EPRI 

Enhancing the Economics of 

Photovoltaic Power Generation with 

Innovative Direct Current Applications

UPEx’ 02 Photovoltaic Experience Conference

Austin Texas, 

November 13, 2002

Prepared by:  

Phil Barker, 

pbarker@epri-peac.com

Dave Crudele, 

dcrudele@epri-peac.com

EPRI PEAC Corporation

EPRI 

Acknowledgement

This project is funded by:

Illinois Clean Energy Community Foundation (ICECF)

Electric Power Research Institute (EPRI)

Salt River Project (SRP)

The authors wish to thank these organizations for the 
funding support. Also, Bob Romo, the ICECF Project 
Manager, and Terry Peterson, the EPRI Project Manager.

This project is funded by:

Illinois Clean Energy Community Foundation (ICECF)

Electric Power Research Institute (EPRI)

Salt River Project (SRP)

The authors wish to thank these organizations for the 
funding support. Also, Bob Romo, the ICECF Project 
Manager, and Terry Peterson, the EPRI Project Manager.

EPRI 

Isn’t PV Already DC?  

What’s New Here?

• The natural form of PV energy is direct current 

(DC)

• Direct current PV installations are already 

widely used for off-grid applications

• What is new here is we are focusing on the use 

of DC photovoltaic energy to directly 
supplement, without an inverter, power to loads 
at sites that already have an AC utility system 
interconnection!

EPRI 

Candidates for PV-DC Assisted 

Applications

• Adjustable Speed Drives (for mechanical 

drive motors and HVAC system)

• Uninterruptible Power Systems (UPS)

• Electric Resistance Heating Applications

• Office equipment

• Lighting Loads

EPRI 

PV Supplemented Resistive 

Heating Loads

PV Array (up to 10 kW)

DC level 
Converter

Rectifier

Conventional 
AC Power Input 
(utility)

DC Bus (250-600 V)

Resistive Heating Element

EPRI 

PV Array

DC level 
Converter

Drive 
Rectifier 
Unit

Conventional AC Power 
Input (utility)

DC Bus (250-600 V)

Adjustable 

Speed Drive 

Inverter

Motor

PV Supplemented Adjustable 

Speed Motor Drives

Overall 
ASD Unit

EPRI 

PV Array

DC level Converter

Rectifier

Conventional AC 
Power Input (utility)

DC Bus 

Computer with 
Compatible Switched 
Mode Power Supply

Computer Monitor with 
Compatible Switched 
Mode Power Supply

Other Compatible 
Office Equipment

DC Lighting 
Equipment

Computer with 
Compatible Switched 
Mode Power Supply

Computer with 
Compatible Switched 
Mode Power Supply

DC breaker

PV Supplemented Office 

Equipment

EPRI 

PV Assisted UPS

Utility Source

INVERTER

Critical Load

Static Switch 
Isolating Device

Battery 

Storage

Solar Array

DC 
Power

AC 
Power

Charge 

Regulator

EPRI 

A 125 kVA Solar Assisted UPS System 

Developed by Niagara Mohawk Power Corp.

EPRI 

Advantages of DC Approach

• Avoiding the inverter might save up to $1/watt (15-20% of 

PV system initial capital cost) 

• Potentially more reliable installation (could save on long-

term inverter replacement and repair costs)

• Avoiding inverter losses may result in a 5-10% power 

production advantage (further lowering cost)

• Limited utility system interaction (no fault current 

contributions, no export of power) will make 
interconnection less of an issue than with AC inverter 
approach

Overall, up to more than a 25% reduction 
in PV lifecycle cost might be achieved in 
certain suitable DC power applications

Overall, up to more than a 25% reduction 
in PV lifecycle cost might be achieved in 
certain suitable DC power applications

EPRI 

Disadvantages of DC Approach

• Loss of load diversity and ability to export to 

utility system means that the application must be 
carefully matched to the load to achieve good 
economics

• Standard loads and building power distribution 

practices, while many can run on DC, are not 
intended or rated for DC – modifications are 
required

A poorly chosen DC application could 
actually cost more than the AC approach!

A poorly chosen DC application could 
actually cost more than the AC approach!

EPRI 

120 Volt, 

60 Hz AC 

Input

Bridge 

Rectifier

+

-

Rectifier 

and 

Filtering 

Filtered and 

Regulated DC 

Output

DC

Regulated 

High 

Frequency AC

60 Hz AC

Unregulated 

and 

unfiltered DC

Filtered and 

Regulated DC

High 

Frequency 

Switcher 

(Chopping 

Circuit)

Switch Mode Power Supply

EPRI 

DC Test Of Switched Mode Supply

EPRI 

Compact Fluorescent Light 

Operating on DC

EPRI 

SMPS Unit for Dell Laptop

EPRI 

Adjustable Speed Drives

• Adjustable Speed 

Drives can be 
employed to convert 
most induction motor 
loads to operate on DC 
power

Typical small 
adjustable speed 
drive units

EPRI 

6 Pole Bridge 

Rectifier

V

A

V

C

V

B

Motor

6 Pole Inverter 

SCR Bridge

3 Phase 208 volts 
60 Hz-AC Input

-

+

3 Phase Variable 
Frequency Output

DC

DC Input 
From PV 
System

+

-

Adjustable Speed Drive with 

Supplemental PV Power

EPRI 

Load Performance with DC

• Many office devices, adjustable speed motor 

drives and other loads can be operated 
successfully with DC power

• Perhaps the main load type that can’t be powered 

are induction or synchronous motors without drive 
units

• Despite success in powering loads with DC, there 

are some issues:

– Surge arresters
– Protection (circuit breakers, switches and fusing)
– Circuits that need AC for timing or voltage doubling 

functions

EPRI 

PV-DC Demonstration Site in Illinois

DuPage Mayor’s and Manager’s Conference Building 
Oak Brook, Illinois

EPRI 

Major Loads at Demonstration Site

About 5 kW

Office 

Equipment

Interior Fluorescents – 4.032 kW

Exterior Downlighting (Metal Halide) – about 1.1 kW

Exterior Parking Lot Lights (Metal Halide) – 2.625 kW

Lighting

Two 5-ton Units

One 3-ton Unit

One 1.5-ton Unit

Rooftop Electric 

Cooling/Gas 

Heating

Total of 9.65 kW of 120 and 240 volt baseboard

Electric 

Baseboard 

Heating

EPRI 

Integrated Gas Heat/Electric 

Cool Rooftop HVAC Unit

Diagram Courtesy Carrier Corp.

EPRI 

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov Dec

Power 

(KW)

Month of Year

Load of two 5-Ton 
HVAC Units

10 kW PV Array

5 kW PV Array

2 kW PV Array

Peak Daily Output 
of Flat PV Array

HVAC Demand Versus PV Array Output

EPRI 

PV Array (2 to 5 kW DC)

5 ton Rooftop 

HVAC Unit

DC Voltage Regulator 
Control (DC-to DC 
Converter Stage)

5 ton Rooftop 

HVAC Unit

Adjustable 

Speed Drive Unit

Adjustable 

Speed Drive Unit

PV Combiner Box, 

Fusing and Lightning 

Protection

AC Power from Utility System 

(240 V, three phase, 60 HZ)

Providing Supplemental DC 

Power to Rooftop HVAC Units

EPRI 

PV Array (6 kW DC)

Auxiliary Port for 

Future DC 

Office Loads

DC Baseboard Heating

5 ton Rooftop 

HVAC Unit

System Load 

Dispatch 

Controller

DC Rated 
Contactor

DC Voltage 
Regulator Control 
(DC-to DC Converter 
Stage)

Control 
Signal

5 ton Rooftop 

HVAC Unit

Adjustable 

Speed Drive 

Unit

Adjustable 

Speed Drive 

Unit

PV Combiner Box, 

Fusing and Lightning 

Protection

AC Power from 

Utility System

Integrated Load Approach

EPRI 

Project Conclusions and 

Next Steps

• Many loads can be supplemented with PV-DC (ASD 

units, electric heating, SMPS units, and day use lights) 

• Load must be carefully matched to the DC output of the 

array to insure good economics. An ideal application 
may realize 25% or more PV energy cost saving!

• There will be a decision soon to move forward with an 

HVAC unit demonstration project in 2003

• The industry needs DC compatible products that have 

two power connection ports (an AC port and DC port), 
suitable AC/DC appliance ratings and building 
distribution systems with DC compatible protection