SERI/TR-254-1727

UC Category: 59a

Measured Performance Results:

Low-Cost Solar Water Heating

Systems in the San Luis Valley

Joel Swisher

January 1983

Prepared Under Task No. 1119.70

WPA No. 302

Solar Energy Research Institute

A Division of Midwest Research Institute

1617 Cole Boulevard

Golden, Colorado 80401

Prepared for the

U.S. Department of Energy

Contract No. EG-77-C-01-4042
Printed in the United States of America

Available from:

National Technical Information Service

U.S. Department of Commerce

5285 Port Royal Road

Springfield, VA 22161

Price:

Microfiche $3.00

Printed Copy $ 4.50

NOTICE

This report was prepared as an account of work sponsored by the United States

Government. Neither the United States nor the United States Department of Energy,

nor any of their employees, nor any of their contractors. subcontractors. or their

employees, makes any warranty, express or implied, or assumes any legal liability

or responsibility for the accuracy, completeness or usefulness of any information,

apparatus, product or process disclosed, or represents that its use would not

infringe privately owned rights.
TR-1727 S=~II.I ----------------------..,;~:......-

PREFACE

The San Luis Valley is a high desert valley in southern Colorado. It is one

of the poorest areas in the state and contains few natural resources.

However, the San Luis Valley is endo~ed with abundant sunshine and with people

comm!tted to harnessing the sun f s energy to provide heat and jobs. These

people have initiated an explosion in solar energy use, resulting in perhaps

the highest per capita concentration of solar installations in the country.

The majority of these installations are low-cost, site-built retrofit systems

that employ locally available materials and labor. Many were built under

government grants and in community workshops, and others were built by private

individuals.

Nearly 1000 solar devices have been installed in the San Luis Valley, shoWing

great promise for fuel savings and local economic self-sufficiency. However,

measured performance results on low-cost systems are scarce, and this type of

information is necessary as more communities implement low-cost solar technology. As part of a program to measure the performance of innovative active

solar heaters, SERI supplied equipment and methodology to monitor seven lowcost solar water heating systems during 1981 in the towns of Alamosa and San

Luis. This report presents the results of this monitoring.

I wish to thank the following individuals for their contributions to this

effort: Don Frey for technical guidance; Jimmie Boswell of SERI for electronics maintenance; and Mark Randall of the san Luis Valley Solar Energy

Association, Andy Zaugg of Hot Stuff, and Arnie Valdez of People's Alternative

Energy Services for installation and operation of the data acquisition equipment. I thank also the seven families who allowed us to monitor their solar

water heaters.

JO~ Approved for

SOLAR E~mRGY RESEARCH INSTITUTE

il1AM(1awp

Meir Carasso, Chief

Building Systems Research Branch

n ~.~d;f:i~ Barr~Manager

Solar Thermal and ~~terials Research Division

ill
TR-1727 S=~I '.1 ------------,------------

SllMMARY

OBJECTIVE

To measure the thermal performance of a variety of low-cost solar water

heating systems that are representative of the designs and components used in

the San Luis Valley of southern Colorado.

DISCUSSION

SERI monitored seven low-cost solar water heating systems in the San Luis

Valley during the summer and fall of 1981. SERI supplied the data acquisition

equipment, performance evaluation methodology, and data reduction software.

Technicians from the San Luis Valley Solar Energy Association installed,

programmed, and operated the data acquisition systems. Measurements included

solar radiation, outdoor temperature, collector inlet and outlet temperatures,

domestic water temperatures, collector fluid and domestic water flow rates,

and auxiliary water-heater energy use. From these quantities, the energy

flows in each system were calculated, including collected solar energy, net

solar and auxiliary heating, and gross auxiliary energy use. Collector and

system efficiencies, coefficients of performance, and solar fractions were

also calculated. 'The monitored systerns represented a variety of low-cos t

solar water heating system designs and components. Five systems had sitebuilt collectors, and four included low-cost, tank-in-jacket heat exchanger/

storage tank components. Two of the systems were air-to-water systems. The

five liquid-based systems included a drain-down design, a propylene glycolcharged thermosiphon system, and three pumped-glycol systems.

CONCLUSIONS

• The pumped-liquid systems performed better than the thermosiphon and airto-water systems.

• The site-built, liquid-based collectors performed as well as the commercial collectors.

• The thermosiphon system performed at low efficiency because of low

collector flow rates.

• Air leakage in the air-to-water systems decreased collector performance

and promoted thermosiphon heat losses.

• The measured efficiency of each system depended on the hot water demand.

• Future work should include further detailed testing of low-cost, freezeproof thermosiphon systems.

v
TR-1727

TABLE OF CONTENTS

1.0

2.0

3.0

Introduc tion ;. .

The Monitoring System••••••••••••••••••••••••••••••••••••••••••••••

Results •••••••••••••••••

1

3

7

3.1 System 1:

3.2 System 2:

3.3 System 3:

3.4 System 4:

3.5 System 5:

3.6 System 6:

3.7 System 7:

4.0 Conclusions ••••

Alamosa Site-Built Water Heating System••••••••••••

San Luis Site-Built Water Heating System•••••••••••

San Luis Site-Built Thermosiphon System••••••••••••

San Luis Air-to-Water System•••••••••••••••••••••••

Alamosa Air-to-Water System••••••••••••••••••••••••

Alamosa Commercial Water Heating System••••••••••••

San Luis Commercial Water Heating System •••••••••••

.....................................................

7

7

10

11

13

13

14

17

4.1

4.2

4.3

4.4

Sys tem Performance ".

Collector Performance •••••••••••••••••••••••••••••••••••••••••

Load Effects•...•••....••..• 1 .•••••••••••••••••••••••••••••••••

Future Work.••••.••.•...•.•...••. "•.••••••.••••••••...•.•.•.•••

17

17

17

18

5.0 References . 19

Uncertainty Analysis••••••••••••••••

Appendix A:

Appendix B:

Detailed System Drawings •• ....................................

..........................

21

31

Appendix c: Horizontal Insolation Measurements ••••••••••••••••••••••••••

vii

35
TR-1727

3-1

3-2

3-3

3-4

LIST OF FIGURES

Operational Characteristics of the Alamosa Site-Built Water

Heater (Site 1).................................................. 10

Operational Characteristics of the San Luis Thermosiphon

l'1ater Heater (Site 3)............................................ 11

Operational Characteristics of the San Luis Air-to-Water System

(Site 4) ......•...............................•.................. 12

Operational Characteristics of the Alamosa Air-to-Water System

(S1 t e 5)......................................................... 14

LIST OF TABLES

2-1

2-2

3-1

3-2

Continuous Measurement Sensors .....•........•.....••.•........•...•

Data Reduction Functions••.•..••.......•.••........••...••..••...••

System Characteristics ....•........................................

Performance Summary•......•...••......••...•••......•......•.......

viii

4

5

8

9
TR-1727 S=~I i.1'" == -----------~------------

SECTION 1.0

INTRODUCTION

This report summarizes the measured performance of seven low-cost solar water

heating systems in the San Luis Valley of southern Colorado. The systems were

moni tared during the summer and fall of 1981. Microprocessor-based data

acquisition systems were supplied by the Solar Energy Research Institute

(SERI) and installed and operated by technicians from the San Luis Valley

Solar Energy Association (SLV/SEA) under contract to SERl. SERI also supplied

the performance evaluation methodology and the data reduction software.

The objective of this monitoring was to investigate the thermal performance

and fuel savings of low-cost solar water heaters, particularly site-built

systems. The San Luis Valley contains a large assortment of such systems, as

well as skilled individuals to assist with the monitoring program. This

presented an opportunity to test the performance of several low-cost systems

and to determine some of the associated benefits and problems.

A variety of low-cost solar water heating system designs and components were

monitored. Five systerns have site-buil t collectors, and two have commercial

collectors. Four of the systems include low-cost, tank-in-jacket heat

exchanger/storage tank components. Two of the systems are air-to-water

systems. The five liquid systems include a drain-down design, a propylene

glycol-charged t hermos Lphon system., and three pumped-glycol systems. The

systems were installed by varying degrees of professional labor, resulting in

a wide range of construction quality.

1
2
S=~I ~., - ___.~.J_.&J_ TR-I]?]

SECTION 2.0

THE MONITORING SYSTEM

Each water heating system was monLtor-ed using a microprocessor-based data

acquisition system (DAS) that was developed for the SERI Residential Class B

Passive Solar Performance Monitoring Program [11. The DAS was adapted to the

water heating application by including fluid flow rate and temperature sensors

in the hardware package and by reprogramming the data reduction software.

Each system had a slightly different combination of sensors and software.

The DAS processed 14 data channels every 15 seconds and stored hourly channel

averages on cassette tape. The sensors, described in Table 2-1, provided the

measurements necessary to calculate a basic energy balance for each system.

Storage tank temperature profiles and other specific component parameters were

not measured.

In addition to collecting and storing raw data measurements, the DAS performed

real-time data reduction, converting the raw channel data into system performance functions. The DAS printed daily summaries of the data reduction

functions. The functions, listed in Table 2-2, included daily performance

factors, which accumulated over a 24-hour period and ~'lere zeroed after each

daily printout, and monthly performance factors, which accumulated over an

entire month and were updated in the daily printouts and zeroed at the end of

each month.

The basic energy flows in the system were calculated using a Btu-meter calculation, integrating the product of flow rate, specific heat, and temperature

difference. The IS-second sampling period provided sufficient resolution to

accurately integrate this type of function, which included the net solar and

auxiliary heating and the gross solar energy collected by pumped-liquid

systems. For air-to-water systems, the air flow rate was measured once, using

hot-wire anemometry, and assumed constant whenever the collector fan operated. Collector flow was not measured for the thermosiphon system because a

flowmeter in the fluid path might have interfered with the flow. However, the

flow rate could be approximated from the energy and temperature measurements.

For electric water heaters, the gross energy input to the auxiliary water

heater was determined from measurements of the electric current drawn and the

AC supply voltage. For fuel-fired water heaters, the on-time of the burner

was monitored, and the fuel flow rate was measured once and assumed constant.

3
TR-1727 S=~I'~.' -----------------------------..:..-..:..--

Table 2-1. COntinuous Mea~urement Sensors

Description Symbol Measurement

Units Sensor

Horizontal radiation

Incident radiation

Btu/h ft 2

Btu/h ft 2

LI-COR pyranometer

LI-COR pyranometer

Outside air temperature AD590 integratedcircuit temperature

sensor

Collector inlet temperature AD590

Collector outlet temperature AD590

Cold water supply temperature

Water preheat temperature

Hot water output temperature

AD590

ADS90

AD590

Collector flow gal Kent positivedisplacement

flowmeter

(pulse-initiating)

Domestic water flow gal Kent flowmeter

AC supply voltage v v voltage transformer

Collector pump/fan current A AMPROBE current

transducer

Water heater current

or burner on-time

A

or h

AMPROBE

or thermal switch

Collector pump/fan on~time h relay

4
TR-1727 S=~I,i.1 ------------------------------------------ -~

Table 2-2. Data Reduction Functions

Description Symbol Measurement

Units Definitiona

Gross solar energy collected

Net solar heating

Net auxiliary heating

Gross auxiliary energy use

Solar system heat loss

Collector pump/fan energy

Hot water use

Net water heating load

Average outdoor temperature

Average cold water temperature

Average preheat temperature

Average hot water temperature

Horizontal radiation

Incident radiation

Average AC line voltage

Collector efficiency

Net solar efficiency

Auxiliary efficiency

Solar fraction

Net coefficient of performance

Collector pump cycles

Qsl

Ecp

Q

w

To

T1

T2

T3

I h

lin

V

ncol

ns o l

naux

Fso l

COPne t

ncyc

Btu

Btu

Btu

Btu

Btu

Btu

gal

Btu

of

of

of

of

Btu!ftZ

Btu/ft 2

volt

Cp IFcCTc o - Tc i)

8.33 JFwCTZ - T1)

8.33 JFw(T3 - TZ)

qw Jt w ( f ue 1 ) ;

3.413 IECwV dt (elec.)

Qgs - Qns

3.413pf IEecp V dt

JFw

Qna + Qn s

(l!t) ITo dt

(1/~) IFw T1

(l/~) IFw TZ

(l!r-~) I Fw T3

IIh dt

IIi n dt

(lIt) JV dt

Qgs!CAc lin)

Qns/CAe lin)

Qna!Qga

Qns/(Qns + Qua)

Qns!Ec p

a Ac collector area (ft2)

C specific heat of collector fluid (Btu/gal of or Btu!ft30y )

p¥ = collector pump/fan power factor

qw water heater auxiliary energy input rate (Btu/h)

t time (h)

5
-- III I "'.•"~ .~,

6
s.=~II*1 ----,-_---..;;.TR-1727 ............-.-0-

- .

SECTION 3.0

RESULTS

Table 3-1 summarizes the physical characteristics of each monitored system.

Appendix A contains detailed drawings of the systems and the sensor

locations. All of the systems use a double-tank configuration. Cold water

enters the solar storage component, where it is preheated by the solar

collectors before entering a second tank, which is equipped with electric or

gas auxiliary heating. The overall performance of each system, summarized in

Table 3-2, was derived mostly from the daily and monthly printouts of the

real-time data reduction functions. The performance factors in Table 3-2 have

the same definitions as the corresponding functions in Table 2-2. Toe

following discussion of the performance of each system is based on these

functions and examination of the detailed hourly data.

3.1 SYSTEM 1: ALAMOSA SITE-BUILT WATER HEATING SYSTEM

System 1 uses two ground-mounted, site-built collectors and a tank-in-jacket

heat exchanger/storage component. The single-glazed collectors contain copper

tubing wired to a flat-black absorber plate, in a parallel configuration. A

propylene glycol solution is pumped through the heat exchanger, which consists

of an uninsu1ated 66-gal (250-L) storage tank surrounded by a slightly larger

jacket, with the heat transfer fluid circulating through the 1/2-in. (1.3-cm)

passage between the tank and jacket walls. The jacket is also uninsulated.

Preheated water from this tank goes to a 52-gal (197-L) electric auxiliary

water heater.

During this study the system performed well, meeting 46% of the 64-gal/day

(242-L/day) hot water demand. The collector efficiency was 33%, the highest

of the seven systems monitored. The net solar efficiency was 23%, with most

of the thermal losses due to the uninsulated outer jacket. If this jacket

were insulated to R11 (0.5 W/m2 °C), the reduction in thermal losses would

raise the net solar efficiency to about 30% and the solar fraction from 46% to

about 50%.

Figure 3-1 shows the collector temperatures and weather conditions during a

three-day span in September 1981. The collector flow rate of 3 gallmin

(0.2 L/s) gives a sufficient fluid capacitance rate (1250 Btu/h OF) (660 ~?/oC)

to maintain the temperature increase across the collector at 40F (7°C) or

less. This keeps the collector outlet temperature relatively low and results

in a high collector heat removal factor and a high efficiency.

3.2 SYSTEM 2: SAN LUIS SITE-BUILT WATER HEATING SYSTEM

System 2 uses a roof-mounted site-buil t collector and an insulated 42-gal

(159-L) storage tank. The collectors are double-glazed with fiberglass and

use tube-on-plate construction in a parallel configuration. A copper hea t

exchanger coil is soldered to the storage tank exterior. The heat transfer

fluid is a propylene glycol solution.

7
System Characteristics

System

Location

System type

Collector fluid

Maxim.w collector flow

(~a1/min)

(scf/min)

Collector area (ft2)

Collector type

Absorber

Clazing

Number of layers

Material

Heat exchanger type

Storage tank volume

(gal)

Auxiliary tank volume

(gal)

Auxiliary energy

Data start date

Data end date

1

Alamosa

closed-loop,

pumped

propylene

glycol

3.0

44

tube on

plate

black

steel

1

glass

tank in

tank

66

52

electric

July 15

Nov. 27

Table 3-1.

2

San Luis

closed-loop,

pumped

propylene

glycol

3.8

36

tube on

plate

black

..teel

2

fiberglass

external

coil

42

42

electric

Sept. 23

Nov. 1

3

San Luis

closed-loop,

thermosiphon

propylene

glycol

0.06

35

tube on

plate

black

steel

2

fiberglass

tank in

tank

66

40

electric

July 29

Sept. 24

4

San Luis

air/water

air

75

64

baffled

air

black

aluminum

1

fiberglass

tank in

tank

40

40

electric

July 22

Oct. 12

5

Alamosa

air/water

air

148

86

straightpass air

black

aluminum

1

fiberglass

tank in

tank

88

30

electric

July 22

Oct. 3

6

Alamosa

closed-loop,

pumped

propylene

glycol

2.1

30

commercial

liquid

selective

steel

1

glass

internal

coil

80

30

natural gas

July 17

Aug. 7

7

San Luis

drain-down ,

pumped

water

6.5

50

commercial

liquid

selective

copper

1

glass

direct

65

40

propane

Sept. 25

Oct. 13

III

III

-'"

.

- ,

1/ I

'-'

Note: gal/min x 0.06309 = L~s

sc~/min x 3.468 x 1~; = ~g/h

ft x 9.290304 x 10 = m

gal x 3.785412 = L
TR-1727 S=~II.I ---------------------------- _

Table 3-2. Performance Summary

System 1 2 3 4 5 6 7

Monitoring period (days) 119 38 48 64 61 16 19

Gross solar energy collected (103 Btu/day) 28.3 15.3 6.1a 24.3 32.5 16.7 26.9a

Net solar heating (103 Btu/day) 19.4 14.4 4.8 11.0 11. 7 15.9 25.9

Gross auxiliary energy use (103 Btu/day) 36.2 24.9 18.2 22.0 18.3 9.1 65.0

Net auxiliary heating (103 Btu/day) 23.0 11.1 5.5 13.8 10.6 6.9 38.5

Auxiliary efficiency 0.64 0.44 0.30 0.63 0.58 0.76 0.60

Hot water use (gal/day) 64 39 16 39 36 53 135

Average cold water temperature (oF) 61 63 63 63 61 88 63

Average preheat temperature (oF) 98 108 99 97 101 124 86

Average hot water temperature (op) 141 142 140 139 136 139 120

Average outdoor temperature (oF) 53 46 63 57 60 68 51

Incident radiation (Btu/ft2 day) 1920 1730 1910 1860 1700 2330 1930

Collector efficiency 0.33 0.25 0.09a 0.20 0.22 0.24 0.28a

Net solar efficiency 0.23 0.23 0.07 0.09 0.08 0.23 0.27

Solar fraction 0.46 0.57 0.46 0.44 0.53 0.70 0.40

Purchased energy per gallon (Btu/gal) 620 750 1130 670 780 240 520

Collector pump/fan energy (103 Btu/day) 3.8 4.2 a 4.0 9.7 3.6 5.2

Net coefficient of performance 5.1 3.4 2.8 1.2 4.4 5.0

aApproximate calculation; not directly measured.

Note: 103 Btu/ day x 1.0548 = HJ/day

gal/day x 3.785412 = L/day

(OF - 32)/1.8 = °c

Btu/ft2 day x 11.348 = kJ/m2 day

Btu/gal x 2.785297 x 10-1 = kJ/L

9
TR-1727 S=~I[II ---------~--------------------------

"\ 11\

I \

':;' f .f~

/ \

j £

I

"

\. 1\ VIIJ~

='=" . V \T~'~/ 1\ ~'i=1 *~1'-=- / J-

........... ,

I!..... ..1--(..1...... v-rl .,

.......... .. ......... ..........

160

40

o 24 48

Time (h) trom 9-1-81, 0:00

"'

"'

300 ::l

CO

c: 200 ,2

Cii

(5

en

c

100 =c

(1)

"0

'0

o c:

72

----Insolation

- • --.• Collector Inlet Temperature

.......................... Collector Pump On

----- Ambient Temperature

------ Collector Outlet Temperature

Conversion Factors: Btu/ttl h x 3.152481 = W/m'

(OF·32)/1,8 = °C

Figure 3-1. Operational Characteristics of the Alamosa

Site-Built Water Heater (Site 1)

roe system met 57% of a 39-ga1/day (159-L/day) hot water demand. As was true

for three others of the seven systems tested, the hot water demand in this

system was not sufficient to fully utilize the solar heater. The net solar

heating was about equal to the standby heat loss from the electric water

heater. Nevertheless, the net solar efficiency was 23%, and the collector

efficiency was 25%. These figures would have been higher with a larger water

load.

3.3 SYSTEM 3: SAN LUIS SITE-BUILT THERMOSIPHON SYSTEM

This system uses a ground-mounted, site-built collector, similar to the

collector in system 2, but in a thermosiphon configuration. A propylene

glycol solution circulates by thermal buoyancy between the collector and the

storage tank/heat exchanger component located in an attic about 6 ft (2 m)

above the top of the collector. Similar to system 1, the heat exchanger is a

tank-in-jacket design, with the heat transfer fluid circulating between the

tank and jacket walls. Due to space limitations in the attic, the tank is

positioned horizontally. The outer jacket is insulated to Rll (0.5 W/m2 °C).

The system met 47% of a meager 16-gal/day (60-L/day) hot water demand. Again,

the small water demand reduced the solar heater performance, and the largest

energy flow in the entire system was the standby heat loss from the electric

auxiliary water heater.

10
S=~I TR-1727 ,t" ----------------------------------..=.:.::........:::..:...::..:.-

The net solar efficiency was 7%J with a collector efficiency of about 9%. The

low collector efficiency was caused by low collector flow J which resulted in

high collector temperatures and large thermal losses. Although the flow in

the thermosiphon loop could not be directly measured, the calculated flow rate

never exceeded 0.06 gal/min (4 mL/s), resulting in a collector heat removal

factor of less than 0.2. The results can be seen in Fig. 3-2 J wher e the

temperature rise across the collector reaches 55°F (31°C).

The low flow and high temperatures are partly due to the high viscosity of the

glycol solution (about six times that of water) and the relatively high

collector inlet temperatures caused by the heat exchanger (compared to a

direct system). The theoretical peak flow rate is about 0.6 gal/min

(40 mL/s) , 10 times the calculated flow, and the maximum collector temperature

rise is only about lOoF (lloe) [2].

The exact cause of the low thermosiphon flow is not clear. The collector is

made with 1/2-in.(1.3-cm)-diameter pipe, soldered in a parallel configuration

for low flow resistance J and the connecting pipe has a I-in. (2.5-cm)

diameter, sufficient to allow much higher flow rates. It is possible,

however, that there is some major restriction in the piping or the heat

exchanger/storage tank component.

3.4 SYSTEM 4: SAN LUIS AIR-To-liATER SYSTEM

This system is an air-to-water system with a roof-mounted, site-built

collector and an integral storage tank/heat exchanger component. The

collector is a Ramstetter-type design with an aluminum absorber and a

corrugated fiberglass glazing [3]. Metal baffles cause a serpentine air

c

e, 130

:::l

C5

Cl. 90 E(1)

I24 48

Time (h) from 8-3-81, 0:00

c

200 .2

C;;

or.n

c

100 ,...

(1)

u

c

----Insolation

- • --. Collector Inlet Temperature

----- Ambient Temperature

------ Collector Outlet Temperature

Conversion Factors: Btu/It" h x 3.152481 = W/m 2

(OF - 32)/1.8 = °C

Figure 3-2. Operational Characteristics of the San Luis

Thermosiphon Water Heater (Site 3)

11
TR-1727 S=~I';" ~---------------------

pattern through the collector. The 40-gal (151-L) storage tank is surrounded

by an air passage through which the air is circulated from the collector.

The system met 46% of a 39-gal/day (148-L/day) hot water demand, with a net

solar efficiency of 9% and a collector efficiency of 20%. The collector

efficiency measurement was relatively uncertain because a 25% collector air

leakage rate made flow measurements difficult. Nevertheless, it is clear that

much of the collected energy was not being used to heat the domestic water.

Conduction losses through the outer tank wall could not account for all the

lost energy.

Figure 3-3 shows a possible explanation for the energy loss. The collector

inlet and outlet ducts are located inside the conditioned space of the

house. One would therefore expect the collector inlet and outlet temperatures

to approach room temperature (about 70oF, or 210C) shortly after the collector

fan shuts off in late afternoon. Instead, the collector outlet temperature

remains well above room temperature, and the inlet temperature approaches the

outdoor air temperature. This suggests a thermosiphon loss mechanism that

operates whenever the collector fan is off. The warm storage tank is in

direct contact with air in the collector loop. This air rises ~lhen heated by

the tank, travels through the collector outlet duct to the collector, where it

is cooled or mixed with outdoor air (via collector leaks), and returns down

the collector inlet duct to cool the storage tank. The air in the inlet duct

is as much as 140F (8 oC) cooler than the air moving up the outlet duct.

Another similar thermal loss mechanism may be induced by leaks in the heat

exchanger, so that indoor air leaks into the duct and is heated by the tank;

c

200 .2

Cti

o

<J)

c

100 =c

Q)

"0

o

720 24 48 c

Time (h) from 7-27-81, 0:00

...~'\ I /\, I>'~

\\ ill \\ ! v\ f \ : :

.J /1 \' 1/ ~,

: .~ :.. !I"'"

tv \. ! .. !/ i

l '0\ 1\\ '. .......... or,l/" \ -,,..... "" ~\ ". f iV V

0- \;\.\ I -, - ..!

/ ...... 'OJ-

~'-rf. f-v-L.. ........... .\"i~':: ~:... ......l\ 1'=11....L.....

165

u,

; 125 ...

::l

Cti

...

Q)

E 85 Q)

I-

-----Insolation

- • --. Collector Inlet Temperature

************ Collector Fan On

----- Ambient Temperature

------ Collector Outlet Temperature

Conversion Factors: Btu/fF h x 3.152481 W/m 2

(0F - 32)/1.8 = 0C

Figure 3-3. Operational Characteristics of the San Luis

Air-to-Water System (Site 4)

12
TR-1727 55:c!1 [Ill ---------------------....;........;.""--

this air then travels out through the collector.

remove energy from the storage tank.

This chimney action can also

Figure 3-3 also shows collector temperatures during fan operation. The large

collector temperature difference (up to 40°F, or 22°C) is caused by the

relatively small fluid capacitance rate, 80 Btu/h OF (42 W/oC) (compared to

the pumped-liquid systems). This results in a lower collector heat removal

factor and higher collector outlet temperatures and thermal losses.

3.5 SYSTEM 5: A.LAMOSA AIR-To-WATER SYSTEM

System 5 is an air-to-water system. It uses a roof-mounted, site-built

collector similar to the system-4 collector, except that the air path is

straight rather than serpentine. The storage tank/heat exchanger component is

located in the basement, with the collector inlet and outlet ducts in

conditioned space.

The system performed similarly to system 4, heating 53% of a 36-gal/day

(136-L/day) load. Again, the solar performance was penalized by a small hot

water demand. The net solar efficiency was 8%, and the collector efficiency

was 22%.

A thermosiphon loss mechanism occurs in this system as well. As Fig. 3-4

shows, the temperature in the collector outlet duct remains above room

temperature when the collector fan is off, while the inlet duct temperature

approaches room temperature. It appears that basement air is leaking into the

heat exchanger and traveling up to the collector (which has a 10% leakage

rate), removing heat from the storage tank. There are backdraft dampers in

the collector inlet duct just below the collector and in the outlet duct just

above the tank. Apparently the damper near the tank leaks somewhat.

Other performance factors were similar to those of system 4, and Fig. 3-4

shows, again, as much as a 400 F (22°C) rise across the collector. One

difference between the two systems is that this system consumes more

electricity for the fan. The system coefficient of performance (COP) was 3.3

based on the collected energy and only 1. 2 based on the net solar energy

used. Since- the fan motor is ventilated by the collector air flow, a

significant amount of the collected heat is actually contributed by the fan.

One reason for the low COP is that the collector fan continues to run in the

afternoon until the collector temperature rise is close to zero. Because the

specific heat of air is low, very little energy is collec ted and the COP

during these hours is quite low.

3.6 SYSTEM 6: ALAMOSA COMMERCIAL WATER lIEATING SYSTEM

System 6 uses two commercial collec tors and a well-insulated 80-gal (303-L)

storage tank. The collectors are single-glazed, with a selective-surface

steel absorber that is formed around the copper tubing. The heat transfer

fluid is a propylene glycol solution.

13
TR-1727 =~-~ ,;.,

,~ ---'-------~-----~---------­

165 N

J:: '" " 0

- 0

<,

-::J

Cii

c:

.2

Cii

'0(J)

c:

C

<ll

:'Q

u

.=

200

300

100

o

24 48 72

Time (h) from 8-17-81, 0:00

.~ ~\ ~\

I I I I \\

8\ I II ~~ /\\

1/ (

\

.:::.:::. ..' i~~:. ,.::::::::0 '--~ "'~ .::::.~:: ::/ I -.... '\:::.: .,.I ,~,

1"---, ld....L...,

, , -,

---lJ..•. ....."'.... '.tL. ...........

125

u,

o

----Insolation

-.--. Collector Inlet Temperature

*********"'** Collector Fan On

----- Ambient Temperature

------ Collector Outlet Temperature

Conversion Factors: Btu/tt- h x 3.152481 = W/m 2

(0 F - 32)/1.8 = 0C

Figure 3-4. Operational Characteristics of the Alamosa

Air-to-Water System (Site 5)

TIle system met 70% of a 53-gal/day (200-L/day) hot water demand, preheating

the water to an average of 124°F (51°C). The high temperature of the water

supply--880F (31 0C)--resulted from nearby geothermal activity. The collector

efficiency was 24%, and the net solar efficiency was 23%.

Data collection at this site stopped when the data acquisition hardware was

seriously damaged during a severe lightning storm. All the sites were soon

equipped with lightning arresters that direct any lightning charge to ground

without damaging equipment. No further lightning damage resulted, despite the

high incidence of lightning in the San Luis Valley.

3.7 SYSTEM 7: SAN LUIS COMMERCIAL WATER HEATING SYSTEM

System 7 uses a commercial collector and a 65-gal (246-L) storage tank in a

drain-down configuration. The collector is single-glazed with a selectivesurface copper absorber.

The system contributed 40% of a 135-gal/day (511-L/day) hot water load. The

net solar efficiency was 27% and the collector efficiency was about 28%. The

high collector efficiency was partly due to the large hot water demand (the

family includes five young children). The average preheat temperature was

only 86°F (300C), whf ch kept the collector temperature low and improved the

efficiency. The average hot water output temperature was relatively low at

14
S=~II.I ----------'------------_----------~_:!:...!....=.L._ TR-1727

120 0p (49°C), partly because the 40-gal (151-L) gas-fired water heater could

not always keep up with the water use. Had the output temperature been 140°F

(60°C), as in the other systems, the solar fraction would have been only 30%.

This system performed remarkably well despite a serious control flaw. Due to

an improperly placed sensor or a faulty controller, the collector pump cycled

on and off an average of 49 times per day, draining the collector each time.

Although most of the cycling occurred during times of marginal solar input, it

wasted a significant fraction of the collected energy and made measurement of

the collected energy impossible. If this problem is corrected in the future,

the system performance should be excellent.

15
16
S=~II.I 'I'R-1727 --------------------------"---------..:;;;;;..;;...~..,;;;..-

SECTION 4.0

CONCLUSIONS

4.1 SYSTEM PERFORMANCE

The system efficiencies listed in Table 3-2 indicate that the best overall

performance was by the pumped-liquid systems. Systems I, 2, 6, and 7 all had

system efficiencies greater than 20%, with solar fractions between 40% and

70%. The lower efficiency of the closed-loop thermosiphon system 3 was due to

low collector flows and a very small hot water demand.

The air-to-water systems 4 and 5 did not perform as well as the pumped-liquid

systems. The low specific heat of air caused lower collector heat removal

factors; the collectors and heat exchangers leaked significantly; and thermosiphon and chimney air circulation mechanisms caused substantial heat losses

when the collector fan was off.

4.2 COLLECTOR PERFORMANCE

The collector efficiencies in Table 3-2 suggest that the site-built liquid

collectors (in systems 1 and 2) performed as well as the commercial collectors

(in systems 6 and 7). The systems containing the commercial collectors,

however, had some performance penalties: the high water supply temperature in

system 6 and the control problem in system 7. It would be unrealistic to

conclude that site-built collectors are as efficient as commercial models in

general; however, these results show that the site-built collectors do perform

adequately.

The air collectors operated with acceptable, albeit uncertain, efficiency.

The air leakage rates, 25% in system 4 and 10% in system 5, decreased collector performance and promoted thermosiphon heat loss mechanisms that

decreased system efficiency.

4.3 LOAD EFFECTS

The performance of each system depended somewhat on the hot water demand it

supplied. A large hot water load resulted in the use of more solar energy and

reduced the storage and collector temperatures, increasing system efficiency. At the sites with small loads, the largest energy flows in the water

heating system were standby tank heat losses. In these cases, additional tank

insulation would probably provide more energy savings at less cost that even

the least expensive site-built solar heater. At the sites with larger loads,

low-cost energy and water conservation measures would augment the value of the

solar heater by reducing the amount of purchased energy and would improve the

cost-effectiveness of the entire system. Hater conservation, however, will

make the solar heater appear less efficient for the reason noted above.

17
TR-1727

4.4 FUTURE WORK

The disappointing performance of the thermosiphon system suggests the necessity for future work in this area. Freeze-proof thermosiphon systems have not

been exhaustively researched, and further testing of low-cost designs would be

useful. This testing should include extensive instrumentation of the storage

component to isolate the various energy flows more precisely than was possible

in this project.

18
S=~II.I --------------------------------~:-=..~- TR-1727

SECTION 5.0

REFERENCES

1. Frey, D.; Swisher, J.; Holtz, M. 1982. "Class B Performance Monitoring

of Passive/Hybrid Solar Buildings. to Proceedings of the ASME Solar Energy

Conference; Albuquerque, NM; Apr. 1982. Golden, co: SERI!TP-254-1492.

Solar Energy Research Institute.

2. Mertol, D.; Place, H.; Webster, T. 1981.

of Liquid Solar Thermosiphons with Heat

Vol. 27 (No.5): pp. 367-86.

"Detailed Loop Model Analysis

Exchangers." Solar Energy.

3. Kornher, S.; et a l , 1981. "Solar Air Collectors."

San Luis Valley Solar Energy Association.

Alamosa, co:

4. Kline, S.; McClintock, F. 1953. "Describing Uncertainties in Single

Sample Experiments." Hechanical Engineering.

19
20
S=~II.I -------------'"---------------

APPENDIX A

DETAILED SYSTEM DRAWINGS

21
_ ·:Il~1

- ~~,

22







30
S=~I IE 1727 '~'I -------'-------------------------~::.J...L.A'_L

APPENDIX B

UNCERTAINTY ANALYSIS

The following calculations relate the estimated experimental errors of the

individual measurements to the uncertainty of the energy quantities derived

from these measurements. The uncertainty calculations are based on representative instantaneous values of each measurement. They provide a guide to

the magnitude of the uncertainties and their sensitivity to individual

measurement errors.

The derived energy quantities are integrated values based on transient

measurements made at IS-second intervals over the span of several months. A

complete time-dependent uncertainty analysis would be an enormous t ask , far

beyond the scope of this project.

The analysis shows that the calculations of the gross auxiliary energy use ,

the net auxiliary heating, and the net solar heating were relatively precise,

with calculated uncertainties of 4% to 6%. The values for the gross solar

energy collected, for both air and liquid systems t had a large uncertainty:

28% to 36%.

Basic Equation for Uncertainty of Derived Quantities*

where

result uncertainty

measured value uncertainty

partial differential of the derived result with respect to an

individual measurement

The calculated uncertainties are expressed as percentages of the derived

results for the major system energy flows. See Tables 2-1 and 2-2 for definitions of the symbols used in the following equations.

Gross Solar Energy Collected by Glycol Sys tems

= [0.02 2 + 0.02 2 + 2(0.2)2]1/2 = 0.284, or 28.4%

Notes: The specific heat has 2% error due to temperature dependence and

uncertainty of the glycol content. The high-temperature Kent flowmeter is

accurate to 2% for values between 0.22 and 22 gal/min (0.014 to 1.4 Lis). The

temperature transducers are accurate to 1°F (0 .6oC) t and a representative

temperature difference is 5°F (3°C).

*See Ref. 4.

31
TR-1727

Notes:

leakage

accurate

(l7oC) •

".' ;.

"=, ---------------------------------------

Gross Solar Energy Collected by Air Systems

8~:: = [e~p y+ 2e~~Y + 4(TC O ~TT

CiYT/2

= [0.01 2 + 2(0.25)2 + 4(0.033)2]1/2

= 0.360, or 36.0%

The specific heat has 1% error. The flow rate error is 25%, due to

and the assumption of constant flow. The temperature transducers are

to l oF (0.6oC), and a representative temperature difference is 30°F

Net Solar Heating or Net Auxiliary Heating

8~: = 8~: = [(8~:Y + e::Y + 2(T2 ~\1)2T/2

= [0.012 + 0.0152 + 2(0.025)2J1/2

0.040, or 4.0%

Notes: The specific heat has 1% error. The Kent low-temperature flowmeter is

accurate to 1.5% for values between 0.25 and 25 gal/min (0.016 to 1.6 L/s).

The temperature transducers are accurate to 1°F (0.6°C), and a representative

temperature difference across the preheat tank or the auxiliary tank is 400 F

(22°C) •

Gross Auxiliary Energy Use (Electricity)

8~:: = [e~~Y + (~V)2T/2

= [0.06 2 + 0.012]1/2

= 0.061, or 6.1%

Notes: The AMPROBE current transducer is accurate to 1 A, and a representative current is 16.7 A (4 kWat 240 V). The AC voltage has 1% error.

32
5=~1 TB-J727 i.' --------------------------------~'--" .........

-'-

Gross Auxiliary Energy Use (Natural Gas or LPG)

~Qga = [(ll.Qw)2 + (ll.tw)2Jl/2

Qga qw t w

[0.03 2 + 0.05 2]112

0.058, or 5.8%

Notes: The energy input rate is accurate to only 3% due to the errors in the

gas flow measurement and in the estimate of the heating value of the fuel.

The burner on-time error is one data scan interval (15 seconds), and a representative on-time is 5 minutes.

33
34
TR-1727 ;~*I ----~---~---------'----- -~'~

APPENDIX C

HORIZONTAL INSOLATION MEASUREMENTS (Btu/ft2 day)*

Month Alamosa

July (15 to 31) 2130

August 2040

September 1720

October 1390

November (1 to 26) 1150

San Luis

1900

1810

1410

*Btu/ft2 day x 11.348 2 kJ/m day.

35
Oocument Control 11. SERI Report No. 12. NTIS Accession No.

o Page SERI/TR-254-1727

4. Title and Subtitle

Measured Performance Results: Low-Cost Solar Water

Heating Systems in the San Luis Valley

7. Author(s)

Joe1 S\I/i sher

9. Performing Organization Name and Address

Solar Energy Research Institute

1617 Cole Boulevard

Golden, Colorado 80401

12. Sponsoring Organization Name and Address

15. Supplementary Notes

3. Recipient's Accession No.

5. Publication Date

January 1983

6.

a. Performing Organization Rept. No.

10. Project/Task/Work Unit No.

1119.70 11. Contract (C) or Grant (G) No.

(C)

(G)

13. Type of Report & Period Covered

Technical Report

14.

16. Abstract (Limit: 200 words)

This report summarizes the measured performance of seven low-cost solar water

heating systems in the San Luis Valley of southern Colorado. During the summer

and fall of 1981, SERI monitored a variety of low-cost solar water heating

system designs and components. Five systems had site-built collectors, and

four included low-cost tank-in-jacket heat exchanger/storage tank components.

Two were air-to-water systems. The five liquid-based systems included a draindown design, a propylene glycol-charged thermosiphon system, and three pumpedglycol systems. The pumped-liquid systems performed the best, with system

efficiencies greater than 20% and solar fractions between 40% and 70%. The airto-water systems did not perform as well because of leakage in the collectors and

heat exchangers. The thermosiphon system performed at lower efficiency because

the collector flows were low.

17. Document Analysis

a.Oescriptors Colorado; Comparative evaluations; Data acquisition systems; Solar

water heaters

b. Identifiers/Open-Ended Terms San Lu i s Va 11 ey; Site - bu i It sol ar water heat i ng syst ems

c. UC Categories

59a

18. Availability Statement

National Technical Information Service

U.S. Department of Comme~ce

5285 Port Royal Road

Sprinqfield. Virq;nia 22161

Form No. 0069 (3-25-82)

19. No. of Pages

44

20. Price

$4.50