PUBLICATIONS

A 1 1 1 4 07L532

NBSIR 84-2980

Research Priorities for Improving the

Effectiveness of Active Solar Hot

Water and Space Conditioning

Systems

Robert D. Dikkers

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, Maryland 20899

William J. Kennish

TPI, Inc. Beltsville, Maryland 20705

C. Byron Winn

William Huston

Solar Environmental Engineering Company, Inc.

Ft. Collins, Colorado 80524

December 1984

-ac—

100

,1156

Prepared for U.S. Department of Energy

°*fice of Solar Heat Technologies

tive Heating and Cooling Division

ishington, DC 20585

NBSIR 84-2980

i t 1 RESEARCH PRIORITIES FOR IMPROVING THE

EFFECTIVENESS OF ACTIVE SOLAR HOT

WATER AND SPACE CONDITIONING

SYSTEMS

Robert D. Dikkers

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, Maryland 20899

William J. Kennish

TPI, Inc. Beltsville, Maryland 20705

C. Byron Winn

William Huston

Solar Environmental Engineering Company, Inc.

Ft. Collins, Colorado 80524

national bureau

OF STANDARDS

T TrtBARY

/» ,

/w

December 1984

Prepared for U.S. Department of Energy

Office of Solar Heat Technologies

Active Heating and Cooling Division

Washington, DC 20585

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director
ACKNOWLEDGEMENTS

The authors gratefully acknowledge the cooperation and contributions of the

various members of the solar industry (manufacturers, designers,

contractors, researchers) who participated in the two meetings held in

August 1983. The preparation of a draft report on research issues for

advanced systems (basis for Section 3 of this report) by Charles Kutscher,

Solar Ehergy Research Institute (SERI), is also appreciated. The support

and comments of Morris Skalka and Robert Hassett, Active Solar Heating and

Cooling Division, Department of Energy (DOE), and the assistance of Larry

Salamone and Karen Lister, National Bureau of Standards (NBS), in the

planning and conduct of the August 1983 meetings are also gratefully

acknowledged.

The following individuals provided useful review comments on a previous

draft (February 1984) of this report: William Schertz, Argonne National

Laboratory; Morris Skalka, DOE; Paul Pekrul and William Marlatt,

ETEC/Rockwel 1 International; Freeman Ford, FAFCO; Robert Aasen, Honeywell;

John Avery, Los Almos National Laboratory; Joseph Greenberg and

Thomas Faison, NBS; and Charles Kutscher, SERI.

ii
TABLE OF CONTENTS

Page

Section 1. INTPCDUCTION 1

1.1 Background 1

1.2 Scope 1

1.3 Meeting Participants 3

1.4 Meeting Organization and Methodologies 5

Section 2. RESEARCH RECOMMENDATIONS FOR STATE-OF-THE-ART ACTIVE

SOLAR ENERGY SYSTEMS 5

2 . 1 Introduction. 5

2.2 General Meeting Recommendations

(August 2-3, 1983) 5

2.3 Control Subsystem Meeting Recommendations

(Augut 9-10, 1983) 12

Section 3. RESEARCH RECOMMENDATIONS FOR ADVANCED ACTIVE

SOLAR ENERGY SYSTEMS 17

3.1 Residential Direct Heating and Hot Water-Liquid... 18

3.2 Residential Direct Heating and Hot Water-Air 19

3.3 Residential Open Cycle Solid Desiccant

Cooling, Heating, DHW 19

3.4 Commercial Solid Desiccant/Vapor Compression 19

3.5 Commercial Absorption Cooling 20

3.6 Commercial Rankine Power Generation and Cooling... 20

3.7 Ground Coupled Solar Assisted Heat Pump

Community Heating 20

Section 4. SUMMARY 21

4.1 State-of-the Art Systems 21

4.2 Advanced Systems... 22

Section 5. REFERENCES 22

APPENDIX A. Development of a prioritization methodology

for the system effectiveness research program

at the U.S. Department of Ehergy 23

APPENDIX B. Solar heating and cooling control subsystems 31

iii
*

.
1. INTRODUCTION

1.1 Background

In 1980, a Department of Energy (DOE) program plan was developed to address

the reliability and maintainability (R&M) of active solar energy systems

[1]*. The goal of this R&M program was to "accelerate the removal of

reliability and maintainability as major concerns impeding the widespread

adoption of solar energy systems." Primary objectives of the program were

to:

o Provide all groups that have solar R&M concerns with the information

that is available to the program and that is necessary to alleviate

those concerns.

o Assist the solar energy industry in improving levels of R&M

performance in state-of-the-art solar energy systems, components, and

materials

.

o Assist in the early development of a viable infrastructure for the

design, manufacture, installation, and maintenance of reliable,

maintainable, and durable solar energy systens.

o Assist in the development of appropriate standards, code provisions,

and certification programs relating to the R&M performance of solar

energy systems, components, and materials.

o Develop the information required to support the other activities

within the R&M program.

In addition to reliability and maintainability, the 1980 program plan [1]

recognized that other factors such as thermal performance, cost and site

constraints were also important components of "system effectiveness" (Fig.

1). In order to maximize system effectiveness, the designer or

manufacturer must often consider tradeoffs between these various

factors.

1.2 Scope

In 1983, the DOE Active Solar R&M program was renamed the "Systems

Effectiveness Research" (SER) program. The objective of the program was to

identify, assess and initiate research into those factors which influence

the effectiveness of solar space conditioning systems. As part of the FY

1983 SER program coordinated by the Solar Energy Research Institute (SERI),

the National Bureau of Standards (NBS) was assigned responsibility for

developing research priorities for improving the effectiveness of active

solar hot water and space conditioning systems. To carry out this

responsibility, NBS in cooperation with various industry representatives,

organized and conducted two meetings in August 1983. With support from

TPI, Inc., the first meeting was held at NBS on August 2-3, 1983 and

covered all major aspects of state-of-the-art active solar hot water and

space conditioning systems. The second meeting dealt only with solar

* timbers in bracket refer to references listed in Section 5.

1
• System

Efficiency

• Subsystem

Efficiency

• Component

Efficiency

• Design Cost • System

Availability

• Equipment Cost

• Durability

• Construction

Cost • Maintenance

• Operating

Cost

• Maintenance

Co'st

• Climate

• Load

Characteristics

• System/Load

Integration

Figure 1: Components of System Effectiveness

2
control subsystens and was held at NBS on August 9-10, 1983. Support for

this meeting was provided by the Solar Ehvironmental Engineering Company,

Inc.. Recommendations obtained in these meetings are presented and

discussed in Section 2. Recommended research priorities for addressing the

effectiveness of advanced active solar systems (not dealt with at August

1983 meetings) are presented in Section 3. A summary of recommended

research priorities for improving the effectiveness of state-of-the-art and

advanced active solar systems is provided in Section 4.

1*3 Meeting Participants

Solar industry representatives (manufacturers, contractors, architects,

engineers) who participated in the ranking of recommended research areas at

the August 2-3, 1983 were:

o Donald Bowden

Solar Unlimited, Inc.

Huntsville, Alabama

o Edward Butler

Sunrise Solar Services., Inc.

Suffield, Connecticut

o Brad Davis

Marc Ehergy Corporation

Cinnaminson, New Jersey

o Webb Farber

U.S. Solar Corporation

Hampton, Florida

o Joseph Frissora

Sunmaster Corporation

Coming, New York

o Susumu Karaki

Colorado State University

Ft. Collins, Colorado

Government and government contractor staff participating in this meetinq

included:

o Andrew Parker

Mueller Associates, Inc.

Baltimore, Maryland

o Richard Rittleman

Burt Hill Kosar Rittleman

Associates

Butler, Pennsylvania

o Robert Spears

Reynolds Metals Company

Richmond, Virgin

o Burt Swerdling

Grumman Ehergy Systems, Inc.

Melville, New York

o Raymond Yaeger

Synergic Resources Corporation

Bala Cynwyd, Pennsylvania

o John Avery

Los Almos National Laboratory

Los Almos, New Mexico

o Robert Dikkers

National Bureau of Standards

Gaithersburg, Maryland

o Ed Gray

JRB Associates

McLean, Virginia

o Chuck Kutscher

Solar Ehergy Research Inst.

Golden, Colorado

o Terry Logee

Vitro Laboratories

Silver Spring, Maryland

o William Schertz

Argonne National Laboratory

Argonne, Illinois

3
o Robert Hassett

Department of Energy

Washington, DC

o Oscar Hillig

ETECyRockwell international

Canoga Park, California

o Morris Skalka

Department of Ehgery

Washington, DC

o Dave Waksman

National Bureau of Standards

Gaithersburg, Maryland

o William Kennish

TPI, Inc.

Beltsville, Maryland

The following persons (controls manufacturers, government staff, etc.) took

part in the August 9-10, 1983 meeting:

o Bob Aasen

Honeywell/TSC

Roseville, Minnesota

o Rick Boers

Dynatech Energy Controls

Lakewood, Colorado

o Joseph Mibelli

Rho Sigma, Div. of watsco

Hialeah, Florida

o Dennis Miller

Johnson Controls

Milwaukee, Wisconsin

o Cliff Carwile

Department of Energy

Washington, DC

o Gibb Conover

Independent Ehergy Inc.

East Greenwich, Rhode Island

o Robert Dikkers

National Bureau of Standards

Gaithersburg, Maryland

o Hunter Fanney

National Bureau of Standards

Gaithersburg, Maryland

o Robert Hassett

Department of Ehergy

Washington, DC

o Peter Herczfeld

Drexel Uhiversity

Philadelphia, Pennsylvania

o Bill Huston

Solar Environmental Engr. Co.

Fort Collins, Colorado

o Daryl Myers

Solar Ehergy Research Inst.

Golden, Colorado

o David Parks

Heliotrope General

Spring Valley, California

o Paul J. Pekrul

Rockwell Intemational/ETBC

Canoga Park, California

o Kent Reed

National Bureau of Standards

Gaithersburg, Maryland

o Shari Rossi

Vitro Laboratories

Silver Spring, Maryland

o Morris Skalka

Department of Energy

Washington, DC

o C. Byron Winn

Solar Environmental Eng. Co.

Fort Collins, Colorado

o Peter Jacobs

Novan Energy, Inc.

Boulder, Colorado

4
1.4 Meeting Organization andJfethodolpqiejg

Approximately one month prior to the August 2-3, 1983 meeting, various

background information was sent to the invited meeting participants. This

information package included a discussion of system effectiveness, a listing of proposed research areas, and a proposed methodology for

prioritizing research areas. Detailed information on the proposed

prioritization methodology and the methodology which was used at the

August 2-3, 1983 meeting is discussed in Appendix A.

Similarly, prior to the August 9-10, 1983 meeting, a background paper on

solar heating and cooling control subsystems was prepared and distributed

to the invited participants. This paper (Appendix B) contained a review of

control theory, a literature review and recommended research areas. Hie

prioritization methodology used at the earlier meeting was not employed at

the August 9-10, 1983 meeting. Instead, recommended solar control research

activities were developed on the basis of individual presentations and

working group discussions.

2. RESEARCH RECOMMENDATIONS FOR STATE-OF-THE-ART ACTIVE SOLAR ENERGY

SYSTEMS

2.«1- .introduction

The purpose of this section is to present recommended research priorities

for inproving the effectiveness of state-of-the-art active solar energy

systems which were developed at the two meetings held in August 1983.

Section 2.2 presents the recommendations from the general meeting held on

August 2-3 and Section 2.3 discusses the recommendations from meeting held

on August 9-10 which focused on solar control subsystems.

2.2—Sffleral Meeting, Recommendations . (August. 2-3.r_J.983 ) Eighteen proposed research areas which were considered and ranked are

listed in Table 1. The prioritized list of research areas as developed by

the industry participants is shown in Table 2. Out of the eighteen

research areas evaluated, the top three ranked areas were: (1) installer

training? (2) designer training? and (3) packaging and installation

methods. As shown, industry participants believed these important areas

contribute to systems effectiveness and should be addressed by the solar

community. Energy distribution was the only other area which federal

government support was deemed to be inappropriate or not needed. A brief

summary of the key points relating to each research area is presented in

the following paragraphs.

Installer Training. The training of competent installers has been a major problem in the past. Poor or incorrect installation of various

subsystems, such as controls, has accounted for a significant loss in

performance and system failure. Improvement in this area has considerable

potential for enhancing field performance and a relatively high probability

of success. The cost is relatively small and the time for its effect to be

felt upon the industry should be quick. Although this area ranks as the

top priority, most major manufacturers train their own distributors and do

not feel that this is an area where DOE could contribute significantly.

5
Table 1. List of Proposed Research Areas

(Not prioritized)

1 . Absorber Development

- Selective Surface Materials

2. Glazing Development

- Thin Film

- Degradation of Polymers

- Reduction of Thermal Losses

3 . Controls

- Control Strategy Development

- Reliability

- Calibration Techniques

- Sensors

4 . Storage

- Liners

- Stratification Methods

- Phase Change

5 . Machines

- Absorption Chiller Development

- Desiccant System Development

6. Energy Distribution

- Heat Exchangers

- Piping - Leakage

- Paras itics - Punps/Fans

- Low Cost Piping

- Flexible Coupling, Sealants,

Expansion

7. Heat Transfer Fluids - Freeze Protection

- Air Vapor Blockage

8. Systems Performance

- Hot Water

- COoling

- Side by Side Comparisons

9. Modeling and Simulation

- Components

10. Design Tools

11. Test and Evaluation Procedures - Materials

- Systems

Plate Design

- Durability Under Impact - Anti-Reflective Coatings

- Variable Speed Pumps & Fans - Standardized Strategies - Small Programmable Controllers - Monitors

- Tank Durability

- Annual Cycle

- System Losses

- Rankine Development

- Heat Punps

- Flow Balancing - Ducts - Leakage

- Low Cost Ducting

- High Quality/Low Cost Air Flow

Controllers

- Corrosion

- Refrigerants

- Heating - Field Data Acquisition

- Load Profile

- Systems

Components

6
Table 1. List of Proposed Research Areas (CONTINUED)

(Not prioritized)

12. Designer Training Programs

13. Packaging and Installation Methods

14. Installer Training Programs

15. Consumer Awareness Programs

16. Insulation

- Collector

- Storage

17. Technology Transfer

18. Mirror Reflectivity/burability

Piping

7
Table 2. Industry Ranking of Systems Effectiveness Research Areas

Overall

Federal

Government

Weighted Support

Research ftrfifl Sgpr.e* &EB£gPliafc3.

1. Installer Training 161.1 No

2 . Designer Training 157.1 No

3. Packaging and Installation Methods 155.0 No

4. Controls 154.3 Yes

5 . Insulation 150.1 Yes

6 . Design Tools 150.0 Yes

7 . Systems Performance 148.4 Yes

8. Technology Transfer 145.2 Yes

9. Energy Distribution 143.8 No

10 . Absorber Development 140.9 Yes

11 . Storage 140.2 Yes

12. Modeling and Simulation 137.3 Yes

13. Test and Evaluation Procedures 135.5 Yes

14. Consumer Awareness 132.2 Yes

15. Heat Transfer Fluids 128.9 Yes

16. Mirror Reflectivity and Durability 126.8 Yes

17 . Glazing Development 114.9 Yes

18. Machines 94.0 Yes

* See Appendix A for the methodology used in developing these weighted

scores.

8
Designer Training. Another major contributor to poor performance of

installed systems has been the lack of experience or training of the

designers of the system. Often the projects were first efforts for the

designer and therefore mistakes were made. Although this applies primarily

to larger systems where a designer is more heavily involved, it has also

been experienced in smaller residential systems.

Industry has overcome much of the problem in residential applications

by going more toward packaged systems. Even space heating systems are

becoming more packaged. Commercial systems will continue to be a problem

as long as the number of jobs is few. Design courses are available

throughout the country but are not attended because of the lack of market

pressure. There is not any significant role for DOE to play in the

training of designers although the development of design tools and other

technical resource material for designers is appropriate as discussed

later. The solar community can assist when the need forces designers to

solicit help.

Packaging and Installation Methods. Installation has been problem in

active solar applications since the industry first began to develop. The

need to piece together complicated systems and the lack of trained

installers has resulted in many poorly functioning or non-functioning

systems in the field. In response to this problem, manufacturers have gone

to packaged designs and simple installation methods. This has been most

successful on small systems (domestic hot water and small space heating)

where close control over the product distribution network is exercised by

the manufacturer. This is an area which must be addressed by any solar

company that expects to be successful but does not require support from

DOE.

Controls. The control subsystem of a solar system include those

active and passive elements which govern the operation of the system.

Sensors, controllers, actuators, dampers, vents, drain valves, etc. are all

components of the controls subsystem. Virtually all of these components

have experienced problems in the past.

Sensors have had one major problem which is yet to be resolved—sensor

drift. The output of a sensor is not necessarily stable over time.

Therefore, measurements, and especially differential temperature

measurements, become inaccurate and result in improper system operation.

The lack of a reliable and accurate low flow, non restricting flow meter

has been a problem primarily for measuring system performance but this does

not currently play a significant role in most control systems. This may

change as more sophisticated variable flow control strategies are developed

in the future.

The controller itself has had failure problems due to manufacturing

and environment problems but these problems seem to be disappearing as

experience is gained. Better units are now available with much better

reliability. Back dampers, vents and other control devices have caused

problems but these are also being solved through the natural evolution of

the industry.

DOE could provide support in two major areas. First, recommended test

methods and evaluation procedures which would aid industry in developing

reliable controllers and sensors should be prepared. In this regard, other

9
agencies should be surveyed for related research. Second, DOE should

investigate the use of advanced control strategies to determine if

substantial gains in overal l HVAC and solar system performance could be

realized. This will include the use of variable collector flow to preserve

storage stratification, use of offpeak rates, and in general overall system

optimization. This work will probably not be pursued by industry. (Note: Additional detailed research recommendations on solar control subsystsns

are also presented in Section 2.3.)

Insulation. The long term integrity of insulating materials used in

solar systems, especially those exposed to higher temperatures, needs to be

determined and probably improved. However, this offers little potential

for cost reduction or performance improvement. If the demand is

sufficient, industry will carry out this role. Other technology activities

should be monitored by DOE to identify insulation advances. In addition,

compilation and dissemination of available information on recommended

insulation materials and installation practices for various applications

(collectors, piping, thermal storage) would be helpful to the solar

industry.

Design Tools. Many of the installed systons in the field today are

poorly designed. Poor collector area/storage volume ratios, pump sizes,

HVAC interface requirements, etc. have all led to inefficient operation.

Because of the trend toward packaged systems, much of the need for this

type of information has been alleviated. However, for field assembled

systems there still remains a need for better design guidelines.

With respect to sizing and performance analysis many companies develop

and disseminate design tools. DOE's most appropriate role should be the

validation of these design tools, which has not been completely

accomplished to date.

Systems Performance. Although system performance testing of advanced

concepts is extremely important, the monitoring of field systems seems to

be relatively non-productive. Not only are the monitoring programs

extremely expensive but the results have been of little use by

practitioners in improving cost or performance. Even when system

performance data is reported, the uncertainty and format of the data render

it virtually useless. In future field monitoring programs, more emphasis

should be placed on the performance of components and subsystems so that

manufacturers can determine those areas which work well. The possibility

of a manufacturer replicating a system configuration exactly as well as the

environmental conditions under which it operated are almost nonexistant and

therefore the overall system data are not as important as component or

subsystem data.

System and component testing should support the design tool

development and validation efforts, test and evaluation procedure

development activity, and modeling and simulation work. System testing

should not, however, generate performance data only for the sake of

creating the data but should be more strongly driven by the other research

activities. DOE should support this work, as needed since industry is not

typically in a position to do so. Technology Transfer. As noted in other areas previously discussed,

adequate information may exist but is not being successfully disseminated

10
to potential users. This, in general, is the case in active solar energy

applications. Tremendous quantities of information are generated annually

but fail to reach the designers, manufacturers, owners and installers in a meaningful format. Because of the current fragmented industry, this

function will not be performed outside of DOE. A concentrated effort in

gathering available technical information and disseminating it to the

industry is needed.

Energy Distribution. A problem which has degraded the potential

performance of active systems in the past has been the lack of adequate

integration with the conventional system. This includes the auxiliary

back-up subsystem as well as the distribution subsystem. Much of the

problem is due to poor design which should get better as more experience is

gained. Although problems do exist, DOE funded research is not justified.

Anticipated improvements will come from training, experience and technology

transfer.

Absorber Development. The performance of state-of-the-art absorbers

and selective surfaces is high and could be improved little. The cost and

durability of the absorber materials, however, must be improved but

industry will do this on their own. Turbulence enhancers and non-linear

a/e coating materials to avoid high stagnation temperatures are areas of

possible research but realistically offer little potential benefit.

Storage. Three fundamental problems appear to exist with storage:

durable liners for liquid tanks, heat loss from storage (air and liquid)

and improved stratification in liquid systems. Information on which liners

perform well needs to be disseminated based upon field performance. Better

techniques for insulating storage containers need to be developed. Often

the problem of excessive heat loss is the result of poor installation

practices. The improvement of stratification in tanks may require further

research although maximum use should first be made of available

information.

Model ing and Simulation. Few of the systems currently installed

require the use of detailed modeling and simulation. Most of the research

involving parameter and configuration studies for conventional solar

systems has been done and one can expect to gain little through additional

modeling. The exceptions to this are simulations which may be required to

develop improved design tools and to evaluate the performance of tested

systems for rating purposes. Work is done in this area will not be

performed by industry but might be performed by academicians without

significant support from DOE.

Test and Evaluation Procedures. With DOE support, considerable

progress has been made in the development of standard test methods and

evaluation procedures for various materials, components and solar hot water

systems. Several of these test methods are being used in industry rating

and certification programs. However, there is a need for converting the

results of the standard tests to long-term thermal performance estimates

and ratings. Because of the unique characteristics of different systems,

rating methods have been developed which are inappropriate for all system,

such as those which use noon instantaneous efficiencies or clear day data

only. Research is required to define improved rating procedures as wel 1 a. methods for converting test results into thermal performance estimai

Industry has not successfully accomplished this and may build in sysla;.

11
biases if left to their own. DOE should play an active role here in

cooperation with the various rating and certification groups.

Consumer Awareness. Although consumer awareness is a major factor in

the ultimate market success of solar products, it has little to do with

system effectiveness research. DOE should, however, recognize the adverse

effect that published poor performance results have had in the past. These

results are extrapolated by consumers to all solar systems thus making it

difficult for quality systems to penetrate the market. This is another

reason why the test programs should be carefully designed. Information on

systems performance should point out the successes as well as the problems.

Heat Transfer Fluids. The development of new fluids with all the

desirable characteristics required by indirect solar systems (i.e„ nontoxic, non-boiling or freezing, non-corrosive, etc.) would be helpful to

seme manufacturers but does not represent significant potential for cost or performance improvements.

Mirror Reflectivity and Durability. Due to the unique potential

advantages of systems using evacuated tube collectors with reflectors,

research on mirror materials, emphasizing reflectivity, durability and

cost, should be pursued. Because of the small size of the industry segment

manufacturing evacuated tube collectors, this type of basic materials

research will probably not be adequately performed without DOE support.

Glazing Development. Performance improvements due to glazing

development are doubtful for state-of-the-art flat plate collectors. The

development of low cost, thin film collectors was not considered here

because it was assumed to be a part of an advanced system development

program. The use of polymers rather than glass offers some potential for

improvements in cost, weight and safety but durability remains a major

problem. The small potential gains do not warrant an expensive polymer

development program.

Machines. The development of more cost effective and better

performing machines relates primarily to solar cooling. Because of the

extremely high cost and poor performance of these components, considerable

potential exists for improving the situation. However, it is doubtful that

changes to current cooling system machines will ever successfully reduce

cost sufficiently. A Second Law analysis has also indicated that bench

test performance of the single effect machines are as high as can be

expected, thus only field performance can be improved.

2.t3_.CQhtrpJ^subsy.stm.M.e!g.t.ing. (August, 9.-l.Q.«JL9fi31

2.3.

J

L. Pg&JLfimJttBaa

Control subsystem problem areas that inhibit the effectiveness of active

solar energy systems are listed and prioritized in Table 3. An indication

is also given as to whether the problem area should be addressed by DOE or manufacturers. DOE-sponsored controls research would help to:

1. ensure that satisfactory products and methods are developed; and

2. quantify the benefits of controller products (hardware and logic)

so that industry can make appropriate decisions regarding the

design and manufacture of controllers.

12
Table 3. Problem Areas for Active Solar Control Subsystems

Priority

Suggested Activity for

DOE Manufacturer

Problem Area or Solution Needed

1 # Control strategies to provide optimum

performance of system types (including

sensitivity analysis)

.

2 # More complete models and their

validation.

3 # Algorithms and implementations to

result in correct set point or mode

selection.

4 # System fault detection and indication

concepts.

5 # Recommended practices for testing and

installation.

6 # Sensors fail catastrophically.

7 # Sensors are incorrectly placed.

8 # Sensor calibrations drift.

9 # System integrators/installers do not

understand the operating benefits of

potential control functions. Need

education.

10 # Installation practices are poor.

11 # Customer Service.

12 # Strategies to meet consumer and

utility needs have not been

identified. Potential problem.

13 # Control system usability by

consumer-man/machine interface.

# Incorrect strategies.

# Controllers fail or cause failures.

# Controllers and related component

hardware are not designed for the

application - fail to follow good

design practice.

# Lack of standard specifications.

13
2.3.2 Objectives

Recommended objectives for a DOE-sponsored controls research program are as follows:

1. Develop control strategies and more accurate models for inproved

system performance.

2. Develop standard test methods and evaluation procedures for:

a. Controller hardware

b. Controller interfaces

3. Perform sensitivity studies to determine the effects of sensor

drift, sensor placement, etc.

4. Develop system fault detection and indication (hardware and

software)

.

5. Improve the reliability of sensors.

6. Improve the understanding of system integrators and installers.

7. Develop standard specifications for sensors and controllers.

1».3, 3 Status. ..of. Current- Res-^aish

A background paper on control research has been prepared and is included as Appendix B of this report. It includes a review of control theory, a

comprehensive literature review, and recommendations for further research.

The interested reader may wish to refer to appendix B for further details.

However, a concise review of current research activities pertinent to the

above-men

t

ioned objectives is contained in this section. These activities

address key areas, and are the basis for recommended future research

activities.

A. CONTROL STRATEGIES FOR IMPROVED PERFORMANCE

Recent simulation work conducted at the University of Wisconsin, Madison,

Wisconsin, has indicated that significant improvements in solar domestic

hot water (SDHW) system performance may be realized by decreasing the

collector flow rate in order to achieve a greater degree of thermal

stratification. The increase in collector efficiency due to lower

collector inlet temperatures more than offsets the decrease in the

collector heat removal factor due to the lower flow rates. Flow rates less

than one-fifth the normally recommended flow rates are being considered.

The lower flow rates may also result in lower parasitic losses.

The results of the simulation studies have been verified to a certain

extent by experiments performed at the SDHW test facility at MBS. During

tests of six independent SDHW systems, it was found that the highest

fractional energy savings were realized by a single-tank thernosypon direct

system and that the high perforamnce was due to low collector flow rates

14
that, in turn, resulted in thermal stratification of the storage medium.

This is very inportant work in that it indicates that the system designers,

manufacturers, and installers may not be using the best flow rates and, in

fact, are perhaps off by a factor of two or greater. When parasitic losses

are considered, the flow rates being used at present may be too high.

Although the results obtained so far are significant, tne work is not

complete. The optimal flow rates should be determined for a wide variety

of system types, both for space heating and SDHW systems and in various

locations. In particular, indirect systems should be analyzed, and

parasitic losses should be considered in all analyses. The effects of the

hot water load profile on the optimal flow rate should also be examined,

along with devices that promote stratification. It is inportant that this

area be investigated in more detail since the recommended flow rates are so

far different from those presently being used in typical installations.

As mentioned in the Appendix B, adaptive control has inportant applications

in solar heating when the building parameters are not accurately known.

Adaptive controllers utilizing real-time estimation techniques, although

not as common as the digital and analog controllers, have been developed

and evaluated in a commercial scale building. Further analysis and

modeling is needed to determine the cost effectiveness of the increased

performance of these controllers.

B. TEST METHODS FOR (DNfTROTI.FIRS

Recent tests performed at the Solar Energy Research Institute (SERI) have

revealed various problems with controller hardware. Sensors failed to meet

published specifications after going through characterization and

stagnation tests. At low temperatures, the responses of the thermistors

varied by as much as 4 degrees F. The self-heating of the thermistors

caused changes on the order of 2 degrees F. Hie quality of construction of

the thermistors caused certain types of thermistors to be seriously

affected by the stagnation tests.

Functional tests of the controllers showed deviations from the published

manufacturers' specifications. The differential temperatures for the

starting and stopping of the pump varied with the temperature of the

sensors. When the sum of the deviations from specifications for the

sensors and controller is considered, it is possible for the value of the

collected energy to be negative.

Hiese tests are inportant to the controller manufacturers. However, the

tests should be expanded to include more controller types and additional

environmental conditions. A review committee, comprised of manufacturers

and DOE/SERI/NBS personnel, should be formed to provide advice regarding

the tests to be performed.

In addition, sensitivity studies should be conducted in order to ascertain

the effects of sensor drift, sensor placement, and controller set points on

system performance for the types of systems presently being used. *In the

SERI tests, a failure was considered to have occurred if the sensor drifted

out of its specified accuracy. This may not have led to any degradation in

* One recently published study is "An Analysis of the Effects of Active

Solar Energy System Control Sensor Degradation on System Performance,"

R. B. Farrington, W. Short, SERI/TR-253-2185, July 1984.

15
system performance. For example, the two degree drift of sensors due to

the self-heating may not have an impact on the performance of the

controller if both the collector and the storage sensor have the same drift

rates (the absolute temperature measurement would be in error, but the

relative measurement between sensors would be within specifications).

Controller manufacturers vary in recommended sensor location. Location of

sensors and controller set points must be considered as a set. Collector

sensors are normally placed on the absorber plate or in the fluid return

immediately adjacent to the collector. Storage sensors are normally placed

near the bottom of the storage tank (either inside or out) or next to the

storage tank in the return fluid line to the collector. The sensor

response time for the various sensor locations, combined with the optimum

controller set points, needs to be reviewed to ensure proper performance

under real world environmental and material conditions.

C. FAULT DCTBgriQIj

Several manufacturers presently include monitoring and display systons with

their controllers. This is a marked improvement from the initial bang-bang

controllers which usually had a small LED to tell when the system was

collecting energy. The present day monitoring controllers still rely on

the thermistor to provide the technician with temperature readings for

system analysis. None of the existing controllers have the capability to

determine if the sensors have drifted since installation, a possible

problem identified by SERI.

There are controllers that can record maximum and minimum temperatures,

number of times the pump was turned on, measure the approximate heat output

of the solar system, and identify shorts or open thermistor circuits, lowflow or no flow conditions, reverse thermosyphon flow and low supply

voltage. These controllers are a step in the right direction toward builtin-test-equipment and fault-detection systems. However, these systems rely

on proper sensor installation and accurate sensor performance. Faultdetection systems need to be independent of the controller sensor and be

capable of verifying the performance of the controller.

D. MODELING

The prime method for evaluating various control strategies and determining

the accepted practices for controllers is the modeling of the system based

on real world examples. First, the component and the system models are

developed; the driving functions, control objectives and the control

execution are determined; and sensitivity of the model established by

computer simulation.

The model then needs to be verified and validated by testing at facilities

such as SERI, MBS or other qualified laboratories. Environmental limits of

operation could be determined that would assist in developing

specifications for the controllers and their sensors. Simulations could be

run that would assist in determining the validity of fault detection

concepts. Controller set points could be provided for the various sensor

locations.

16
2.3.4

Recommended Research Activities

Recommended research activities that are required in order to solve control

subsystem problems (Table 3) and address the objectives stated earlier are

listed below.

Task. 1, Control Strategies

1.1 Determine the optimal mass flow rates for solar space heating and

DHW systems.

1.2 Investigate the controller function based on stagnation

temperature (see Appendix B, p. 55).

1.3 Determine the effects of cycling on system performance (see

Appendix B, p. 56).

1.4 Determine partial excitation characteristics of pumps and blowers

in relation to proportional controllers (see Appendix B, p. 56).

1.5 Examine the performance of multi-rate circulators (see Appendix

B, p. 57).

1.6 Examine proportional, proportional-integral-differential (PID),

and adaptive controllers for the distribution of energy (see

Appendix B, p. 59).

Task 2. Test Methods

2.1 Develop standard test methods and evaluation procedures for

controller hardware.

2.2 Develop standard test methods and evaluation procedures for

controller interfaces.

TasX .3, . Sensitivity Studies

3.1 Determine system performance sensitivities to sensor drift.

3.2 Determine system performance sensitivities to sensor placement.

3.3 Determine system performance sensitivities to set point.

Task 4. Fault Detection Methods

4.1 Develop fault detection methods for anti-freeze systems.

4.2 Develop fault detection methods for water systems.

4.3 Develop fault detection methods for sensors.

3 . RESEARCH RECOMMENDATIONS FOR ADVANCED ACTIVE SOLAR ENERGY SYSTEMS

In a recent DOE Active Program Research Requirements (APRR) Study [2] a number of advanced heating and cooling systems were identified as pramisincj

concepts for the year 2000. These systons are listed in Table 4.

17
Table 4. Primary APRR Advanced Systems

1. Residential Direct Heating and Hot Water, Liquid

2. Residential Direct Heating and Hot water. Air

3. Residential Open Cycle Solid Desiccant Cooling, Heating, DHW

4. Commercial Solid Desiccant/Vapor Compression Hybrid Cooling and

Heating

5. Commercial Absorption Cooling

6. Commercial Rankine Power Generation and Cooling

7. Ground Coupled Solar Assisted Heat Pump Commuinity Heating (96 Apts.)

While the APRR study identified costs and performance of these systems,

reliability issues were not addressed. Accordingly, it is the purpose of

this section to identify research requirements for addressing potential

reliability problems with these advanced systems.

In many cases, these advanced systems will face the same reliability

problems as conventional systems. Since these problems are addressed

elsewhere, this section will focus only on new reliability issues unique to

the advanced systems. Each system will be discussed separately with both

system and component reliability issues addressed in each case. It should

be understood that since these advanced systems have not yet been built,

the identification of reliability issues involves a considerable amount of

hypothesis and speculation. Wherever possible, this speculation will be an

extrapolation of what we know about the reliability problems of existing

systems [3],

1.1 . Re?

This system consists of a closed collector piping loop with drainback to

the storage tank whenever the pump shuts off for freeze protection. A

significant feature of this system is the use of plastic pipe, chlorinated

poly (vinyl chloride) (CPVC) or polybutylene (PB), in place of more

expensive copper pipe. These plastics are typically limited to 200 F and

cannot be attached directly to a stagnating absorber plate. Usually acetal

or brass fittings are used to connect lengths of PB pipe while CPVC pipes

are typically joined by adhesives. The reliability of these joints is an important research area. Several different collector concepts have been proposed to lower costs.

Thin film collectors must be capable of withstanding stagnation

temperatures (as must any of the collector concepts) and must be able to

withstand the local steam pressure produced when the collector is filled.

For drainback systems not employing a trickle flow, the collector must be

able to withstand the system vapor pressure and the elevation head present

at the base of an array of collectors connected in series. Since a major

18
thrust is to use materials less expensive than copper, the corrosion of

these materials must be addressed. This is largely a systan reliability

concern since proper inhibitors must be enployed in the collector loop. For

collectors using plastic materials the long term effects of ultraviolet

radiation and high temperatures must still be addressed as well as

compatibility with corrosion inhibitors. Thermal stability of adhesives

and selective surfaces are also critical.

Several of the advanced concepts call for the use of a heat pipe heat

exchanger on the load (DHW) side to reduce pumping power. Since this is a new concept, the reliability of this component needs to be addressed.

Several of the liquid systems are to employ low cost storage tanks. A

drainback system need not employ a tank that can withstand 125 psig. A

lower pressure tank would need only to withstand about 15 psig in a closed

drainback configuration for a residential application and could potentially

be less expensive than a 125 psig tank. Whether such a tank would be as

reliable as conventional pressurized DHW tanks needs to be determined.

Plastic tanks would have problems similar to those of plastic piping.

3.2 Residential Direct Heating and Jfot_Water - Air

This advanced system would be a typical air heating system but would employ

a less expensive air collector, such as a fibre mat absorber with plastic

glazing. Although the biggest question about this type of collector is

performance, reliability issues associated with the details of assembly

need to be addressed.

As in the advanced liquid systems, a new heat pipe load side heat exchanger

warrants investigation. Other than the teat exchanger and collectors, this

air system is essentially conventional and one can expect the typical

reliability problems associated with air systems, e.g. leaks.

3-. 3_ Residential Open Cycle Solid Desiccant Cooling. Heating. DHW

This is an open cycle system of the Munters type which uses a desiccant

wheel to dry air which can then be evaporatively cooled. Even the

"conventional" version of this system is still experimental. The advanced

system would employ a parallel passage dehumidifier wheel. Reliability

issues include the maintaining of passage spacing with time and the long

term durability of the parallel passage base material (e.g.. Mylar) and the

desiccant material when exposed to temperature and humidity cycles.

Maintenance of the seal between supply and return air is also a critical

factor. Although a rotary heat exchanger and evaporative cooler are

conventional components, their maintenance in a residential systems

environment needs to be addressed.

Because this is a multi-component air system, leakage can be expected to

emerge as a major reliability concern. Also the control system in any

combined heating/cooling system is relatively complex and can be a source

of problems.

.1^.4- .Commercial Solid Desiccant/Vapor Compression

This system combines the dehumidification of a desiccant system and the

sensible cooling of a vapor compression subsystem. Potential reliability

19
problems for the desiccant subsystsn are similar to the previous system.

The addition of two more components to the air stream increases leakage

sites, and the control system is somewhat more complex. Research is needed

to develop low cost leak-free dampers and simplified controls. Since the

vapor compression unit is modified and not simply off-the-shelf, its

reliability would be in question.

1*5—Commercial Absorption Cooling

This system uses a solar-fired absorption chiller which contains an

integral gas backup boiler. Because of the high firing temperature of the

chiller, high performance collectors (e.g„ of the evacuated tube/CPC type)

are required. These are still in the development phase and durability of

the tubes and reflectors are serious questions. The integral absorption

chiller/gas boiler is new, especially when one considers that the chiller

design is thermodynamically advanced (e.g., double effect). Further

complication results from the need for a heat rejector in the collector

loop.

As in any absorption cooling system, but particularly for this advanced

design, controls can be expected to emerge as a major reliability concern. This system is extremely complex because of the effects of chilled water

and cooling water temperatures, interaction of gas backup, and use of

storage (hot side and/or cold side).

3.-6 Commercial Rankine Power Generation and Cooling

This system uses solar heat to run a Rankine cycle engine which drives the

compressor of a heat pump to supply cooling and also spins a

motor/generator to produce electricity. Both the Rankine and vapor

compression cycles would share the same evaporatively cooled condenser.

Like the absorption chiller system, the Rankine cycle system requires high

collector delivery temperatures. The SERI low cost parabolic trough

concept is considered the collector of choice. This collector is cabledriven on the rim, and its accuracy is affected by the durability of the

cable. Like any trough the reliability of the tracking/drive system is

critical and the reflector surface and substrate can degrade with time. If

the system is designed such that boiling takes place in the collectors, the

reliability of water quality control comes into question.

A major area of reliability concern is the Rankine cycle engine itself,

particularly the seals. Development of a long lasting turbo-compressor

will be needed for this type of system to work. As in the case of the

absorption chiller, control complexity is high, particularly if variable

speed pumps and fans are used.

3.J_ . .ground. Coupled Solar.Assisted. Heating

This system uses a dual source heat pump (low cost collectors and the

earth) for the heating of community buildings. An array of deep wells

containing U-type heat exchangers is employed, and the ground is used as

storage for seasonal carryover. Thin film plastic collectors open to the

atmosphere are used with a lined sheet metal tank for storage, plastic

piping, and drain-back protection.

20
Reliability issues are similar to those of liquid direct heating systems

with the additional complications of the heat pump and ground-coupling.

Ground heat exchangers have traditionally faced problems associated with

various soil conditions.

Degradation of the contact between the heat exchanger and the ground, and

freezing of the ground can have a significant impact on heat transfer.

Drying of the soil due either to thermal cycling or shielding of rainfall

by surface insulation can reduce performance. Underground leaks are also a

potential reliability issue. Since the heat exchange wells are covered by

insulation and the collector array, access for maintenance can be a problem. As in the case of the liquid direct heating system, reliability

of thin film plastic collectors is a serious issue.

4. SUMMARY

4.1 . ..St^t.grofr.tht-.Aft .Systems

Based on recommendations presented in Section 2, the following research

areas (listed in priority order) are recanmended for DOE support in order

to inprove the effectiveness of state-of-the-art active solar hot water and

space conditioning systems.

1. Control Subsystems. This includes investigation of control strategies,

development of test methods and determination of system performance

sensitivities and fault detection systems.

2. Materials . For various materials (e.g. insulation, piping, ducts,

tank liners, heat transfer fluids, reflectors, glazing, absorptive

coatings), research should be conducted to develop:

o Durability data under various in-use conditions;

o Test and evaluation procedures to aid in long-term performance

predictions; and

o Material selection guidelines to assist in choosing materials for

various system applications.

3. Systems Performance. In regard to the collection and evaluation of

field performance data, more emphasis should be placed on subsystems

or component performance, cost, reliabililty and maintainability.

Field and laboratory testing of systems should continue, as

appropriate, to support modeling and simulation, design tools, and

test and evaluation procedure development.

4. Technology Transfer. Available research data and information is not

being successfully disseminated to potential users. Accordingly, more

government effort is needed to gather available information and to

transmit it to industry in a useful and timely manner.

5. Design Tools. The development and validation of better design tools

and guidelines is another high priority area which should be

emphasized in the DOE research program.

21
4.2 Advanced Qystems

In regard to the advanced active solar energy systems presently being

studied by DOE (Section 3), the following research areas (not listed in

priority order) are recommended for DOE in order to address important

systems effectiveness factors (thermal performance, cost, and

durability/reliability)

.

1. Materials

o Plastic piping (including joints)

o Plastic glazing and absorber plates

o Dessicants (solid and liquid)

2. Components

o Thin film collectors

o High temperature collectors (including reflectors and tracking

devices)

o Heat exchangers

o Controls

o Open-cycle regenerator/collectors

o Absorption chillers/heat punps

o Dessicant chiller/dehumidifiers

5. REFERENCES

1. "Program Plan for Reliability and Maintainability in Active Solar

Heating and Cooling Systems," Mueller Associates, DOE/CS/3 60 10-01,

October 1980.

2. "Active Program Research Requirements (APRR)," Final Report, Vol. 1,

W. B. Scholten, J. H. Morehouse, Science Applications, Inc.,

DOE/SF/11573-T1 (Vol. 1), October 31, 1983.

3. "A Summary and Assessment of Historical Reliability and

Maintainability Data for Solar Hot Water and Space Conditioning

Systems," Gary J. Jorgenson, Solar Energy Research Institute,

SERI/TRl-253-2120, May 1984.

22
APPENDIX A

DEVELOPMENT OF A PRIORITIZATION MEffiCDOLOGY

FOR THE SYSTEM EFFECTIVENESS RESEARCH PROGRAM

AT TOE U.S. DEPARTMENT OF ENERGY

23
1.0 iotroduction

The System Effectiveness Research (SER) Program has emerged from an

earlier DOE program which dealt with reliability and maintainability of

solar systems. The new definition of SER, however, encompasses additional

factors which impact on a system's ability to provide cost effective

service. Figure 1 illustrates the many elements which combine to determine

the system effectiveness. The two key elements are thermal performance and

annualized cost. These two factors combine to determine the cost per

million BTU's the system realizes.

The goal of the SER program is to conduct research which will inprove

this cost effectiveness figure for solar systems. Because of the large

number of proposals for research which are encountered relative to a

limited budget, it is necessary to prioritize the areas of research based

on a rational set of criteria. This appendix describes a methodology for

accomplishing this prioritization and discusses its preliminary use during

the August 2-3, 1984 NBS meeting. It is suggested that the methodology be

refined and used in future planning cycles.

1*1.- Elementg_Qf. .. the. SER .gr.ogr.an Figure A-l suggests that there are two key factors to system

effectiveness; thermal performance and cost. Many elements comprise each

of these areas. To facilitate the evaluation process the elements under

thermal performance have been separated into two categories; those which

represent improvements to a state-of-the-art properly operating system and

those which represent improvements of field installed systems so they can

operate closer to the expected performance.

Annualized costs can also be thought of as being comprised of two

factors; those which reduce the cost of the system and those which extend

the life of the system. A discussion of each of these areas is presented

below in section 1.2.2.

1.2 Method for Prioritizing Research^Areas

Research, by definition, involves the acquisition and interpretation

of new knowledge. This suggests that the prioritization of research

activities will inherently be fraught with uncertainty of success and

benefit. However, this does not necessarily mean that the prioritization

cannot be undertaken systematically with bounds applied to at least the

potential benefit of each activity. The methodology developed for this

study seeks to establish these bounds and provide initial evaluation of

priorities. The methodology and conclusions can then be reviewed by other

researchers and managers to obtain consistently formatted input.

The methodology consists of a number of phases. The first deals with

the identification of factors which contributed to either a reduction of

thermal performance from an ideal system or the incurrence of a cost. As

shown in Figure A-l, these two areas are the elements which comprise the

"system effectiveness".

The second phase involves the establishment of bounds for improvements

in each of these areas. This provides an appreciation of the relative

24
SYSTEM

EFFECTIVENESS

THERMAL

PERFORMANCE

Sources of Degradation

• Parameters

. Nonideal design parameters

Installation effects on parameters

. Changes in material properties

• Nonoptimal Design

. Design tool error

. Wrong parameter values

. Site constraints

. Errors in weather data

. Control strategy

• Downtime

Component Failure

. System Replacement

• Inefficient Operation

. Leaks

. Control system malfunction

. Load profile

Sources of Performance Improvement

• Collector

. Absorber

. Glazing

• Back and edge coefficients

• Storage Losses

• Load Profile

• Control Strategy

Figure A-l. Factors in

SYSTEM

COSTS

Sources of Incurred Costs

• Component Materials

. Collectors

. Controls

. Storage

. Plumbing (pipes, valves, etc

. Pumps

. Heat exchangers

. Fluids

. Conversion Equipment

(heat pumps, chil lers)

. Support structure

. Ducting

• Overhead and Profit

• Installation

• System Design

• Maintenance (component life)

• Replacement (system life)

• Parasitics

Pumps

Fans

. Controls

. Other Equipment

System Effectiveness

25
potential for various activities. It is expected that these bounds will be

a function of application and system type.

The third phase involved subjecting each area to screening factors

which determine if work in this area falls into the general purview of the

DOE Solar Heat Technology System Effectiveness Program.

The fourth phase is the evaluation of each area of research which

passed the screening criteria with respect to a number of evaluation

criteria. The results of this evaluation can then be used to rank the

research activities.

1.2.1—Identification of Factors

Figure A-l illustrates the typical factors which affect the therm1

performance and annualized cost of an active solar system. Hie importance

of each of these factors varies from system to system and application to

application. It is recommended that the list shown in Figure 1 (page 2) be

made system specific for future planning cycles by consulting industry and

research personnel.

X±2l*2—Establishment of. Bounds

The lists in Figure A-l provide an overview of the factors which

affect cost and performance, but they do not indicate the potential gains

which could be achieved by performing research in any one of the areas. An

understanding of the limits of improvement for any one activity area allows

the development of rational priorities amongst activity areas. For

example, research on selective surfaces for flat plate absorbers might be a

very interesting area to fund research but if the research is directed

toward improving thermal performance then the limit to improvement over

current systems might be determined to be quite low.

It was not possible to establish these bounds for thermal performance

inprovement for all system types in such a short period of time. However,

what was determined was that for properly operating, state-of-the-art

active solar systems of most types and applications, the system efficiency

should be 35-45%. Field data of numerous sites indicates that installed

systems are realizing an efficiency of 0 to 41% with most below 25%. (See

Table A-l). This discrepancy indicates that substantial gains might be

possible in improving performance toward the expected performance. The

areas where performance degradation is taking place must be identified and

serve as a basis for future research prioritization.

In a manner similar to the breakdown of performance degradation

described above, the costs over the lifetime of a system should be broken

out to identify what costs could be reduced and by how much. If

distribution networks are imposing high overhead costs on solar systems,

then a simple reduction in collector costs may not accomplish proportional

reductions in system costs. Thus a thorough understanding of cost and

pricing structures within the solar industry should be prepared to better

prioritize projects aimed at cost reductions. Several studies (Booze

Allen, DHR) have performed such a study but these need to be updated and

performed for more system types to allow project prioritization.

26
Table A-l. Measured System Performance of NSDN Sites

(% Efficiency)

Space Heating & DHW

Liquid Ai_r DHW Space H&C/DHW

25 13 41 7

21 13 35 5

18 13 34 4

12 10 33 3

12 8 32 3

12 7 31 2

11 5 28 2

11 3 28 0

10 3 27 0

7 26 0

6 25 0

5 24 -1

5 24

2 23

22

21

13

13

13

12

4

AVERAGE 11 8 24 2

27
Projects aimed at extending the life of the system is an effective

method for improving the cost effectiveness of the systems. Component

reliabilities for field installed systems must be understood to base

prioritizations in future plans.

The thermal performance and cost data described above will serve as guidelines in developing research priorities. However, there are a

number of factors which require more subjective input. To accomplish this,

screening and evaluation criteria have been developed. The evaluation

criteria allow the integration of the quantitative performance and cost

data described above with the subjective factors such as probability of

success, etc. These criteria are described, in the following sections.

1.2.3 Screening Criteria.

A research activity undertaken by a program within the DOE should meet

general suitability requirements imposed by the stated mission of the

Department. The current administration has indicated that the DOE should

be involved in long-term, high risk research which would not be

accomplished by other organizations if not funded by the DOE. This broad

policy statement can be put into two "Screening Criteria" which must be

passed before an activity can be considered for funding. These screening

criteria are:

o High Risk/Long Term Payoff

o Not Performed by Industry or Elsewhere

These criteria are self explanatory and reflect the current

administration's policy.

1.2.4 Evaluation Criteria

Assuming that a number of research areas are identified which pass the

Screening Criteria it then becomes necessary to evaluate the relative worth

of each area to DOE in achieving its system effectiveness objectives. Hie

following Evaluation Criteria are recommended in this methodology:

o Potential Contribution Toward Achieving

Cost Reductions (5)

o Potential Contribution Toward Achieving

Performance Improvements (4)

o Probability of Success (3)

o Cost to Program (2)

o Time to Complete Research or Reach

First Major Milestones (3)

These criteria reflect the inportance of an activity area but t^iey ?re

not necessarily of equal importance. To account for this, weighting

factors are used to indicate the relative inportance of one criterion to

another. The weighting factors used in this evaluation are shewn above in

parenthesis.

28
To arrive at a total weighted score for a research area the criteria

score's (0 - 10) are multiplied by the weighting factors, and then the

products are sunned.

2.0 USE OF METHODOLOGY (AUGUST 2-3, 1983)

Major factors (thermal performance and cost) affecting the

effectiveness the various types of active solar energy systems was the

initial topic at the August 2-3, 1983 meeting. Although the pre-meeting

information package contained a preliminary quantification of: (1) various

sources which cause degradation or improvement in system performance and

(2) system costs, the industry participants believed the current data base

on system performance and costs was inadequate and could not be used at

this time for establishing research priorities.

The second item discussed was a list of preposed research areas which

would be considered for prioritization. The final list which was developed

by the participants contained 18 research areas (Table 1). Proposed

evaluation criteria and weighting factors for ranking the list of research

areas was the next item considered. After discussion, the proposed

evaluation criteria and weighting factors were revised. The revised values

are shown in Table A-2 and were used by the industry participants to

establish the prioritized list of research areas at the August 2-3, 1983

meeting (see Table 2).

Table A-2. Evaluation Criteria and Weighting Factors

Used At August 2-3, 1983 Meeting

Weighting

Evaluation Criteria Ractocs

1. Potential contribution toward achieving cost goals 4

2. Potential contribution toward achieving performance 4

goals

3. Probability of success 3

4. Cost of program 3

5. Criticality to technology success 4

6. Time to conplete research or reach first major 2

milestone

7. Durability and reliability 4

29
The following guidance was also given in determining criteria scores:

Criteria Number Comment

1,2,7 0 = Strong negative importance

5 = Neutral

10 = Strong positive inportance

3 10 = Strong positive inportance

0 * Poor probability

4 0 = High cost to DOE

10 = Low cost to DOE

5 0 = Not critical

10 = Critical

6 0 = Long time

10 = Short time

30
APPENDIX B

Solar Heating and Cooling

Control Subsystems

by

C. Byron Winn

and

Peter Armstrong

June 1983

31
Table of Contents

Page

1. Introduction 33

2. Review of Control Theory • ••• 34

3. Controls Literature Review 47

4. Recommendations for Further Research. ... 54

References 64

32
1 . INTRODUCTION

This paper has three main sections: "Review of Control Theory,"

"Literature Review," and "Recommendations for Further Research." The

objective of the "Theory" section is to provide readers with a common

knowledge base of control concepts and nomenclature. Some readers may skip

most of this section by quickly scanning it for underlined words and noting

their usage. The objective of the "Literature Review" section is to show

where research in solar heating and cooling controls now stands. While the

list of cited references is fairly comprehensive the discussion is necessarily limited, being mainly an effort to enumerate the control topics that

solar researchers have addressed and show how the work of different

researchers is related.

The objective of the "Further Research" section is to stimulate

thought and discussion. It is a frankly biased catalogue of promising

areas of controls research that would benefit the solar industry and, in

some cases, the electric utilities and other providers of back-up energy.

Readers are invited to suggest additional research topics.

33
2 . REVIEW OF CONTROL THEORY

The control of solar energy systems is quite similar to the control of

any dynamical system that is subjected to disturbances. A great deal of

work has been done in this area and it may be applied to controllers of

solar energy systems. Before examining specific types of controllers used

in solar energy systems we shall first review some basic control theories.

Consider first an example of a classical control problem as illustrated in

Figure 1.

The objective is to vary the energy input to the heater in order to

control the enclosure temperature T_. Usually the enclosure temperature is

Et controlled in order to maintain some desired temperature, T^. The deviation of T from T_ should be small regardless of the magnitude of disturh u bances, G (sunshine), T^ (outside temperature), and (wind speed). There

are several ways to do this.

The simplest form of control is " open loop 1

* control. In this case

the heater is energized on some schedule regardless of the current enclosure temperature. Obviously this type of control would not result in very

high comfort levels.

An improved type of open loop control is possible by measuring G, T»,

and V^. A predetermined function of these variables is then computed and

the furnace is energized if the function exceeds some predetermined threshold. This type of closed loop control will perform well if the function

and threshold are properly selected (not an easy task) and provided no disturbances (such as open windows) are introduced. Also the need to measure

these variables is a burden.

The controller is simplified and the comfort level is improved by

using " closed loop " control, as illustrated in Fig. 2.

34
Disturbances (TA , G ,VW , etc.)

Feedback

Fig. 2. Feedback control systems.

u

Fig. 3. Bang-bang control.

nnnnmm

u

To

TIME, t

Fig. 4. Time variation of temperature (bang-bang

controller).

35
Bang-Bang Control

In this case the thermostat serves as the first element of the controller and compares the actual room temperature, as sensed by the thermostat, with the desired room temperature, as set in the thermostat.

The final control element, the furnace, has only two "states' 1 - full

on or off - depending on the state - closed or open - of a switch (relay)

operated by the thermostat. When the temperature at the thermostat falls

below the desired temperature, the switch is closed, causing the furnace to

operate, and consequently increasing the room temperature Tc . When T_ h E

exceeds the switch is opened and the furnace is shut off.

This would also be an undesirable controller since it would lead to

rapid " cycling " (on and off) of the furnace. That is, the controller

would turn the furnace on whenever the enclosure temperature is sensed to

be less than the desired temperature, no matter how small the difference.

This could result in a rapid increase in T^, depending on the furnace output and the ambient conditions, such that T

£

exceeds T

Q

and the furnaoe is

turned off. This type of controller is referred to as a bang-bang controller. It has only the two states, on or off. It is represented in Figure 3. The building temperature response that would result from use of

this controller is illustrated in Figure 4.

The enclosure temperature is maintained at the desired value but at

the cost of excessive cycling of the furnace. The cycling rate may be

reduced by introducing a " deadband " into the controller. The effect of

the deadband is that the controller will not turn the furnace on until the

enclosure temperature drops below the " bottom of the deadband : " the controller will then keep the furnace on until T_ increases to beyond the

E " top of the deadband " , This is illustrated in Figure 5. The oontrol

36
Fig. 5. Time variation of temperature (bang-bang

controller with deadband).

u

u max“T

h

-DB

Fig. 6.

-< » i —

!

1 r

Tn'o ^ + ~ WO 2

Bang-bang control with deadband.

Fig. 8. Proportional controller (with saturation and

offset).

Fig. 9. Block diagram of PID controller.

37
function (often referred to as the "control law" ) is illustrated in Figure 6, and may be expressed analytically as

u =

(

U

MAX ' VT

D -

”DB/*^

( 0 , T

e-Td

> DB JZ ( 1 )

The bang-bang controller with deadband does not maintain as precise

control of T as does the bang-bang controller without deadband, but it

E

results in much less cycling. However, it also results in "overshoot;"

that is, the enclosure temperature will exceed the top of the deadband and

fall below the bottom of the deadband. Normally this is of little consequence in building heating systems. However, with the advent of buildings

having large " thermal capacitances ." the problem of overshoot becomes

more significant. Cycling and "

o

vershoot " are characteristics of bangbang controllers, regardless of the system being controlled.

Proportional Control

The overshoot may be eliminated by using " proportional control ." In

the case of proportional control, the output of the furnace is proportional

to the difference between the enclosure temperature and the desired temperature. Thus, if the room temperature is very low, the furnace will work

hard to overcome the temperature error. However, as T_ approaches T_, the

E U

output of the furnace will decrease, thereby preventing overshoot from

occurring to the extent it occurs with a bang-bang controller. The control

law for the proportional controller may be written as

and is illustrated in Fig. 7. The slope of the line, k , is referred to as

( 2 )

P

the controller gain

The principal disadvantage of the proportional controller, as

38
illustrated, is that it would cause the furnace to operate continuously as

long as T is less than T . This problem can be avoided by including an

" offset ,

as illustrated in Fig. 8. In this case, the controller will

not turn the furnace on until T^, is less than by the amount OS. The

control is proportional to the magnitude of the temperature error,

'ED I T -T_ I,1 for TED-T_ < -OS. It then remains constant at the value Umm

until T_ exceeds T_,. There will still be some overshoot with this conE D

troller

.

Integral Control

An additional type of control that may be used is " integral control . " An integral controller is designed to increase the controller output (the furnace output in our example) in proportion to the time integral

of the error. That is, the longer the enclosure temperature remains below

the desired temperature, the more the furnace output will be increased.

This is represented analytically as

« " k

I E

(t) -Vt)]dt

<3

o

An integral controller does not result in a fast response, but does have a

stabilizing effect on system response.

Derivative Control

If quicker responses are desired, then " derivative control " may be

used. A derivative controller leads to a control output that is proportional to the rate of change of the error. That is

U k

D dt (TE

T

D

) * (4)

Thus, the controller output increases as the time rate of change of the

error increases. Derivative controllers lead to quick responses, but tend

to be unstabilizing and are strongly affected by noisy signals.

39
PIP Controllers

Hie above control types may be combined to result in a " proportional-integral-differential controller , " known as a PID controller. The control law is expressed as

A PID controller can potentially reduce energy consumption in three

ways. First, it reduces overshoot in heating the space (an improved method

of thermostat "anticipation.") Second, if it is desired to implement a

proportional actuator, the PID controller could increase the average steady

state efficiency of a given size combustion heat exchanger by operating the

burner at less than nominal rating most of the time. And third, the PID

controller may increase transient efficiency by reducing on-off cycling and

reduce flue losses by causing the heat exchanger to be at a temperature

that is well below the nominal design temperature at the end of each "on"

cycle

.

Using the concept of a transfer function, which is defined as the

ratio between the output and the input, both expressed in the frequency

domain, the equation for the PID controller may be written as

The " transfer function " is shown in block diagram fora in Figure 9.

Design of Controllers

There are two basic approaches to the design of controllers. The

( 5 ) o

( 6 )

first is referred to as trial and error, while the second is referred to as

the analytical design approach. These are briefly described below.

40
Trial-and-Error Design

The ** trial and error method " is illustrated in Figure 10. The tr:

and error method is the method that is most often used in the design of

control systems. This is further illustrated by Figure 11, which depicts

unity feedback control system. In this figure, the output from the system,

c(t), represents the controlled output. The desired response is

represented by the input to the block diagram and denoted as r(t). The

controller is a proportional feedback controller and is represented by the

gain, k , acting on the error signal, e(t). The desired response could be

P

interpreted as the enclosure temperature in our example. In the classical

trial and error design procedure, one would select a set of performance

specifications and then attempt to adjust the controller gain so that the

performance specifications would be satisfied. A typical set of performance specifications relative to a step input to a second order system is

illustrated in Figure 12. For the building heating problem discussed previously, one would be primarily concerned with overshoot and final error.

The trial and error design process tends to be inefficient and expensive

and, when one selects a set of performance specifications, one has no way

of knowing whether or not those performance specifications can be satisfied.

Some general guidelines, drawn from experience, can ameliorate these

problems somewhat by suggesting a reasonable initial control strategy.

Design Guidelines

Almost all the temperature control problems in solar applications arc

heat transfer problems, and are characterized by long time constants and

slow reaction rates. Distance-velocity lag (also known as dead tine) is

common. The measurement lag can pose a serious problem. The measurement

time constant depends on the mass and surface area of the sensor or ; r >>c

41
Fig. 10. Trial and error design procedure.

42
io—1—[ZD

Fig. 11. Block diagram of a •'classical" proportional

controller.

43

Final

Value

of

Error
assembly, the fluid being measured, and its velocity past the probe.

Processes dominated by one large capacity, for example, storage tanks

in air heating systems, can be controlled with on-off controllers.

Proportional-plus-reset control is used in smaller capacity systems

where load changes are large and where distance-velocity and measurement

lags are important. Most hydronic collectors and shell and tube heat

exchangers fall into this category. Derivative control becomes helpful

provided the distance—velocity lag is not the dominant secondary dynamic

element, for example, as with an air heating collector.

Controlling temperature by mixing hot and cold streams is more nearly

a blending problem than a heat exchange problem. Good mixing and fast temperature measurement are the keys to simplifying the control job.

Proportional-plus-integral controllers should be used.

In general, the following guidelines can be used in selecting controllers. Use proportional control where

o the cycling action, due to on-off control, is undesirable;

o set-point changes are small or infrequent; and

o the steady-state deviation between the set point and the process

variable, that is, the offset, can be tolerated.

Use integral control where

o the offset must be reduced or eliminated, and

o the set point changes are frequent.

Do not add integral control when

o start-up overshoot must be eliminated, and

o the process can be controlled with high-gain proportional

control.

Add derivative action to proportional control when

o the distance-velocity lag (for example, dead time in the pipes) is

smaller than either of at least two linear lags (for example, storage

tanks) in the process loop.

44
Do not use derivative control if

o the distance-velocity lag is significant;

o the process is noisy.

It is not at all uncommon for considerable amounts of money to be

spent on computer simulations in attempting to select a set of gains for

selected controllers in order to meet a set of performance specifications.

It is often more efficient to use an analytical design process in which the

guesswork is eliminated and where one can determine directly the control

strategy that will result in optimizing system performance.

Analytical Design

In applying the " anal ytical design ** process, one selects a cost

function to be optimized. Hence, it is often referred to as " optimal control . "

A typical formulation of the optimal control problem is the " 1 inear

re gulator " problem. This type of control is applicable to situations

where the objective is to bring a control variable, such as (T - T) , to

L U

A linear regulator for this application could be designed to minimize

the weighted sum of the root-mean-square deviation from the set point and

the mean energy input:

where the deviation from the set point, E(t), is related to the controlled

energy input, U(t) (and to other factors, such as weather and occupant

behavior, which are both treated as noise) by a set of differential equations that governs the dynamic behavior of the heated space and backup

heating plant. The chief parameters of concern in these governing cquazero

dt ( 7 )

45
tions are the room parameters, the resistance to heat loss (R) and the

thermal storage capacitance (C)

.

One of the issues raised by the integration of backup heaters with

solar heated buildings is the interaction of controls. In particular, passive solar heated buildings present a number of problems. First, the " time constants " (T = RC) of rooms in such buildings are usually much

longer than in conventional buildings because both R and C are much

greater. The thermal capacitance effect alone has a major significance:

any overshoot of the setpoint by the backup heater controller represents a

relatively large quantity of energy absorbed by thermal capacitance elements. This overshoot will then persist (decay very slowly) because of the

large time constant of the heated space. It is therefore apparent that

significant energy savings can be realized by reducing overshoot and by

reducing thermostat deadbands.

The quadratic criterion represented by equation (7) helps to reduce

overshoot because increasing deviations, E(t), are penalized with progressively increasing severity. The solution to (7) will also yield a control

law with small final error because the first term penalizes deviations of

the set point deviation. Since these penalties are integrated over time

the resulting control law will involve some integral feedback which will

tend to reduce final error. Another useful feature of the optimal regulator is reduced peak demand. The quadratic penalty on U(t) tends to limit

parasitic or auxiliary power.

46
3 . CONTROLS LITERATURE REVIEW

Some of the major review papers on solar energy controls include:

Kutscher et. al. (1982) and Su and Castle (1979) both emphasizing solar IPH

control; Winn (1982) covering mainly residential systems, the determination

of cycling rates for on/off controllers and control laws for proportionalonly control of collector circulation based on the Pontryagin maximum principle; Kent, et. al. (1980) on residential DHW controls; and Dorato (1979)

on application of on/off singular linear quadratic and periodic optimal

control theory to solar energy systems. Control and measurement references, germane to, but not dealing specifically with, solar energy applications include Tierstein (1978), Roots (1969), Johnson (1964) and Shinsky

(1974)

.

Selection of offset and deadband parameters (i.e. (AT- it and AT^^)

ON OFF

for differential, on/off control of collector blowers or pumps is discussed

by Davis (1975), Schiller, et al. (1980), Duffie and Beckman (1980), Alcone

(1981) and Winn (1982). On/off control strategies are compared to proportional control strategies using simulation by Herczfeld and Fischl (1977)

and Lewis and Carr (1978) and Schiller et al. (1980) and in field tests by

Schlesinger (1978).

Some of the problems encountered with complex solar heating and cooling systems controlled by multi-function set point and differential on/off

controllers are discussed by Guynes (1978) and Karaii and Armstrong (1978).

The simulations of systems with such complex controls are discussed by

Orbach et. al. (1978), by Armstrong and Bechtel (1979) and by Piessens et.

al. (1981). Design of and operating experience with a hardware programmable (not microprocessor based) controller used to "breadboard" complex

control logic based on set point and differential comparisons has been

reported by Bechtel et. al. (1979).

47
One of the earliest papers on optimal utilization of solar energy in

space heating and cooling applications was by Winn, Johnson and Moore

(1974) . A control-theoretic approach was used to examine the problem of

minimizing the use of auxiliary energy required for space heating and cooling. It was shown that the auxiliary heat supplied to a solar heated house

may be minimized by the use of a proportional plus integral state feedback

controller. It was demonstrated that the controller that leads to minimizing the auxiliary fuel may be obtained from an application of optimal regulator theory. The possibility of implementing the optimal controller on a

microprocessor was also discussed. The gains for the optimal feedback controller must be determined by solving a matrix Riccati equation, which

proves to be a nontrivial problem. This situation was examined in more

detail (Johnson and Winn, 1975) in order to develop efficient algorithms

that may be used for efficient management of a building's energy budget and

that may be implemented on a microprocessor. In a simplified analysis of

the problem of controlling temperatures in buildings (Winn and Hull, 1978)

a comparison between several different heuristic control strategies and an

optimal control strategy was presented. The optimal control strategy that

minimizes the integral over time of the sum of the building energy requirements and a measure of occupant discomfort was developed and it was shown

that energy savings could be realized through implementation of the optimal

control strategy. The analysis was extended (Winn and Hull, 1976) to

include sensitivity studies and was further extended (Winn and Hull, 1977)

to include analysis of the singular control problem.

An early study relating to the impact of solar systems on utilities

(Soot, Goldbach, and Winn, 1975) examined the problem of solar domestic

water heating in the northwestern part of the United States and the poten48
tial impact on Pacific Power and Light Company.

The above-mentioned control studies dealt primarily with the problem

of controlling the solar heating or cooling system from the point of view

of minimizing fossil fuel consumption and maintaining occupant comfort.

The principal analytical tool that was used in these studies was that of

simulation.

Winn (1982) compares the process of selecting controller parameters by

trial and error (using simulation to evaluate performance) with penalty

function techniques of analytical control synthesis. For example, Johnson

(1964) describes a general technique for determining on/off control parameters using a quadratic penalty function. Somasundarum et. al. (1978) made

some simplifications to Johnson's approach to obtaining an explicit solution for control of a large solar heating and cooling plant. One of the

first attempts to apply the penalty function approach to solar heating systems was by Kovarik and Lesse (1976) . Other application studies were by

Johnson and Winn (1975), Auslander, et. al. (1979), Blodgett, et al.

(1978a) and Winn and Hull (1978). One of the problems with the optimal

control approach, pointed out by Auslander et. al. (1979) and Blodgett et.

al. (1978b) is the effect of errors in plant parameter estimates on controller performance. The solution to this problem, adaptive control, is

discussed later in this review.

Another problem that must be faced regardless of whether the control

parameters are selected by trial and error or analytical optimization is

simplification in the plant model. For example Schindwolf (1981) shows how

to select collector loop flow control parameters based on a lumped capacitance collector model. Orbach et. al. (1981) however show that control

parameters derived under the assumption of a lumped capacitance collector

49
may be considerably in error. Wright (1981) presents a detailed method for

determining control parameters based on a distributed capacitance collector

model. Lumped load models, on the other hand, have been found satisfactory, e.g. Winn and Hull (1978, 1979) and Rink (1981). However, for the

control of auxiliary heat for rooms in passive solar heated buildings, distributed capacitance models may be needed (Winn, 1982).

Some controllers have been built to implement a PID optimal regulator

control law. These controllers were installed and tested as reported by

Eisenberg et. al. (1977) and by Winn and Hull (1977).

The utility load factor problem was addressed by Winn and Duong,

(1977). An optimal control strategy was formulated to reduce the amount of

auxiliary energy used and to avoid peak load hours for solar structures.

The proposed strategy consisted of three interrelated operations. First, a

short-term weather forecasting model based on the Kalman filtering approach

was used to obtain the predicted values of the weather parameters (solar

radiation, wind speed and ambient temperature). The amount of auxiliary

electric energy required to be stored in order to satisfy the predicted

heating load requirement for the next day was then determined from the

estimated amount of useful solar energy collected, the estimated heating

load, the amount of energy available in the storage tank, and the expected

losses by heat transfer processes. Finally, a linear programming scheme

was used to determine the optimal electric energy consumption sequence and

a self-tuning regulator was used to minimize the occupant discomfort.

Additional papers that dealt with this problem were (Winn and Duong, 1978),

(Lorsch, Oswald and Crane, 1978), (Debs, 1978), (Moe, 1978), (Eltimsahy and

Santos, 1977), (Debs, 1979), (Dorato, 1979) (Sebald, et.al., 1980), and

(Winn, 1982). Weather prediction models suitable for implementation in

50
microprocessor based controllers have been studied by Sinha and Sharma

(1975) and by Hamlen and Hamlen (1976).

The studies mentioned above dealt primarily with solar beating and

cooling systems and only secondarily, in some instances, with off-peak

storage type devices. The possibility of using beat or cool storage and

off-peak electricity for charging of tbe storage bas not escaped consideration. A Luenberger observer was developed (Hays, Parkinson and Winn, 1979)

for accurately predicting ambient temperatures for up to 24 hours in

advance and this bas been built into tbe controller for an off-peak beat

storage device and operated in one of tbe Colorado State University solar

bouses. Some of tbe hardware and software details for this device were

presented in (Winn and Robinson, 1979) . Additional energy storage systems

were discussed in (Kalhammer, 1979), (Lorscb, 1980), (Winn and Winn, 1982)

and (Winn et. al., 1982).

In addition to tbe controllers, one must have an efficient mechanical

system design, and efficient energy conversion hardware. There have been

many papers written relative to tbe subject of efficient energy conversion

equipment. In (Grot, 1975) the feasibility of combining various appliances

that are now constructed separately into integral assemblies which would

permit more efficient energy design, utilizing waste beat and minimizing

tbe impact of appliance operation on heating and cooling systems, was considered. Single zone, single zone variable volume, multi-zone, dual-duct,

terminal unit, etc. systems are described in (Sbib, 1976) and tbe interrelationships between tbe fan systems / bydronic systems, the energy convert

ing machines and tbe building are presented. Papers dealing specif icali

with tbe hardware aspects of controllers in building climate control sys

terns include (McNall, 1976) and (Baitz, 1976) in which recommendations f r

51
government sponsored projects of R and D for energy conservation are

presented. Also, in (Vanderweil, 1976) a review of the different types of

HVAC systems, including fan coils, unit ventilators, closet units, induction units, self-contained units and incremental heat pumps was presented.

Baumann (1979, 1981) describes how pump and valve characteristics affect

flow control techniques in fluid loops. Another paper (Gajjar, 1976)

described low cost techniques for monitoring and utilization of energy in

rooms and zones of structures, and using these techniques to automatically

control the temperature of the zones to effect a savings in energy. Heating system controller parameters that influence system efficiency and

operating costs and that can be field adjusted, but for which no criteria

for optimal adjustment have been developed, were discussed in (Bonne, et.

al. 1976). In addition both retrofit and new equipment aspects were

covered, together with a breakdown of the operating costs and savings due

to changes in fuel and electricity consumption. In another paper (Jones,

et. al., 1976) a two stage analysis was conducted in which the energy

requirements for an existing office building were examined in detail and

the most promising opportunities for conservation were identified and then

the potential impacts of the various possible conservation measures were

evaluated. In another paper (Mrnka, 1976) various types of controllers

were analyzed and their characteristics were described. Enthalpy controllers, anticipatory controllers and olfactory controllers were discussed. An

examination of sensor requirements for building control and energy management was presented in (Johnson, 1976) . An analysis of the requirements for

on-line management and supervisory strategies to control the interaction

between system elements in HVAC systems was presented in (Anderson, 1976)

.

Performance characteristics and relative costs of control elements such as

52
dampers, valves for liquid and vapor, switches, relays, pneumatic and electric evacuators, and communication network requirements were discussed in

(McNall and Buchanan, 1976)

.

As mentioned earlier, adaptive control may have important applications

in solar heating and cooling because the plant parameters are not always

accurately known. The parameter estimation techniques necessary to implement adaptive control are discussed by Pryor et. al. (1980) and Pryor and

Winn (1982). Adaptive controllers utilizing such real-time estimation

techniques, and computing in real time the parameters necessary to implement an optimal regulator control were developed by Farris and Melsa (1978)

and MacDonald et. al. (1978) and evaluated after implementation in a commercial scale building by Farris and Melsa (1980).

Designs of controllers for industrial process heat applications have

been reported by Su and Castle (1979) and Gerwin (1980) . Flow control for

a high temperature (central receiver) system that could also be applied to

low and medium temperature collectors is analyzed by De Rocher et. al.

(1982).

Hardware designs for microprocessor based controllers are reported by

Johnson and Winn (1975) and by Kent et. al. (1978) and their implementation

and testing are reported by Winn and Hull (1977), Eisenberg et. al. (1977),

Moen and Lane (1978) and Hays et. al. (1978).

53
4. RECOMMENDATIONS FOR FURTHER RESEARCH

From the foregoing literature review a number of areas for further

research have been identified. Some of the suggested research involves

taking previous research topics to their logical completion. In these

areas the tasks are fairly well defined. Other topics involve inquiries

where the magnitude of improvements in cost or performance are harder to

guess in advance. These areas of research will necessarily require of

investigators a desire to embark on an uncharted course, defining new tasks

and abandoning unpromising control concepts or analytical methods as the

work progresses.

Research on Controls for Collecting Solar Energy

Much research has focused on preventing cycling and maximizing solar

energy collected or maximizing the difference between solar energy collected and parasitic energy.

Bang-Bang Controls

Davis (1975) gave an expression for differential control set points

that maximized the duration of collector operation subject to no ON/OFF

cycling under steady state conditions:

In the foregoing expressions AT^pp i s usually determined from the generally

Alcone (1981) corrected the expression for heat removal factor:

^collected

accepted constraint, COP = pumping

>_ 1, which implies:

54
AT > OFF ~ qpC

P

where pC is the heat capacity per unit volume of the collector fluid, Ap

P

is the collector loop friction loss and i\ is the pump efficiency. Obviously the collected energy is maximized where there is no slack in the

above constraints (i.e. they become equality constraints). In hydronic

systems AT is often on the order of 0.5°C. This is as great or greater OFF

than the error bounds typically exhibited by thermistor temperature measuring devices. The effect of errors in determining whether or not the collector temperature rise exceeds the desired ATrtT?r, (e.g. probe error,

thermistor/resistor bridge (mismatch) errors) has never been properly

assessed.

Lunde has suggested a method for mitigating these errors by using

stagnation temperature instead of collector outlet temperature. With this

scheme the COP = 1 constraint gives the same expression for AT

Q

^:

• AT m/C

___Q.fi- v e_

AP /„Pc

p

- F^A

c D

l

In principle one can make AT... - ATrt TM arbitrarily small since (in contrast

to collector outlet temperature) the stagnation temperature is not atfected

by the pump state (ON or OFF). In practice, however, there is some thermal

coupling between the collector fluid and the stagnation temperature sensor

(i.e. the sensor suggested by Lunde actually measures some weighted average

collector outlet and stagnation temperature). Although the effect of a

AT , - AT^„„ of 1°C or so should be small it has never been properly ON OFF

assessed. It will also be necessary to conduct field tests of this

improved control strategy before it will be accepted by the industry.

55
Winn (1982) has shown that the above constraint on AT__,/AT-_ is con- ON OFF

servative because some cycling is permissible (also inevitable since collectors do not operate in steady state) and because thermal capacitance

effects result in rather slow cycling when it does occur. Winn derives a

cycling time of

9(t) = ?

m

mC /A )ATnM p c ON

G( t) (ta) - U

L

(T

s (t) - T

A

(t))

f

r

+ 1

Thus the number of pump cycles per day is:

f

£ N = / e(t) dt

J t o

Assuming constant T and T and that

s a G(t) = G

?i Sin(^ t), 6:00 = t < t < t f = 18:00

the expression for N may be evaluated analytically to give N as a function

of G„ and AT..,, A suitable value of AT.... that is less than the value given M ON ON

by AT_ X7 /Art „„ may then be selected.

ON OFF

The effect of such reductions in AT^ on performance and cost should

be evaluated in further research. A set of charts for specifying AT^

might also be useful.

Proportional and PIP Control

Proportional control is used much less than ON/OFF control in controlling collector blowers and pumps. This is due mainly to lack of information about efficiencies of pumps with motors run on less than rated excitation and thus to uncertainty that parasitic power can be much reduced.

Work is proceeding at SERI into this and the related problem of controls

and pumps for closed drain down systems where initial head is much greater

than steady state head. It will be useful if the partial excitation

56
characteristics of a number of pumps and blowers are characterized in this

work.

A control scheme that combines some of the features of proportional

control and of bang-bang control involves the use of multi—rate circulators. A multi-speed blower controlled by a microprocessor programmed to

maximize the difference between collected and parasitic energy was tested

in C. S.U. Solar House 2 by Hays et. al. (1979). In hydronic systems it is

practical to run two or more pumps in series or parallel in order to

achieve the range of desired flow rates. (This scheme provides additional

protection against collector stagnation due to pump failure as well.) A

relationship for specifying differential control thresholds for multi-rate

circulators based on the steady state criterion for no cycling was derived

by Armstrong (1979)

:

AT

AT > m . J

m . C

J-P1-exp (-7

A D . ,

.

Vicp

where m^ is the mass flow rate with the multi-rate circulator operating at

til

* its j rate, m_. is the next higher available rate, and the A U., J+l c j

j=0,l,..., are the overall collector loss conductances (corresponding to

F'A UT for a collector loop with just one flow rate) at the various flow

C L

rates, m^ . The thresholds for transition to lower circulation rates,

AT. , come from the COP criterion that is used to specify AT for the

J J (Jr 1 simple bang-bang control, thus:

Ap,

AT. i±L

»j+1PC

p

Note that this accounts for the nonlinear relation between pumping power

57
and flow rate which unadorned proportional control does not.

A proper comparison of the costs of and long term performance resulting from use of bang-bang, proportional, and multi-rate controls for collecting solar energy should include analysis of systems with a range of

collector, heat exchanger and storage parameters, and systems operating in

a range of different climates. Current circulator efficiency curves, especially germane to proportional control, should be used as well as hypothetical efficiency curves that can be anticipated with improved motors and

motor speed controllers. Variable speed control involving a nonlinear

relation between AT and pump excitation power should be analyzed as well.

A related topic that has received virtually no attention is the use of

a PID input stage in a bang-bang or multi-rate output controller.

Winn and Winn (1981) have derived the optimal control that maximizes

the difference between collected energy and parasitic energy when parasitic

power is not directly proportional to m:

t f

J = f (

C

1 Q - P)dt

1 u

o

d where P = C^m .

Using an approximation for F

K

F'

. A UT

i , 2 c L

2mC

the resulting optimal control is:

C.fF'

2

A UT 1 c L

2aC.C

2 P

1/a+l

where f, the available energy, is zero if (Tos- T ) is negative or, if (T

o - T

s

) is positive, determined from either

58
f =. mCp (T

0 - T

s )/Fr

if the collector fluid mover is on, or

f = A U.(T - T ) c L o s

if the fluid mover is off.

This control is optimal for mixed storage systems. Note that can

reflect the ratio of pumping energy to collected energy values. This may

be useful if time-of-day rates apply to parasitic energy.

Recent work conducted at the University of Wisconsin, Madison, has

indicated that significant improvements in SDHW system performance can be

realized by decreasing the collector flow rate in order to achieve a

greater degree of thermal stratification. The increase in collector efficiency due to lower collector inlet temperatures more than offsets the

decrease in the collector heat removal factor due to the lower flow rates.

Flow rates less than one-fifth the normally recommended flow rates are

being considered. The lower flow rates also result in lower parasitic

losses. This area should be investigated in more detail since the recommended flow rates are so far different from those being used in typical

installations at this time.

Research on Controls for Distributing Energy to the Load

In active space heating systems, Winn and Hull (1979a, 1979b, 1979c)

have shown that auxiliary energy use can be reduced simply by reducing

overshoot in control of temperatures in the heated space. Optimal PID control laws were used to obtain the control improvements over conventional

bang-bang controls. Even greater reductions in auxiliary energy can be

expected from auxiliary control improvements in passive buildings because

of the larger thermal capacitances and longer time constants. One problem

59
in developing an optimal control law, however, is the variability of time

constants in different heating zones due to the use of moveable insulation.

This may be a promising application for adaptive control. The use of

parameter estimation to implement such adaptive control should be investigated and the resulting effects on auxiliary energy use, peak demand, and

overall heating costs in passive buildings should be evaluated.

Reduction of high coincident demand for auxiliary energy that conventional SHACS impose on utilities has been studied by Winn et. al. (1982).

One result of this study was an optimal off-peak storage control law that

was then compared, by simulation of the solar space heating system, to a

conventional control law and to a suboptimal control law. The resulting

solar load fractions, f, and total annual costs of supply, c, were:

Conventional Suboptimal Optimal

(no off-peak) off-peak control off-peak control

f .51 .52 .49

c $2,932 $2,749 $2,596

The cost reductions were due mainly to reduced utility capacity costs (i.e.

end user demand charges).

Further investigation of the optimal off-peak control is needed

because it assumes that the total load and solar energy collected over the

on-peak period are known in advance. This typically requires a 12 hour

weather prediction. An evaluation of the effect of weather forecast errors

should be made.

60
System Simulation Studies

At the present time there is a need for a simulation program that is

useful in studying control strategies for solar heating and cooling systems. The TRNSYS program is perhaps the most widely used solar system

simulation program at this time but, although a controller subroutine has

been added to the program, it is not sufficiently general and versatile to

allow for the study of numerous control strategies of interest. For example, proportional controllers, PID controllers, adaptive controllers, etc.

cannot readily be examined using the present TRNSYS program. SIMSHAC, a

simulation code developed in 1973-74, was developed for the purpose of conducting control studies, but has not been updated since 1978. EMPSS and

EMPS-2 were recently developed to study the impacts of solar heating and

cooling systems on the electric utilities, but these programs require

modifications to examine controllers. DOE-2 can be used for limited controls studies but is also neither specific enough, nor general enough, to

be very useful in studies of controllers.

It is recommended that one of the existing codes be modified to the

extent that it may be used in controls studies so that improved strategies

may be investigated. It is further recommended that experiments be performed in an existing well-instrumented systems test facility so that the

code may be validated and various control strategies may be examined in a

complete systems environment.

Hardware Studies

SERI has recently conducted a study of solar domestic hot water systems in which commercially available controllers were purchased and then

tested in a laboratory environment . The tests revealed problems with sensor accuracy, sensor failure, controller accuracy, failure to meet

61
specifications, etc. Also, the tests were limited in scope and did not

include a wide variety of controller types. It is recommended that the

problems with reliability of SDHW controllers that were identified in the

SERI study be resolved and that additional testing be performed, both on

the controllers that were examined in the first phase of the work and on

additional controllers.

The recommended research activities are summarized in Table 1.

62
TABLE 1. Summary of Recommended Research Activities

CONTROLLER FUNCTION/TYPE

Recommended Research Collecting Distributing

Bang-Bang Prop, and PID Bang-Bang Prop, and PID

Quantify

Investigate

Parasitic Losses XX XX

Controller

Based on Stagnation

Temperature

X

Determine the Effects of

Cycling on System Performance and Cost

X X

Develop Charts for

Specifying AT

QN

X

Determine Partial

Excitation Characteristics

of Pumps and Blowers

Examine Multi-Rate

Circulators

X X

.Test Relationship for

Differential Control

Thresholds

X X

.Compare Costs and

Performance

X X

.Test the Use of the

PID Input State in

Mixed Storage Systems

X

.Test the Performance

of Systems at Very Low

Flow Rates

X X

Test Controllers that

Reduce Overshoot

X

Test Off-Peak Storage

Controllers

X X

Examine the Use of Weather

Prediction in Controls

X X

Develop Control Systems

Simulated Program

XX XX

Testing for Reliability XX

63
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70
U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Cate BIBLIOGRAPHIC DATA

REPORT NO.

December 1984 SHEET (See instructions) NBSIR 84-2980

4. TITLE AND SUBTITLE

RESEARCH PRIORITIES FOR IMPROVING THE EFFECTIVENESS OF ACTIVE SOLAR HOT WATER

AND SPACE CONDITIONING SYSTEMS

5. AUTHOR(S)

Robert D. Dikkers, William J. Kennish, C. Byron Winn, William Huston

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No. DE-AIQ1-76PR06010

8 . Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, Stote, ZIP)

U. S. Department of Energy

Office of Solar Heat Technologies

Active Heating and Cooling Division

Washington, DC 20585

10. SUPPLEMENTARY NOTES

2 Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

As part of the FY 1983 Department of Energy Systems Effectiveness Research Program,

the National Bureau of Standards (NBS) was assigned responsibility for developing

research priorities for improving the effectiveness (i* e *> thermal performance, cost

reliability and maintainability) of active solar hot water and space conditioning

systems. To carry out this task, NBS, in cooperation with various industry

representatives, organized and conducted two meetings in August 1983. The first

meeting covered all major aspects of active solar hot water and space conditioning

systems. The second meeting dealt only with solar control subsystems. Based on information obtained from these meetings , recommended research priorities for

improving the effectiveness of active solar energy systems are presented.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by se^/co •. Control subsystems; materials; reliability; research priorities; solar heating; sol.ir

hot water; test and evaluation procedures; thermal performance

13. availability

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