages.

For the boost converter feedback, the design was used from [3]. The feedback controller consists

of a FET driver circuit and a low pass filter. The filter keeps out any high frequency noise from the

switching action, which is at about 20kHz. The driver circuit compares the voltage divided output

voltage to a reference voltage and changes the duty ratio for the FET based on if the output is too low or

high. The boost converter was designed based on the input range, from about 12V to about15V for the

green voltages and the rectifier voltage, and the desired output voltage of about 19.5V. For the

converter, both the value of the capacitor and the inductor had to be determined. Equation (2.7.1) was

used to determine the capacitor needed based on the allowable amount of voltage ripple for the output,

and equation (2.7.2) was used to determine the inductor value based on the allowable amount of output

current ripple. In (2.7.1), V is the inductor voltage when the FET is off which is the input voltage, L is

the inductance needed, Δi is the allowed current ripple and Δt is the amount of time in one switching

cycle that the FET is off. For (2.7.2), I is the capacitor current when the FET is off which is the desired

output current, C is the capacitance needed, Δv is the voltage ripple and Δt is the amount of time in one

switching cycle that the FET is off. The initial design calculations called for a 19.5uF capacitor and a

560uH inductor. Again, the inductor had to be handmade, and the number of turns needed was

calculated using equation (2.5.1). For the design required inductance, it was determined that about 81

turns were needed.

(2.7.1)

(2.7.2)

Figure 2.7.1 shows the schematic for the complete boost converter circuit including the feedback

controller circuit. The V1 in the schematic is the input voltage to the converter and represents the green

energy voltage or grid voltage from the rectifier depending on the operation mode. The resistor on the

output R13 represents the converter load which is the batteries in the model system. It should be noted

that an extra snubber circuit was used across the FET in the converter. The purpose of the snubber is

reduction of the ringing effect of the switching action of the FET. It is a simple circuit that reduces the

ripple in the output that is due to the 20kHz switching action of the FET.

Figure 2.7.1: Boost Converter Schematic

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2.8 Controlled Relays

The controlled relays act as the interface between the system controller and the system itself. The

final design uses the same optocouplers used in the inverter circuit. They were used because the PIC

only outputs a 5V signal, which is too low to drive a power FET, and they can handle the high currents

of the circuit. The relays provide a break in the circuit between the boost converter and the rectifier,

between the green energy input and the boost converter, and between the batteries and the inverter.

Depending on the mode of operation, the relays allow the PIC to connect and disconnect the parts of the

circuit needed for the current mode of operation. For example, in the grid charging mode the batteries

are disconnected from the inverter and the boost converter is disconnected from the green energy input

and connected to the grid input. In order to decrease the PIC output voltage to the 1.2V required to turn

on the optocoupler, a 470 ohm resistor was connected in series. This also serves to limit the current,

which is crucial to overall system efficiency.

2.9 Green Energy Options

The part of the system would include any green energy generation such as solar cells or

geothermal energy. These energy sources are the preferred charging source for the battery bank. In the

model, a simple DC power source was used to represent a green energy source such as a solar cell. It

represents the DC voltage and current that would be available from a solar cell.

2.10 Grid Power

This is the grid connection to the house for regular power. It will be simulated by a 10:1

transformer that will produce 12Vrms for the model from the 120Vrms of a wall plug in the lab. The

grid power gets converted to DC power by the rectifier in order to charge the batteries. The transformer

used was a simple plug in transformer that provided the about 12Vrms, actual output was measured at

13.8Vrms, to simulate the grid power.

2.11 Home Energy Needs

These are the systems in the house that need power to work. Anything a resident would use that

needs electrical power is included here. The model simulates these loads with a power resistor that is

connected to the output of the inverter.

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3. DESIGN VERIFICATION

3.1 Individual Part Verification

A complete, full-scale system will not be able to be built and tested due to high costs. However, a scaled

down model was built, complete with all the functioning parts involved but at a much smaller amount of

energy stored.

The individual parts of this system were built and tested in the lab. Each part was tested

individually and then combined to form the whole system. The entire system was then tested for proper

operation and switching. The two converters in the system were built and tested individually to show

that these parts work on their own. Most of the control system is program based so it was put under

simulated conditions before it was connected to the real system model.

3.1.1 Grid Transformer

The transformer circuit was tested to make sure it produced the desire output. It was connected to the

wall outlet power and the output connections were connected to the scope to make sure the output

waveform is at 60Hz as desired. This circuit was tested with a range of currents from 0A to about 1A

current, which is the range of the model, to confirm proper operation under all conditions.

As expected, the output of the transformer was about 13.8Vrms which is close to the 12Vrms

that it is rated at for 120Vrms input. The waveform was at the 60Hz that it is supposed to be, and the

transformer could handle the current loads of the circuit.

3.1.2 Rectifier

In order to test the functionality of the rectifier, it was tested by itself before being added to the

system. The rectifier was connected to the grid transformer as the input that it would be connected to in

the final model. Originally, the rectifier was connected to the function generator in the lab. However,

this did not work as a test source because the function generator does not provide any current. Instead

the circuit was tested with the grid transformer while monitoring the current to ensure no damage to the

rectifier. The rectifier was then loaded with a power resistor that represented the boost converter and

battery pack load in the final model. In order to verify correct operation, the oscilloscope was connected

across the load and capacitor to see the DC output waveform and the output voltage was measured with

a multimeter.

The test showed that the rectifier circuit was behaving properly. The input transformer was

working fine and the current levels were measured to be within the safe operating bounds. The output

waveform, shown in figure 3.1.2.1, has almost no ripple as expected with the large capacitor in the

circuit. There are very small voltage spikes shown in the waveform, but they had no effect in the circuit

and were just due to the switching of the diodes in the H-bridge. The voltage measured across the output

of the rectifier was measured to be about 16V with the load, and about 14.5V when connected to the

boost converter in later tests.

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Figure 3.1.2.1: Rectifier Output Waveform

3.1.3 Battery Bank

The battery bank was tested in both charging and discharging modes. The DC power source was

used to provide a steady 16V to the batteries to simulate. A multimeter was used to observe the current

flow into the batteries and the corresponding voltage increase. As the batteries charge, their voltage

increases steadily until they reach full charge. The battery bank was then connected to a power resistor

to verify proper discharging. Again, a multimeter was used to observe current flow out of the batteries

and the corresponding drop in voltage. As expected, a discontinuous drop of the voltage was observed as

progressively lower resistance loads were connected.

3.1.4 Current Limiter

The current limiter was tested using a set DC voltage into a resistive load. The triple output DC

source allowed the voltage to be modified up to 25V, and displayed the current it provided on screen.

The load resistance was then decreased in order to ensure that the current was properly limited to .33A.

The test results showed that when voltage was approximately 19.5V (the output of the boost converter),

the current was maintained at just above .33A. This held as the resistance was lowered from 50 ohms to

about 10 ohms. The test concluded here, since the actual performance requirements of the circuit are

much less demanding then the test showed the current limiter circuit was capable of.

3.1.5 System Controller

Before the functioning of the serial interface, the testing of the system controller was done with

power supplies and the multimeter. The first version of the code featured only relay switching depending

on control signals, so the control signal would be fed and the pin that controlled the appropriate relay

would be connected to the multimeter to observe proper switching. Once the relay circuits were

completed, they were connected and the drop in resistance across the MOSFET part of the optocoupler

was observed as the appropriate control signal went high.

Once serial communication was functioning, debugging became much simpler. Simple text flags

could be placed in the circuit to show that the PIC had switched modes when the time changed from

9:59pm to 10:00pm. Also, the value read by the ADC could be output to the screen to ensure the current

monitoring circuit was properly working. The final step of the verification was the connection of the

entire circuit and the observation of everything properly switching.

3.1.6 Inverter

The inverter circuit was tested modularly in order to confirm proper operation before connecting

it to the final circuit. The initial test of the inverter involved connecting a DC power supply to the input

of the circuit, providing the 60Hz clock used for the switching function of the optocouplers, and using a

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power resistor as the load on the output. The next test of the inverter used the batteries as an input, while

monitoring battery current to make sure nothing would be damaged. For both individual tests, the output

rms voltage was measured with the multimeter and the output waveform was observed with the scope to

make sure the inverter was creating the AC waveform desired. With the DC source test, the source was

set to the same voltage that is provided to the inverter by the battery.

After the testing of the circuit, proper operation was observed from the inverter. For both the DC

source test and the battery test, the inverter produced the desired output waveform. Figure 3.1.6.1 shows

the output waveforms of the voltage and current across the load resistor. This resistor was set to the

same load that was used in the final model system test and demo. The output voltage for the tests was

14.7V AC measured by the toolbox multimeter and 13.75Vrms measured by the power lab power meters

during the final system testing. The inverter was later tested with a smaller load to see the effects of

higher currents, and the output voltage dropped slightly but the circuit worked overall even with the

higher currents. In the figure, the light blue waveform is the voltage waveform across the load, and the

green waveform is the current waveform through the load. It should be noted that the signal is AC but

not quite as sinusoidal as desired. The circuit could be improved by increasing the inductance in order to

further smooth out the waveforms.

Figure 3.1.6.1: Inverter Output Waveforms

Figure 3.1.6.2 shows the clock signal waveform as well as the switching waveforms for the

inverter from the drive circuit. The 60Hz clock was used as an input from the function generator to the

inverter drive circuit. This circuit consisted of a logic NOT gate that output clock and clock bar signals

to create the alternating switching needed for the inverter to create an AC output. This circuit also

included two 10Ω resistors, one for each signal, to prevent the optocouplers from dragging down the

switching signal voltage so that the FETs with still switch. In the figure, the dark blue waveform is the

input clock signal, the light blue waveform is the clock switching signal to inverter switches one and

four, and the pink waveform is the clock bar signal to switches two and three.

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Figure 3.1.6.2: Switching Signal Waveforms

3.1.7 Boost Converter

The boost converter circuit was tested using a range of input voltages between 12V and 16V.

This was tested to ensure correct 19.5V output which is fed to the batteries in the final model. The boost

converter was connected to a DC power supply for modular testing in order to verify proper output and

operation as well as make sure the current in the circuit was not too high. The DC source was also used

to verify the functionality of the feedback circuit by making sure the output voltage remains the same

with different input voltages. The output was connected to a power resistor and the voltage was

measured across this load with a multimeter.

The test showed the boost converter was functioning properly, but small changes in the gains for

the feedback circuit were needed to obtain the correct output desired. The input voltage was varied from

12V to 16V in one volt increments to determine proper operation. The output voltage was measured to

be 19.5V for the various input voltages. An input of 12V produced a slight drop in output voltage to

about 18.9V, but this was expected since 12V is at the bottom of the proper operation range. The scope

was connected to the gate signal to observe the changing duty ratio for the different inputs. Figure

3.1.7.1 shows the switching signal for the input voltage equal to the rectified grid voltage, and figure

3.1.7.2 shows the switching signal for the input voltage equal to the green input voltage of 14V. It can

be seen that the duty ratio for the grid voltage is smaller than the green duty ratio because the grid input

voltage is higher and so less boosting is needed.

Figure 3.1.7.1: Grid Voltage Gate Signal Figure 3.1.7.2: Green Voltage Gate Signal

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3.1.8 Tolerance Analysis

The system controller is the essential part of the whole system. Without proper initialization of

the controller, the whole system would stay in the off state. Once the controller is properly initialized,

then the system runs properly. If the controller does not function properly, the relays connecting the

parts of the circuit will stay off and the circuit will not do anything. All of the parts of the system are

essential to proper operation. However, the system controller brings them all together and runs them in

the correct order. If the system controller does not work properly, the system would have to be manually

operated which is slow and very inefficient. Therefore, it is essential to the system that the controller

works perfectly.

3.2 Full Model System Testing

The overall system testing was completed in several steps. First, the battery charging circuit was

tested by connecting the grid transformer, rectifier, boost converter, current limiter, and battery pack.

These connections would prove that the batteries could be charged by the charging components of the

model. For this test, the grid current, battery current and battery voltage were measured. The currents

were measured to make sure there were no shorts and the battery voltage was measured to make sure it

was increasing which proves that charging is occurring. This test verified that the battery charging

portion of the system was working properly.

The next part of the system test was to connect the discharging parts of the circuit to determine

proper load powering operation. The batteries and inverter were connected to verify this. This verified

that the system could properly discharge the batteries.

The next major testing on the circuit was to connect the charging and discharging parts of the

circuit together through the relays. The system controller was not added in yet and the relays were

operated by manually feeding them the 5V needed to turn them on. During this part of the test, a few

issues were discovered. First, the AC voltage from the grid to the load could not be blocked by the

optocoupler so that part of the model had to be removed in order to prevent interference between the

inverter and grid power. In future work on the model, an electronically operated mechanical disconnect

would be needed for this functionality. Another issue discovered was that even when the green input

relay was off, the green voltage would leak through the relay and be seen across the boost converter at

the same time the grid power was there. This interference caused improper functioning so a second relay

was put in line with both the green input and the rectifier to completely isolate the two sources from

each other. These two issues called for moving the relays around slightly to keep proper operation of the

rest of the circuit. After these bugs were worked out, the system was powered up and manually switched

to see that all the parts of the circuit worked while they were all connected. After this testing was

complete, the operation of the hardware side of the circuit was verified.

The final stage of testing in the whole model was to add the controller to the system and test with

the rest of the circuit. The controller was initialized and proper operation of the whole circuit was

verified along with all the functionality. Various system voltages and currents were then measured in the

power lab to verify proper operation and calculate efficiency. Table 3.2.1 shows the various data

collected from the circuit during the different mode tests. It should be noted that in the battery voltage

measurement, the increasing note means that the battery was charging and the decreasing note means the

battery was observed to be discharging. Table 3.2.2 shows the efficiency data and calculation results for

the circuit. It was determined that the discharging mode of the circuit was very efficient with both low

and high current loads. The charging efficiency from the grid, which is the important efficiency when

figuring money savings, was not as high as desired at only 62%. This is mostly due to the current limiter

dumping excess current which is extremely wasteful. Further work needs to be done on finding a better

way to limit current. However, the current was really only limited in the model because of its small

15

scale. The full scale system would probably not need as much limiting because the battery pack would

be more robust and so the system would be more efficient.

Table 3.2.1: Full Model System Test Data

Mode Tested Discharge Mode Green Charge plus Load

Grid Charging

Green Charge

Pout (W) 3.88 3.93 0 0

Iout,rms (A) 0.277 0.28 0 0

Vout,rms (V) 13.9 14.03 0 0

Vout,AC,toolbox (V) 14.42 14.5 0 0

Vbattery (V) 16.265 (decreasing) 16.43 (increasing) 16.617 (inc.) 16.719 (inc.)

Vboost,out (V) 15.974 19.323 19.172 19.257

Pin,grid (W) 0 0 8.36 0

Vin,grid,rms (V) 13.74 13.71 13.12 13.71

Iin,grid,rms (A) 0 0 0.85 0

Vin,green (V) 14 14.01 14 14.01

Iin,green (A) 0 0.54 0 0.51

Table 3.2.2: Full Model System Efficiency Data

Efficiency Test

Grid to Battery

Battery to Load

Battery to Half Load

Pin (W) 8.25 4.22 7.73

Iin (A) 0.841 0.28 0.523

Vin (V) 13.11 16.4 16.1

Pout (W) 5.08 4.04 7.13

Iout (A) 0.305 0.282 0.525

Vout (V) 16.75 14.06 13.47

Efficiency 0.62 0.96 0.92

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4. COST ANALYSIS

It should be noted that the cost analysis is based on the cost of the parts needed for the model version of

the system and not the full size system that would be implemented.

4.1 Parts

Table 4.1.1: Project Parts and Costs

Parts Unit Price Quantity Total Price

12 mF Capacitor 15.00 1 15.00

CPC 1080J Optocoupler 6.80 10 68.00

1N4001 Diode 0.32 5 1.60

300uH Inductor 732-1430-ND 3.31 1 3.31

Microcontroller PIC16F877A 6.72 1 6.72

MAX232 Chip 1.02 1 1.02

Not Gate Chip SN74S20N-ND 1.50 1 1.50

3.2V Battery LFP-26650-3300 9.20 5 46.00

BJT BD433 0.15 2 0.30

47uF Capacitor 445-2905-ND 2.04 1 2.04

2.2 Ohm resistor P2.2W-1BK-ND 0.34 1 0.34

263 Ohm Resistor RNF14BAE264R-ND 0.18 1 0.18

1u, 10 uF Capacitors 0.50 10 5.00

Wire Set 10.00 1 10.00

560uH inductor core 1.00 1 1.00

20uF cap. And other caps. 0.50 7 3.50

UC3843PWMIC 5.00 1 5.00

various resistors (10, 470, 67, 10k, 100k 8.2M, 2.7M) 0.10 20 2.00

Op Amp 2.40 2 4.80

2.2V zener diode 0.10 1 0.10

1N4150 diode 0.03 1 0.03

20MHz crystal oscillator 0.70 1 0.70

variable resistors 1.00 4 4.00

Total 182.14

4.2 Labor

Labor cost = 2 workers x ($45/hr) x 2.5 x 200 hours = $45,000

4.3 Total Cost

Total Cost = 45000+182.14 = $45,182.14

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5. CONCLUSION

5.1 Project Completion

The final model was produced and was completed with all of the basic functionality that was

expected of it. The battery could be successfully charged from the grid power input as well as the green

energy input. The load could be powered through the stored energy in the batteries alone as well as

powered by the green energy when available. The system controller could run the operation of the

system with proper decision making based on current system conditions. The green energy was used to

charge the batteries and power the load whenever the controller detected it to be available. The grid

power was only used when the batteries needed to be charged and green power was not available. The

system controller could properly switch from discharging to charging mode when the time changed from

day to night.

5.2 Challenges and Successes

One major challenge in the project was correctly driving the FETs in the inverter with the clock

signal. The original power FETs used could not be driven with the 5V signal of the clock. Initially, it

was attempted in different ways to drive the power FETs with smaller BJTs and FETs, but these

attempts were unsuccessful. The problem was solved with the optocouplers. These were power FETs

that are driven by a LED with only requires 1.2V and is isolated. This means that the clock signal could

drive them and all high side gate problems were also eliminated.

Another major issue that came up was the voltage of the green energy leaking to the rectifier

output while the green energy relay was off. The rectifier output was also leaking to the green energy

input when the grid relay was off. To solve this problem, two optocoupler relays were connected in

series with the two negative sides of the switches connected in order to block voltage both ways. This

kept the voltages from leaking into the circuit when the relays were off.

The other issue related to this problem was that the optocouplers could not block AC voltage.

Even with two in series, the AC voltage was leaking through. This meant that a direct connection from

the grid to the load could not be established. This eliminated the grid powering the load during charging

mode operation of the model. However, with more time and an electrically controlled mechanical relay,

this issue could be resolved.

Initially the capacity monitoring of the batteries was handled using voltage monitoring of the

batteries. This caused problems because the battery voltage decreased discontinuously when large loads

were placed on the system, which meant that the capacity would be misread when the load changed. To

solve this problem, the system had to be initialized with the unloaded battery voltage and then current

monitoring was used. A 1 ohm resistor between the battery negative and ground allowed the PIC to see a

voltage equal to the current draw, and use this to calculate discharge rate. Since the PIC cannot see

negative current, the charge rate is not handled by the ADC and is instead calculated using the

knowledge that the charging current is always .33A.

An additional problem with the control was getting the serial interface to function properly. It

was found that TTL serial sends 0V for a 0 and 5V for a 1, while PC serial sends 13V for a 1 and -13V

for a zero. This meant that conversion was necessary, and this conversion is handled by the MAX232

chip. This is a very simple chip, and it handles all of the voltage conversions necessary for both

transmission and reception.

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5.3 Future Work

Work on this project would continue to make it a more standalone product. The next step would

be to build on-board clock generators for the inverter and the PIC. This would make the system self

contained. The grid to load relay issue would also be worked out. The next major step would be to start

the implementation of the full scale system. The major power components of the project would need to

be scaled up to accommodate the larger power of the full system. Also, the green energy input would

need to be tested with an actual full scale green energy source. Further work to increase the system

efficiency would be ongoing throughout the project.

5.4 Ethics

The major ethical concerns for this project are the cost issues. Both the battery bank and the

green energy sources that add a huge cost savings from the grid for this system are very expensive. The

customers of this project would need to be told how long it would take to have the system pay for itself,

which is longer than might be expected. Also, the efficiencies of the project need to be improved before

being sold as a final project. The customers would need to be informed of all the details before being

sold to in order to let them know that the system does not yield free energy.

Since most parts of the project would be built in-house, no major licensing issues are present.

Some of the chip licensing may have to be addressed for the PIC, but all the designs are original and so

no outside permission would need to be obtained. Purchasing of the PIC controller and its software

yields the license to use the PIC and software with the original design of the system controller without

need to consult Microchip.

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APPENDIX – PROJECT PICTURES

All of the figures in the appendix are pictures of the different parts of the final model. They are

label according to the picture.

Figure A.1: Boost Converter Circuit Figure A.2: Battery bank and Grid Transformer

Figure A.3: Controlled relays Figure A.4: Inverter Circuit

Figure A.5: System Controller Circuit

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Figure A.6: Full Model without Batteries or Transformer

Figure A.7: Full System Model

21

REFERENCES

[1] "Current Limiting Circuit | VIDISONIC." VIDISONIC | Hardware & Software Design Resources.

VIDISONIC, 2008. Web. 20 Feb. 2011. <http://www.vidisonic.com/2008/07/10/current-

limiting-circuit/>.

[2] Hoffmann, Lukas. "PIC 16F877A Tutorials for Pitt Robotics Club." Pitt Robotics Club. University

of Pittsburgh, 2010. Web. 20 Feb. 2011.

<http://www.pitt.edu/~sorc/robotics/Lukas%20PIC%20Tutorial.doc>.

[3] Krein, Philip T. ECE-469 Power Electronics Laboratory Information and Guide. 2.4nd ed. 2010.

Print.

[4] Krein, Philip T. Elements of Power Electronics. New York: Oxford UP, 1998. Print.