Florida Scientist, QUARTERLY JOURNAL of the FLORIDA ACADEMY OF SCIENCES VOL 39-3-1976

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Florida
Scientist
Volume 39

Summer, 1976

^x

No. 3

CONTENTS

ACADEMY SYMPOSIUM

Solar Energy
Introduction by the Chairman

Bruce

Nimmo

Bruce

Nimmo

129

Testing of Flat Plate Solar Collectors

and Solar Hot Water Systems
Energy in Florida

The Role of the Florida Solar Energy Center
in Solar Energy Systems Research and
Practical Application of Solar

Commercialization

Delbert B.

Douglas E. Root,

Ward and

Paul

J.

130

Jr.

138

Nawrocki

173

Solar Research at the University of Florida Solar

Energy and Energy Conversion Laboratory
Herbert A. Ingley and George W. Shipp

Solar Energy Research at the Georgia Institute
of Technology
Albert P. Sheppard and J. Richard Williams

181

188

Solubility Studies of Refrigerant-Carrier Fluid Pairs for

Solar

Powered Air Conditioning Applications
R. D.

Evans and

J.

K. Beck

The Florida Academy

of Sciences,

Membership Information

'Copies of this issue may be obtained for $5.00 postpaid from the Florida
East Rollins Street, Orlando, Florida 32803.

Academy

199

206
207

Citation for Robert Nathan Ginsburg

of Sciences, 810

QUARTERLY JOURNAL OF THE FLORIDA ACADEMY OF SCIENCES

FLORIDA SCIENTIST
Quarterly Journal of the Florida Academy of Sciences
Copyright © by the Florida Academy of Sciences, Inc. 1976
Editor:

Department

Harvey A. Miller
of Biological Sciences

Florida Technological University

Orlando, Florida 32816

The Florida Scientist
Inc., a non-profit scientific

is

published quarterly by the Florida

and educational

association.

viduals or institutions interested in supporting science in
tions

may be

Academy

Membership
its

is

of Sciences,

open

to indi-

broadest sense. Applica-

obtained from the Treasurer. Both individual and institutional members

receive a subscription to the Florida Scientist. Direct subscription

$13.00 per calendar year.
Original articles containing

new knowledge,

or

new

is

available at

interpretation of knowledge, are

sections of the Academy, viz.,
and Planetary Sciences, Medical Sciences,
Physical Sciences, Science Teaching, and Social Sciences. Also, contributions will be
considered which present new applications of scientific knowledge to practical problems
within fields of interest to the Academy. Articles must not duplicate in any substantial
way material that is published elsewhere. Contributions from members of the Academy
may be given priority. Instructions for preparation of manuscripts are inside the back

welcomed

in

any

field of Science as represented

by the

Biological Sciences, Conservation, Earth

cover.

Officers for 1976

FLORIDA ACADEMY OF SCIENCES
Founded 1936
President: Dr.

5809

W.

Patrick

J.

Gleason

Churchill Court

West Palm Beach, Florida 33401
President-Elect:

Department

Treasurer: Dr. Anthony F.
Microbiology Department

Walsh

Orange Memorial Hospital
Orlando, Florida 32806

Dr. Rorert A. Kromhout
Editor: Dr.

of Physics

Harvey

A.

Miller

Florida State University

Department

Tallahassee, Florida 32306

Florida Technological University

of Biological Sciences

Orlando, Florida 32816
Secretary: Dr. H.

Edwin

Steiner, Jr.

University of South Florida

Program Chairman: Dr. Margaret Gilrert
Department of Biology

Tampa, Florida 33620

Florida Southern College

Department of Education

Lakeland, Florida 33802

Published by the Florida Academy of Sciences
810 East Rollins Street
Orlando, Florida 32803
Printed by the Storter Printing
Gainesville, Florida

Company

Florida Scientist
QUARTERLY JOURNAL OF THE FLORIDA ACADEMY OF SCIENCES
Harvey

A. Miller, Editor

Summer, 1976

Vo. 39

No. 3

Academy Symposium

SOLAR ENERGY
Bruce Nimmo, Chairman
College of Engineering, Florida Technological University, Orlando, Florida 32816

The 1976 Academy Symposium

indeed a
timely one for the State of Florida and the nation as a whole. The
crisis of energy availability persists, with many claiming that the
truly difficult

topic, Solar Energy,

problems will be facing us

is

in the near future. Solar

energy, outstanding in extent and environmental acceptability,
rises to

as the

the fore in any discussion of the energy

sun

rises

dilemma

as surely

each morning.

Summary papers were presented

at the

symposium covering

the historical development and present state of the solar industry,

the importance of equipment testing and standards, the activities

of the internationally

known

Solar Energy Laboratory of the Uni-

and scope of the newly formed Florida
Energy Center, and finally a survey of projects under way at
the Georgia Institute of Technology and their relation to major solar research and development projects around the world.
Hopefully, the exchange of information, the bringing together
and summarizing of the literature and the opportunity for discusversity of Florida, the role

Solar

sion will help bring the date for large scale utilization of solar

energy a

little

the world.

closer for the State of Florida, the United States

and

Academy Symposium

TESTING OF FLAT PLATE SOLAR COLLECTORS AND

SOLAR HOT WATER SYSTEMS
Bruce Nimmo
College of Engineering, Florida Technological University, Orlando, Florida 32816

Abstract: The introduction to the marketplace of solar equipment which does not meet performance expectations can have a serious detrimental effect on the industry and solar energy utilization in general. The equipment which industry produces must be efficient and well constructed.
The best method of assuring that this will be the case is to have available a well thought out and executed solar equipment testing program. This paper reviews some flat plate collector and solar water
heater testing literature and describes programs presently in existence. The theoretical basis for flat
plate collector thermal performance criteria

An

is

discussed as are collector diagnostic testing techniques.

issue of considerable interest and importance to both consumers and

manufacturers of solar equipment

The emerging

is

the testing of solar components and systems.

solar industry in the State of Florida, as well as in the rest of the

country, suffers from a lack of uniform, definitive, acceptable procedures for

shoddy or inoperative equipment of poor materials, deway to the marketplace (Schwartzman, 1975).

The equipment which the industry produces must be efficient and well constructed, it must be durable and it must perform the job for which it was pur-

such testing. As a
sign, or both, has

chased.

The

result,

found

its

user of the equipment must be able to specify the performance, both

thermally and structurally, in a meaningful manner so that he will

equipment has been
If

what he can expect

a set of standard test criteria

for user

The

installed

of

not developed, there

is

know

after the

it.

is

great potential

disappointment with consequent destruction of trust and confidence.

result, of course,

is

unfavorable impact on current and future markets and

serious delay in reaching the date

tribution to the state
for consideration

when

solar

energy can make a meaningful con-

and national energy budgets. Legislation

by the Florida

is

being developed

State Legislature to ensure that proper testing

capability will be available in the state (Yarosh, personal communication).

The

discussion of collector testing

collector, the single

most

common

which follows

solar device for

is

limited to the flat plate

both past and present appli-

cations.

Component and System Test Procedures Prior to 1974— Development

of a

and useful solar collector test procedure requires familiarity with the
analytical models which have been developed to enhance our understanding and
predict the performance of such collectors. Two classic works which present
flat plat collector analytical models are those of Hottel and Woertz (1942) and
Hottel and Whillier (1958). These authors have applied basic techniques of thermal analysis to predict the amount of useful heat which can be collected using a

rational

"conventional"

flat plate

or "deck" beneath a transparent cover with the under-

No.

NIMMO— TESTING SOLAR COLLECTORS

1976]

3,

side of the unit insulated to reduce

interpreted to

mean

downward heat

loss.

The

131
useful heat gain

the energy increase of the collector fluid as

it

is

passes through

the collector

The expression

for the collector fluid

may be

energy gain

expressed in terms

of collector parameters as

=

Collector energy

Fluid flow

gain rate

X

Specific heat

rate

X

Inlet to Outlet

Temperature Rise

of fluid

of fluid

By measuring

the individual items on the right hand side of this equation

possible to calculate the useful energy gain.

be expressed

The

collector heat gain rate

it is

may also

as

Collector energy

=

Solar irradiation

gain rate

—

rate

Solar radiation

—

reflected from

Heat losses
from collector

glazing to sky

In working equation format, these relationships

q /A = mc p (T -T
q u /A = F [I(ra) -U L (T -T a
f

J

.

f

e

r

.

i

become
(1)

)

f

.

1

(2)

)]

where
q u /A = rate of useful energy collection per unit collector area
F R = heat removal factor which accounts for the fact that the energy losses
are based on the difference between fluid inlet and ambient temperature instead of average plate temperature and ambient
I

(ra) e

= total incident solar radiation
= effective transmittance (t) and
plate

(s)

absorptance

(a)

product of the cover

and absorber surface combination

U L = overall collector heat loss coefficient
T a = ambient air temperature
T = fluid inlet temperature
T = fluid outlet temperature
f

f

If

i

,

one divides both sides of the second equation above by

xi

and commonly used collector

II

radiation, a useful

= q u /A = F R

[(T« e -

U L (T

f

efficiency,

17,

I,

the total incident

results.

,,-T a ) ]

Collector testing generally aims toward determining either the useful heat

pickup or the collector efficiency.

Two

early suggestions for standard experimental tests of flat plate collector
water heaters were by Robinson and Stotter (1959) and Whillier and
Richards (1961). The first paper discusses four parameters which it is suggested
solar

should be determined for performance ratings:

FLORIDA SCIENTIST

132
1)

Thermal efficiency

[Vol.

of the collector.

2) Aerial efficiency of the collector to reveal efficiency of

(calculated as
3)

A active /A total

space utilization

).

Orientation efficiency determined from the angle between the incoming
rays and a perpendicular to the collector (a moveable collector which con-

would have an orientation efficiency of
Heat Storage Coefficient which reflects the overall heat transfer

tinuously "faced" the sun
4)

39

one).
loss coeffi-

cient from the tank.

The paper by Whillier and Richards made
adoption of a standard

test

a strong plea for international

procedure for rating the performance of solar

flat

on an instantaneous efficiency basis. The carefully controlled
testing technique called for appears to have had considerable influence on the
National Bureau of Standards test procedure (Hill and Kusuda, 1974). Whillier
and Richards make the suggestion that (ra) e and U L can be determined from the
plate collectors

instantaneous efficiency data.

A

description

is

given of the standard test appara-

which was set up as part of the basic facilities of the South African Council
for Scientific and Industrial Research in Pretoria.
Numerous experimental test results for both collectors and solar water heater
systems were reported in the literature during the 1950's, 1960's, and early 1970's
with widely varying levels of sophistication in the experimental work. The works
of Khanna (1968), Czarnecki (1958) and Whillier and Saluja (1965) are representative of the tests performed. Khanna and Czarnecki were concerned with
tus

total

water heater systems (the former under actual "in use" conditions in India,

the latter under simulated conditions in Australia). Khanna's paper lacks numeri-

conducted with corrugated galvanized sheet
These collectors had life times of 2-4 yr depending upon the headers
used. Czarnecki found that for the field trials in Australia the mean yearly contribution of solar energy to the hot water supply ranged from 61% to 81%. Whillier and Saluja placed greater emphasis on the solar collector itself, examining
cal or graphical results for the test
collectors.

not only the details of the collector components but also discussing briefly diagnostic analysis of collector performance including
tive surface degradation.

They showed

bond conductance and

selec-

that weathering can have serious detri-

mental effects on certain selective surfaces.

Doron

(1965), in a brief technical note has described

two

test

methods

(iso-

thermal and varying temperature) used in the National Physical Laboratory of
Israel in the early sixties.

He

points out that there are

two

possible operation

mode when the collector is essentially isothermal and the "heating" mode when the collector fluid enters at one temperature and exits at a higher temperature. Of course, in large collector arrays each
collector may operate with only a small temperature rise and thus be well
modeled by the "boiling" mode even though, in fact, no phase change takes
modes

for collectors; the "boiling"

place.

The Standards

Institution of Israel (1966) established

and published

(S.I.

609)

a standard for solar water heater test methods which led to collector efficiency,

determination of the hot water output of the water heater, and system efficiency.

NO.

3,

NIMMO— TESTING SOLAR COLLECTORS

1976]

Nevins (1974) has described a variety of approaches used

133

in Australia for test-

ing solar collectors. These include simple comparative testing against a "stan-

dard" unit, detailed testing to determine collector
absorption coefficient

The

and

1

,

total

results of a series of

3

loss coefficients

system performance

wk

solar hot

of simulated practical domestic conditions

and radiation

tests.

water system

tests,

under a variety

was described by Chinnery

(1971).

Recent Solar Test Procedure Developments— A number of testing proWorkshop on Solar

cedures were discussed at the National Science Foundation

Collectors for Heating and Cooling of Buildings held in 1974. Lior (1974) pro-

posed a daily heat capacity rating

in lieu of efficiency tests in order to provide

operationally meaningful criterion for the customer.

quantity of heat collected during one day of each
tion

is

It

month

for

needed, at the general locality at which the collector

The National Bureau

of Standards (NBS)

an

indicates the effective

is

recommended

which solar
to be used.
test

collec-

procedures, as

developed by Hilland Kusuda (1974) and discussed by Hill (1975), lead to obtaining the efficiency of the collector under "steady state" conditions. Since the tests
are specified for performance under real sunlight conditions they are in fact

"quasi steady".

The

series of tests

which are run

at different

temperature levels

consist of determining the average collector efficiency for 15

min periods by

measuring the flow rate through the collector and the temperature

rise across

the collector. For each test period constraints are given for cloud cover, ambient

temperature variation,

minimum

insolation rate, incident angle between sun and
and instrumentation error levels. Kelly and Hill (1974)
have presented test procedures which were developed at NBS for testing of thermal storage devices. These tests are designed to determine the heat loss factor
for the unit and the response to step increases and decreases in the entering fluid
temperature. An excellent review of the background for the NBS work is presen-

a normal to the collector

ted in Hill et

al.

(1976).

Lee (1974) prefers the calorimetric test method which avoids some of the
difficulties and instrumentation costs experienced with the mass flow-temperature rise across the collector technique described by Hill above. The calorimetric
procedure is based on a closed system in which the heat from the collector is
stored in a large volume of well mixed water and the time rate of change of the
temperature in the tank is measured to determine the energy rate pickup of the
collector. The primary operational advantage of the technique is that only one
variable need be monitored, the storage tank temperature, as opposed to, say,
the NBS technique which requires measurement of flow and temperature. A potential drawback to the calorimeter approach Is that one never achieves a steady
state for fluid

temperature.

A

result of this

is,

of course, that the thermal capaci-

tance of the collector must be taken into account in the data reduction.

Simon (1974) has described a program carried out

at the

NASA

Lewis Re-

search Center for testing of collectors under conditions of simulated solar radiation.

The advantage

of the indoor solar simulator approach

Equivalent to (Ta)Jn equation

2.

is

that true steady

FLORIDA SCIENTIST

134
state

[Vol.

39

achieved under controlled conditions of environment. The drawbacks

mismatch between the true and simulated solar spec-

is

are expense and potential

trum.

The

latter

is

particularly important in the case of so-called selective sur-

where performance is spectrally dependent. Results indicate the NASALewis simulator has good spectral qualities and the indoor data are well

faces

correlated with those taken outdoors.

Two
ture.

built

system-oriented mobile

test facilities

have been described

in the litera-

The Honeywell (1974) transportable solar laboratory was designed and
primarily as a means of evaluating "on board" solar heating and cooling sys-

tems under various climatic conditions. It also served as a traveling demonstration unit to acquaint the public with solar systems and capabilities. Nimmo and
Larsen (1976) have described the design and development of a mobile solar testing and recording system to be used in the monitoring of "in use" solar hot water

systems in the State of Florida. The design calls for capabilities of making appropriate

measurements on both the older thermosyphon units and the newer

pumped

systems.

Most of the above material related to thermal performances. There are, of
course, numerous other criteria (structural, safety, durability/ reliability, maintainability) which must be considered in collector specification. A document
which addresses itself to many of these other aspects is the NBS report "Interim
Performance Criteria for Solar Heating and Combined Heating/ Cooling Systems
and Dwellings" (NBS, 1975).
Finally, mention should be made of the fact that commercial collector test
facilities have been established at a few locations in the country and that a number of collector manufacturers have taken advantage of the services offered.

From

this

summary

of testing activities,

it is

apparent that the large number

may be grouped as shown
Although other related components such as auxiliary energy supplier,
control devices and energy transport machinery might be added to the component list, these devices typically are common in conventional heating, ventiof thermal

in

Table

lating

performance

tests

reported in the literature

I.

and

air

conditioning design work and have been subjected in

stances to the tests of time and large scale usage.

Table

1.

Thermal performance

testing of solar

Component

System

-No load

-Collectors
-Test type:

components and systems.

flow-

tests

AT or

calorimetric

-Simulated load

solar or

solar simulator

-Purpose:

Thermal system design
or collector diagnostic

-Storage
-Loss coefficient

-Transient response

-In situ test with

ac ua

oa

s

many

in-

No.

3,

NIMMO— TESTING SOLAR COLLECTORS

1976]

If a collector test is

135

being performed to obtain information needed to

2
a thermal performance specification

which has been written

fulfill

for a given sys-

tem, then the amount of useful heat output or the collector efficiency as a func-

would generally be

tion of environmental conditions

hand, the test
cation or

if

is

satisfactory.

on the other

If,

being performed to obtain information for a prescriptive specifi-

the test

is

conducted

sign, the greater detail

is

an

as part of

R and D program

on collector de-

usually required in the conduct of the tests and presen-

tation of report data.

The National Bureau

of equation 2 which

useful in obtaining diagnostic information from collector

tests.

By basing

is

of Standards uses a modified

form

the energy losses from the collector on the difference between

the average fluid temperature and the ambient temperature the following expression results.
T?

= F'(ra) -FU L (T
e

f

.

i

+T

f

.

e

)-Ta

I

= F'(T«) -F U L AT/I
/

e

Streed (1975) has pointed out that by plotting the measured efficiency as a
function of

AT/I

for a range of exposure condition, the collector

can be characterized as shown in Figure

1.

The y

axis intercept

is

performance
related to an

experimental value of (ra) e and the slope related to an experimental value of

UL

3
.

Figure

1

(Simon, 1973) suggests a linear relationship between

tj

and AT/I.

y intercepts F'(Ta)e

Slopes f'U L

.04

.08

.12

Sj +

.16

.20

Tf,e

_
" '°

2
I

Fig.

1.

.24

28

°F hr ft

.32

.36

40

2

Btu

Efficiency for a flat plate collector using water as the transfer fluid (Simon, 1973).

The phrases performance specification and prescriptive specification have the following connotation. A
performance specification describes the results to be achieved, such as BTU's of heat to be provided, a prescriptive specification describes the means to achieve desired results such as use of antireflective coatings (Hartman,
:

1974).
*F', which is the ratio of actual useful energy collected to the useful energy collected if the entire collector
surface were at the average fluid temperature, is termed the collector efficiency factor. F' may be determined
analytically for several common flat plate collector designs (Whillier, 1966).

FLORIDA SCIENTIST

136

However,

it is

recognized that in reality

tion of the collector

UL

is

not a constant but rather a func-

and ambient temperatures. In addition, the product

varies with the incident angle to the collector. (A variation of about

expected from

to

30 degrees). For

linearity of the curve will tend to

common

show up

39

[Vol.

flat

4%

(ra) e

can be

plate collector designs, the non-

at higher

operang temperatures.

Streed (1975) has presented information on the interrelation between sub-

systems (components) and systems.

He

points out that tests covering most aspects

of the performance attributes are required to fully identify the subsystem characteristics for the

system designer to investigate their usefulness in the various func-

tional applications. Streed has suggested the listing

senting

some

of the

more

shown

in

Table 2 as repre-

and pertinent

significant system design requirements

sub-system test parameters.
Table

2.

Relationship between subsystem and system performance.

Subsystem Test

System Design Requirements

Energy Collection
Effective Absorptance

t,

Heat Removal Factor

FK

Loss Coefficient

U,,,k,t

Thermal Response

(C p )c

Efficiency vs.

T&I

rh

Energy Storage
Storage Tank
Hot Water Tank

C

p

(AT)

(C p )s
(C p) mv

Stratification

T/d

Heat Transfer Rates

rh

C

p

T)t

(

Energy Transfer
Circulation

Pump

Pe

Heat Exchanger

r; he

Ta > T

Control

s

Ew

Auxiliary Energy
Collector Tilt Angle

Latitude

&

Collector Angular

Response

Total system testing
test is

is

most effectively done, of course, on

site.

The on

site

uniquely suited to answer the ultimate question, "Is this system a sound

choice from the economic point of view." Naturally,

allowances must be

made

if

the system

is

a prototype,

would be true
water heaters which are beyond the

for the inherent additional costs. This

for essentially all solar systems except solar

prototype stage.

Two

major efforts are presently under way by technical societies

in the

The first
is the work of the American Society for Testing and Materials (ASTM) carried
out under subcommittee E21.10 Solar Energy Utilization. This work was instiUnited States to

assist in

evolving standards for solar testing and cooling.

tuted at a meeting held in Philadelphia in October, 1975. Several working groups
have been established in the areas of materials and thermal performance.

NO.

NIMMO— TESTING SOLAR COLLECTORS

1976]

3,

Two

137

been appointed by the American Society
and Air Conditioning Engineers (ASHRAE). The standards work for thermal testing of collectors is being carried out under subcommittee 93-P and the standards for solar thermal energy storage are being studied
by subcommittee 94-P. A report will be made to the ASHRAE standards committee in mid 1976. There appears to be a feeling that ASHRAE's responsibility
should be limited to thermal testing of collectors while ASTM should have responsibility for durability and reliability of materials. The latter should include
the behavior of the collector under "stagnation", or no flow conditions.
It is clear that the emerging solar industry as well as the emerging solar marketplace is urgently in need of standards and that the professional-technical societies and the federal government are moving to meet the need. Meanwhile,
the potential solar customer should abide by all of the conventional wisdom required when one associates with a new product.
special subcommittees have

of Heating, Refrigeration

LITERATURE CITED
Chinnery, D. 1971. Solar water heating in South Africa. CSIR (Pretoria) Res. Rept. 248:8-24.
Czarnecki, J. 1958. Performance of experimental solar water heaters in Australia. Solar Energy
2:2-6.

Doron,

B. 1965. Testing of solar collectors. Solar

Hill,

1975. Laboratory based activities in solar energy at the National Bureau of Standards.

J.

Standardization

News

Energy 9:103-104.

ASTM

3:20-38.

and T. Kusuda. 1974. Method of Testing for Rating Solar Collectors Based on Thermal
Performance. NBS Rept. NBSIR 74-635. Washington.
E. Streed, G. Kelly, J. Geist, and T. Kusuda. 1976. Development of Proposed Standards for Testing Solar Collectors and Thermal Storage Devices. NBS Technical Note 899.
Washington.
Honeywell. 1974. Design and Test Report for Transportable Solar Laboratory Program. Honey,

well Systems and Research Center. Minneapolis.
Hottel, H. and A. Whillier. 1958. Evaluation of flat plate collector performance. Trans. Conf.
Use of Solar Energy 2:74.
and B. Woertz. 1942. The performance of flat plate solar heat collectors. ASME Trans.
,

64:91.

Kelly, G. and J. Hill. 1974. Method of Testing for Rating Thermal Storage Devices Based on Thermal Performance. NBS Rept. NBSIR 74-634. Washington.
Khanna, M. 1968. The development of a solar water heater and its field trials under Indian tropical
conditions. Solar Energy 12:255-261.
Lee, D. 1974. Solar collector testing by calorimetry. Workshop on Solar Collectors of Heating and
Cooling of Buildings. NSF-RANN 75-019:373-379.
Lior, N. 1974. Solar collector testing and standards. Workshop on Solar Collectors for Heating and
Cooling of Buildings. NSF-RANN 75-019:349-358.

NBS. 1975. Interim Performance Criteria for Solar Heating and Combined Heating/Cooling Systems and Dwellings. National Bureau of Standards. Washington.
Nevins, R. 1974. CSIRO and Australian Experience in Testing Solar Collectors. A report to the U. S.
National Bureau of Standards. Washington.
Nimmo, B. and R. Larsen. 1976. Development of a mobile solar testing recording (STAR) System.
Proc. COMPLES Internat. Congr. Heliotechnique and Development. Dhahran, Saudi Arabia,
1975: in press.

Rorinson, N. and A. Stotter. 1959. A proposed standard test code for the determination of the
efficiency of solar water heaters of the flat collector type. Solar Energy 3:30-33.
Schwartzman, R. 1975. Buyers "Heated" by Solar Unit. Today Newspaper April 28:10c.
Simon, F. 1974. Status of the NASA-Lewis flat-plate collector tests with a solar simulator. Workshop on Solar Collectors for Heating and Cooling of Buildings.

NSF-RANN

78019:391-400.

FLORIDA SCIENTIST

138
,

and

P.

Harlament.

[Vol.

1973. Flat Plate Collector Performance Evaluation:

Solar Simulator Approach.

NASA TM

The Case

39

for a

x-71427.

Standards Institution of Israel. 1977. Solar Water Heaters: Test Methods. SI609. Tel Aviv.
Streed, E. 1975. The Relationships Between Tests on Components Separately and Tests on Performance of Solar Total Systems on Installations. Presented at the Conference on Standards
for Solar Heating and Cooling, ASTM Headquarters. Philadelphia. October, 1975.
Whillier, A. 1966. Low Temperature Engineering Application of Solar Energy. American Society
of Heating, Refrigerating and Air Conditioning Engineers Inc. Chapter III.
and S. Richards. 1961. A standard test for solar water heaters. Proc. Conf. New Sources
,

Rome. Paper 5-97:111-113.
and G. Saluja. 1965. The thermal performance

of Energy.
.,

of solar water heaters. Solar

Energy

9:21-26.

Florida Sci. 39(3): 130-138. 1976.

Academy Symposium

PRACTICAL APPLICATION OF SOLAR ENERGY
IN FLORIDA
1

Douglass

E.

Root,

Jr.

Engineering Consultant, Applied Solar Energy, 16 Interlaken Road, Orlando, Florida 32804

Abstract: Promotion of cheap electricity and gas for mass-produced water heaters in the post
World War II era led to the demise of the solar energy industry in Florida. The recent OPEC oil crisis
precipitated renewed interest in solar energy for many uses. Solar water heaters are a mainstay for
domestic users and construction materials, design parameters and designs are provided with notice of
limitations of each type. Flat plate collectors have advantages over concentrating collectors for domestic use but are more limited
Beyond domestic water heating,

in applications

where temperatures more than 180°F are required.

and cooling a
house hut these uses are not yet cost-competitive with fossil fuels. Recovery of potable water by solar
stills has been important in the past and is re-emerging with some urgency as population pressure
creates increasing demands on available supplies. Entry of new disciplines into consideration of
solar energy holds promise for expanded exploitation of our only continuously renewed natural resource—the sun.

The

solar energy can he stored for cooking or for heating

two years have seen the re-emergence of a fledgling solar industry
Companies large and small are vying for an elusive and ill-defined
solar hardware. Some of them manufacture components, some total

past

in Florida.

market for

solar systems.

In

some ways

this activity conjures

up

visions of times past, for during the

development in
For reasons to be discussed subsequently, that industry faltered during
the 1960's and by 1970 the last vestiges of it had all but disappeared. But in other
ways, the current re-emergence bears no resemblance to the past, for the reasons
which have led to renewed activity in solar manufacturing, research and development are broader in scope and more persuasive in nature than the pressures
which led to the solar activities of 1900 to 1960.
first

half of this century a small solar industry enjoyed healthy

Florida.

'Copyright 1976 Douglas E. Root, Jr., Orlando, Florida. All rights reserved. No part of this article may be
reproduced by any means, nor transmitted, nor translated into a machine language without the written permission of the author.

No.

3,

ROOT— APPLICATION OF SOLAR ENERGY

1976]

139

it may be well to trace the rise and
which reached their peak in the 1950s.
History— Inland Florida of the early 1900's was largely rural and agricultural. The road system, until well into the 1920's, was only partially passable
much of the year because of seasonal inundation of the many and broad flood
plains throughout the state. Rural electrification was largely limited to the rail
and navigable river corridors until the Florida land boom of 1925-27. Natural
gas was not natural to most of Florida until Florida Gas installed a major gas distribution network in the 1960's.
By the 1920's plumbing was moved indoors, and it was natural that the old
barber shop signs which read "Bath— 10^ (first water— 20$)" would begin to disappear. "Hot water in every room" appeared on billboards advertising Florida
accommodations and it became fashionable to bathe more often than once a
week. These changes in social customs required a source of low temperature
energy and Florida's citizens had more sunshine than money available to them

To place current

fall

interest in perspective,

of previous solar activities in Florida

in those days.

Early Patents:

parency of

A

patent peripheral to the basic principle involving the trans-

glass to the solar

was issued

radiation

to

spectrum and

D. Rice in 1867 (U.

its

S.

opacity to long

Davis of Pasadena, California, "invented certain

water heaters," according to records

in solar

wave length heat

patent no. 68459). In 1889, Charles

new and

useful

improvements

in the U. S. Patent Office.

II.

Many

World War

other patents covering solar devices were issued between then and

These early patents included:

Number

Date

2

966,070°

Aug.

Patentee and Address

2,

1910

1,003,514

Sept. 19, 1911

1,014,972

Jan. 16, 1912

1,042,418

Oct. 29, 1912

1,056,861°

Mar. 25, 1913
Sept. 30, 1913
Apr. 21, 1914
Oct. 9, 1917
Dec. 18, 1917
Mar. 5, 1918
Jan. 13, 1920

1,074,219
1,093,925
1,242,511°
1,250,260

1,258,405
1,338,644°

Wm.

Bailey, Monrovia, Calif.
Roundtree, Cotton, Calif.
T. F. Nichols, Bay, Ariz.
A. H. Evans, Freeport, 111.
T. W. Walker, Monrovia, Calif.
J.

L. L.

F. A. Skiff, Pakta, Mass.

H. L. Foresman, San Dimas, Calif.
Wm. J. Bailey, Monrovia, Calif.
G. Wilcox, Buena Park, Calif.
D. A. Harrison, Los Angeles, Calif.
E. D. Arthur, Arcadia, Calif, and W. G.
Carther, Arcadia, Calif.

1,473,018°

1,672,750
1,837,449°
1,849,266°

1,853,480°

Nov. 6, 1923
June 5, 1928
Dec. 22, 1931
Mar. 15, 1932
Apr. 12, 1932

F. E.

W.

Danner, Willows,

Calif.

Christiansen, Miami, Fla.

C. F. Kunz, Phoenix, Ariz.
F.

J.

Bentz, Miami, Fla. (storage tank)

H. A. Wheeler, Miami, Fla. and F.

Miami,
Nov. 22, 1932
Nov. 29, 1932

W.

1,889,238

2,065,653°

Dec. 29, 1936

H. M. Carruthers, Miami, Fla.

1,888,620

J.

Bentz,

Fla.

F. Clark, Philadelphia, Pa.

Assigned to Automatic Electric Heater Co.,
Pollstoren, Pa.

^Patents

marked by an

heaters. (Veltfort, 1942)

asterisk (°) are of the type of construction generally

used

in

conventional solar water

FLORIDA SCIENTIST

140

C. G. Abbot, Washington, D. C.

C. G. Abbot, Washington, D. C.

Dec. 27, 1938
May 28, 1940
Jan. 25, 1940
May 28, 1940
July 23, 1940
Sept. 3, 1940

2,141,330
2,202,019
2,205,378
2,202,756*

O. H. Mohr, Oakland, Calif, (cooler)
C. G. Abbot, Washington, D. C.

S.

Cline, Miami, Fla.

2,247,830

July, 1941

H. Cally, Miami, Fla.
Berry, Orlando Park, Fla.
(45% to E. T. Buron, Winter Park, Fla.)
C. G. Abbot, Washington, D. C.

2,249,642

July 15, 1941

E. T.

2,208,789*

2,213,894°

Patents meant

little

B.

E.

J.

was

skilled in building

heaters were built by

Turner

to the early builders of solar water heaters.

population living in Florida during the
sity,

39

O. H. Mohr, Oakland, Calif.

July 5, 1938
Oct. 18, 1938

2,122,821°

2,133,649

[Vol.

first

The agrarian

quarter of the 20th century, by neces-

and repairing mechanical devices. Many solar water
to provide hot water on the farm or in groves

home owners

where the sun provided the only readily available or affordable source of thermal
energy. They most often were made up of blackened iron sheets to which the
builder had clamped iron pipes arranged as in Fig. 1. Those early heat collector
decks were housed in thick wooden boxes which were usually covered with glass
3
(Veltfort, 1942). A collector area of about 12 sq ft per person was required. The
whole south-facing system was ground mounted and connected to a large storage
tank (20 gallons per person served) resting on an elevated platform. Judicious

arrangement of the connecting piping allowed natural circulation of the sun
heated water. Internal convection caused the stratification of the hottest water
near the top of the tank, and the systems provided domestic hot water around the
clock for thousands of

homes

in Florida

by the

1920's.
hot water to house
gate valve
air inlet

drain cock

hot water from collector
solar storage tank

gate valve

cold water to collector
solar heat collector

water drain cock
collector drain holes

Fig.

1.

Sketch of a thermosyphon solar water heater (after Partington, et

al.,

1975).

As many of the events reported here occurred before the American scientific community had embraced the
International System of Units and because much of the rest of it deals with such hardware as pipe and hot water

tanks which are sized in inches and feet or U. S. gallons, traditional English engineering units are used except
where research data cited were reported in metric units.
!

No.

3,

ROOT— APPLICATION OF SOLAR ENERGY

1976]

141

Early Manufacturers: Soon, roofing and plumbing contractors discovered

added a lucrative new
product line to their shelf item hardware. During the 1920's and 1930's dozens
of them began to manufacture and to refine flat plate heat collectors. A few companies were formed for the sole purpose of making and installing solar systems.
By the 1940's many heat collectors were being made with copper heat decks to
which copper tubes were soldered at about 6 in intervals. They still depended
upon the natural circulation caused by the density difference of the water in the
hot and cold legs of their piping, but they were more efficient than their predecessors. The wooden box had given way to a more durable one made of metal
which contained insulation in the form of sheeting such as "Celotex" or even
that the

making

mineral

wood

of "storeboughten" solar water heaters

or sawdust (Hawkins, 1947). In the northern part of Florida, the

were often covered with two sheets of glass instead of one. This produced
an insulating dead air space which allowed units in the colder parts of the state
to operate at acceptably high temperatures even in winter. Large solar storage
tanks were available from several manufacturers, solar water heaters were being
incorporated into some new housing, and commercial units began appearing on
hotels and apartments. Just as plumbing had been moved indoors several decades
earlier, the solar heat collectors were moved from the yard to the roof where
they were less subject to accidental glass breakage and where shade from growing shrubs and trees was less of a problem. The solar storage tanks were sometimes housed in an imitation chimney structure or in the attic of the building
itself. Figure 2 shows such an installation which has been in use for more than
units

Fig. 2.

A

typical roof

mounted thermosyphon

solar

water heater

in Florida.

FLORIDA SCIENTIST

142

[Vol.

39

25 years. Figure 3 shows a ground mounted solar water heater which has been
in use for more than 50 years. Figures 4 and 5 are from illustrations contained in
the report on solar water heaters which Veltfort prepared in 1942 for the Copper

and Brass Research Association.

Fig. 3.

A

ground mounted 4 X 14

structure at the

ft

solar heat collector.

The

storage tank

is

housed

in the

wooden

and

their

manu-

left.

Veltfort (1942) listed the following solar water heater firms

facturing locations:
Solar Water Heater Co.
Tampa, Florida

Bentel's Solar Heater Inc.

Miami, Florida
Bollinger

Company

Sol-Ray Engineering
546 Ballough Road

Lake Worth, Florida

of

Tampa

Company

Daytona Beach, Florida

Hot Spot Solar Heater Company
Dixie Highway
West Palm Beach, Florida

Sun Heater Company
307 South Morgan Street

Tampa, Florida

Kunz Bros. & Messenger
2 Avenue and Jackson St.
Tampa, Florida
Pan-American Solar Heater,

Sun Ray Water Heater Co.

Sarasota, Florida

Inc.

2730-34 Northwest 2nd Ave.
Miami, Florida (with Plant No. 2
at 212 W. Bay St., Savannah, Ga.)

Water Heater Company
310 Northwest 25th St.
Miami, Florida (Branches in Nassau,
Jamaica, Bahamas, Puerto Rico, Cuba, and Manila)
Solar

United States Foundry
Miami, Florida

& Mfg.

Universal Solar Heater

Company

1121 San Marco Blvd.
Jacksonville, Florida

Corp.

"

No.

3,

ROOT— APPLICATION OF SOLAR ENERGY

1976]

aK^~~^)

143

EBB

Pictured above is Day & Night
Storage
Boiler extending
through the roof and camouflaged as a chimney

and

ible

popular

installation

conditions
boiler

—

a feas-

method of
where

used

do not permit the
be placed
under

to

gable, as

often

shown at

left.

SPECIFICATIONS
Day and Night Solar Water Heater
STORAGE BOILER

SUN COIL
Approx.

s,«

No.

Capacity

Weight

Over All
Dimension

Weight
Installed

and Filled

8

4'6"xl0'0"

205

lbs.

40

5'0"x2'4"

900

10

4'6"xl2'5"

2S01bs.

60

=i'0"x2'8"

1100

lbs.

335

80

6'0"x2'8"

1450

lbs.

120

6'0"x3'2"

2100

lbs.

4 6"x 4

12

'

1

'

1
1

lbs.

lbs.

SUN
consists of %" seamless copper tubing
with return bends. The coil box is 4" thick and has 3
strips of lk" material on back to allow for air space

COH

The

between box and

rooi.

RECOMMENDATIONS
Typ* of Building

Storage Boiler

Size Sun Coil

Small bungalow

No.

8

40 gal.

or 10

Residence

No. 12

2-bath residence

80 gal.

3-bath residence

2

No. 10s

80 or 120 gals.

Extra large residence

2

No. 12s

120 gal.

Day
Fig. 4.

60 gal.

No. 10 or 12

bath and shower)

(1

A typical

fc-

Niqht Water Heater

pre World

Early Literature:

War

Mangon

II

catalog sheet of the

Day

Co.,

Ltd.

& Night Solar Water Heater Co.

(1880) in France, Ericcson (1884), the designer of

the steel ship Monitor, and Willsie (1909) were

among

those

who made

early

contributions to the solar literature. As time passed, significant contributions

FLORIDA SCIENTIST

144

ROCK WOOL-^

DOUBLE GLAZED,
WITH AIR SPACE

K

FLAT COPPER TUBING

CORRUGATED BACKING

[Vol.

^ HORIZONTAL

39

TANK

REYNOLDS METALLATION WITH

TWO AIR SPACES FOR

INSULATION

Pan American Solar Heater, lnc
Fig. 5.

Solar Water Heater Co. sold thousands of the units shown here during
and 1950's. Many of them were installed in government housing projects establow income groups.

The Pan American

the 1930's, 1940s
lished for

were made by Farrall (1929), Carnes (1932), Alt (1935), Brooks (1936), Abbot
(1934, 1939), Merle (1940), Moon (1940), Cottony (1941), and Hottel and Woertz
(1942).

work at Harvard and Massachusetts
Technology received the financial support of the Godfrey Cabot
Foundation during the mid-1930's, at which time there were only two other such
programs. One was conducted in Algeria, the other at Tashkent in Russia (Hottel
and Howard, 1971). The University of Florida and the University of Wisconsin
initiated research programs within the next few years and it looked as if the practical application of solar energy was off and running. By 1951, there were more
than 50,000 solar water heaters in use in the Miami area alone (Hottel, 1955).
University Participation: Solar research

Institute of

BUT, something went awry—
By

the mid-sixties public interest in purchase of solar water heaters had

all

but disappeared throughout the entire United States and the marketplace no
longer supported the industry. Accordingly, innovation for product improve-

ment ceased as companies failed or reverted to previous product lines such as
plumbing and roofing. Interest in solar energy continued among a few scientists
such as Farrington Daniels who was directing much of his attention to solar stills
to create fresh water— something which holds future promise in Florida as demands on our fragile and limited aquifers intensify with exploding population.
Daniels (1956) had an extraordinary perception of energy problems clearly

No.

3,

ROOT— APPLICATION OF SOLAR ENERGY

1976]

145

World Symposium on Applied Solar
November, 1955. In any event, as popular

interest in applied solar energy declined in the U. S., scientific and popular interest developed abroad, especially in Australia, Israel, France and Japan.
Several causes have been suggested for the decline in American interest.
Scott et al. (1974) summarized their south Florida survey findings as shown in
stated in his keynote address at the First

Energy held

Table

in Phoenix, Arizona, in

1.

Table

Reasons for discontinuing the use of solar hot water heaters.

1.

Condition

Damage caused by
Insufficient hot

Too expensive

tank leaks

water

to repair tanks

Replaced before

Percent

Frequency

Frequency

16

24.0

15

23.0

10

15.0

6
4

9.1

3
3

4.5

2

3.0

2

3.0

1

1.5

failure, unit

considered too old

Poor position, shading
Not safe, worrisome
Booster problems
Too much rust or sediment
Needed maintenance, bother

Damaged by

Absolute

storms

Already had conventional unit
Other

6.1

4.5

1

1.5

3

4.5

Tanks mounted above ceilings were generally set in drip pans to which drain
were connected. However, the drain lines often became clogged with trash
during their 10-20 yr of dormancy prior to tank failure. This meant wet and often
falling plaster was the first warning many home owners had that their attic
mounted solar tank had sprung a leak. As Scott's survey reports, this turned many
solar water heater owners toward electric or gas units. Eric Farber, long time
director of the Solar Energy Laboratory at the University of Florida, noted additionally that the mass purchase of fully automatic washing machines and dishwashers during the late 1950's and into 1960's put too much demand on many
solar hot water systems (Farber, personal communication, 1974).
High initial cost had always been a deterrent to the sale of solar water heaters.
lines

They remained batch produced by
and gas water heaters,

The

as

solar systems stayed

relatively small manufacturers, but electric

demand grew, became mass produced by major firms.
expensive while the electric and gas units became very

inexpensive during the 1960's.
of advertising may have been a contributor to declining public
During the sixties, electrical energy became so abundant that
power companies spent millions urging the public to buy "gold medallion all
electric homes" or to "live better electrically." The power companies offered
cash rebates to those who installed electric hot water heaters. During that period

The power

interest,

too.

the Florida

power

(the

homeowners averaged paying about $0,017 per

1976 average

is

ca. $0.04).

KWH

for electric

FLORIDA SCIENTIST

146

[Vol.

39

Probably because of the combined negative effects of the problems caused
by tank leaks and others cited by Scott and Farber, and positive pressures for
change exerted by the utility companies and conventional water heater manufacturers, few solar water heaters were sold in Florida from 1960 until about
1975 and most of the manufacturing capability was lost.
On a national level, scientific interest dwindled. Researchers had found what
appeared at the time to be a safe and nearly inexhaustible source of powerlight

water nuclear reactors. By 1965, only a handful of university level research
continued their interest in the solar field. Farber at the University of

scientists

Florida, Daniels

and Duffie

at the University of Wisconsin,

and Yellott in Arizona, Hottel at M.

and Telkes

in

New York

(later

Lof in Colorado,

Bliss

at the University of Minnesota,

Jordan
Delaware) were notable among these.
I.

T.,

Fortunately, as noted previously, growing interest outside the United States

on the part of numerous scientists in Australia, Israel, South Africa, Japan and
France helped keep scientific investigation alive. The activity in Australia was
great enough to provoke the moving of the headquarters of the International
Solar Energy Society to Melbourne in 1970. (The Society had been formed in
the mid-1950's in Phoenix, Arizona, as the Association for Applied Solar Energy
and it had sponsored the First World Symposium on Applied Solar Energy in
that city

November

1-4, 1955.)

As the 1960s passed, solar water heaters disappeared from the roofs in Florida—and in Arizona, New Mexico and southern California, too, for that matter.
Solar manufacturing companies dwindled to a very few, and solar scientists in
general devoted their efforts to other fields of research. The interest level in the
practical utilization of solar energy can be traced as an ever descending curve
across the last 5 yr of the sixties and by 1970 interest was almost gone.
Something else was almost gone by 1970, too, but few recognized the danger
signals.

Cheap energy was almost gone—
That

fact

is

well understood now. In a speech delivered in Washington, D.

C,

January 30, 1976, to the American Institute of Aeronautics and Astronautics,

ERDA,

stated that proven petroleum re-

serves will last about another 35 yr, but that

uneven geographical distribution

Robert

Fri,

Deputy Administrator

will cause shortages in

for

Western bloc nations

to occur far short of that time.

That's a sobering statement, since Mr. Fri has access to the most accurate in-

formation available on that subject. It is not likely that petroleum can ever again
be looked to as a source of cheap energy.
Near cessation of construction of new nuclear power plants is documented

in an article on atomic power which appears in the February 16, 1976, issue of
U. S. News and World Report. Supplemental electrical energy from that source
is going to amount to considerably less than had been expected.
ERDA's National Energy Research and Development Plan is excerpted and
highlighted in a special issue of Information from

ERDA

dated November 21,

No.

ROOT— APPLICATION OF SOLAR ENERGY

1976]

3,

147

It is apparent from this publication that the homeowner can expect little
from constantly increasing power costs before at least 1985. That is to say,
little relief in terms of the new technology which ERDA expects to develop and
market. Hottel and Howard (1971) have analyzed, in depth, most suggested long
range solutions to the growing world energy demand.
A Near Term Prospect for Relief— What about an old technology? What
about the solar water heater? What about solar building heating? What about
solar operated air conditioning? In Florida homes 85% of the energy consumed

1975.

relief

is

for these three items (Farber, 1975).

By way

of answering those questions, let us review the Solar

Conference of

February 13-14, 1976, sponsored by the Associated Plumbing and Mechanical
Contractors of Florida in Orlando. That it was attended by people from 22 states,
including Alaska and several Caribbean countries

is interesting, though not surThere was something surprising about that conference, however. Some
42 manufacturers displayed solar devices ranging from differential thermostats

prising.

to

complete

solar heating systems for buildings.

The

three largest manufacturers

Two of
Two large

of conventional water heaters in the world displayed solar storage tanks.

the three displayed several different types of complete solar systems.

and respected aerospace firms displayed

flat

plate collectors

and systems

for

water heating and hydronic building heating.

The

plumbing supply house

largest

and complete

solar

nation's largest

aluminum and copper

aluminum panels with
heat collectors.
flat

in Florida displayed solar heat collectors

water heating systems which

One

it

manufactures.

fabricators displayed both

integral tubing systems

made

A

of the

exclusively for use in solar

of the largest glass manufacturers in the

plate solar heat collectors.

One

copper and

Texas manufacturer of

air

world displayed

conditioning com-

ponents displayed a tracking solar collector which utilizes a rectangular Fresnel lens

and

is

said to

produce water or steam

at

temperatures suitable for power-

ing absorption air conditioners.

These were not handmade prototypes on display. These were production
for delivery now.
Considered collectively, these displays indicate that those planning the eco-

models ready

nomic future of some very large companies, with widely divergent interests,
have reached a similar conclusion. Their conclusion would seem to be that the
practical application of solar energy is going to make a comeback in the U. S.
Availability of Solar Energy— Even superficial study of insolation data
(Fig. 6, 7 and Table 2) establishes that sufficient solar energy is available, in most
of the United States, to heat a house and its hot water or to power its air conditioner without using
tion. Insolation

more than

half the surface of the roof for solar heat collec-

data measured on a horizontal surface tends to give the south-

west, and to a lesser extent, the entire south a distinct advantage in terms of the

quantity of solar energy impinging on the surface. However,
figures are corrected to reflect the quantity of energy

if

the insolation

which impinges on a prop-

erly sloped surface, the apparent locational differences are smaller. Figures 6

and 7

illustrate this.

FLORIDA SCIENTIST

148

[Vol.

39

2000-

7^K

1500-

;

•

y

:

/

\
/
JFMAMJJASOND

500-

JFMAMJJASOND
Month
(left).

,

X.

1000-

Fig. 6

"">;"••*.

Month

Monthly distribution

of

mean

BTUs per
based upon mean daily

daily insolation in

The solid line indicates levels for a horizontal surface
The dotted line indicates levels for a south facing surface elevated from

sq

ft

Miami, Florida.

in

solar radiation for 1964.

horizontal to an angle equal

4
to the latitude plus 10°.

Fig. 7 (right).

Monthly

distribution of

mean

daily insolation in

Massachusetts, based on data parallel to that in Fig.

The

solar climate has not

What

1960's.

is

BTUs

per sq

ft

in

Cambridge,

6.

changed since the "old" solar industry died in the
renewed interest in the sun's energy from dying

to prevent the

out again? While the solar climate hasn't changed, the energy climate has

changed— markedly.
Energy in the 1950's and 1960's was so cheap it was basically free. Energy
consumption was not even considered in most manufacturing operations. Gasoline price wars were the rule rather than the exception for the last half of the
sixties. Utility companies used every technique known to merchandisers to sell
more electricity and natural gas at giveaway prices. That was the energy climate
surrounding the waning Florida solar water heater industry of yesteryear.
There is simply no hope that such a situation will occur again. Oil rich nations have realized they are custodians of an exhaustible asset which requires

Some responsible scientists hold that nuclear
up about as many problems as they have solutions.

millions of years to replenish.

power

plants have turned

Fluidized bed coal generating plants will help, but their construction will take

time and the cost of operation
able

oil

and gas

is

expected to be higher than the currently oper-

fired generating plants. Environmentalists are less than over-

joyed at the prospect of renewed strip mining of coal to fuel them.

Wind power,

ocean thermal gradients, biocon version and direct production of electricity from
sunlight— all of these things offer promise in the future, but the hardware

is

nei-

ther available for mass purchase nor economically competitive yet (Harrenstien,
1976).
The values given represent a first approximation approach to the actual amount of insolation received. Geometrical considerations dictate that a sloping surface intercepts more direct or beam radiation than a horizontal
one during the winter season. During the winter, overcast days are few and probably more than 80% of the total
insolation is direct. Because of rain showers, however, much of the summer insolation any surface receives is
diffuse. Tilting the surface may increase or decrease its ability to receive diffuse radiation on overcast days, cloud
reflected radiation when cumulus clouds abound and ground reflected radiation from lakes and fields. Many
investigators have suggested methods for evaluating both the beam and diffuse radiation received by variously
oriented surfaces. Among them are: Moon (1940), Hottle & Woertz (1942), Brooks (1952), Becker and Boyd (1957),

Bliss (1961), Liu and Jordan (1967), Duffie and Beckman (1974), and Hottel (1976).
4

i

No.

3,

ROOT— APPLICATION OF SOLAR ENERGY

1976]

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