SERI/TR-721-575 

UC CATEGORY: UC-59 

ANNUAL-CYCLE THERMAL ENERGY 

STORAGE FOR A COMMUNITY SOLAR SYSTEM: 

DETAILS OF A SENSITIVITY 

ANALYSIS 

FRANK BAYLIN 

ROSEMARY MONTE 

SANFORD SILLMAN 

JULY 1980 

PREPARED UNDER TASK No. 5525.00 

Solar Energy Research Institute 

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FOREWORD 

This document is part of a coordinated effort at the Solar Energy Research 

Institute (SERI) to examine all aspects of energy storage technologies having applications in solar systems. Storage systems are perceived as being 

critically important to many solar energy applications. 

This research examines applications of annual-cycle thermal energy storages (ACTES) to solar space heating and domestic hot water systems. This 

effort is the forerunner of a more thorough analysis of the value of ACTES 

technologies (such as large constructed tanks, aquifers, in-ground pits, and 

solar ponds) to solar energy systems. The data in this report are designed 

to aid the planning efforts of the Chemical and Thermal Energy Storage 

program in the Office of Advanced Conservation Technologies at the U. S. 

Department of Energy. 

The authors wish to express appreciation to a number of associates who 

contributed both information and critical reviews of this document. Frank 

Hooper and his associates at the University of Toronto developed the code 

upon which this work is based. C. J. Swet, a private consultant, and 

Michael Holtz and Charles Wyman of SERI all reviewed the completed docurn ent and provided valuable advice. 
5 TR-575 

SUMMARY 

This report presents results and conclusions of a simulation and sensitivity analysis of 

community-sized, annual-cycle thermal-energy-storage (ACTES) solar energy systems. 

The analysis which is based on an hourly simulation is used to (1) size systems in 10 

locations, (2) identify critical design parameters, and (3) provide a basic conceptual 

approach for future studies and designs. This research is a forerunner to an economic 

analysis of this particular system (based on large constructed tanks) and a general 

analysis of the value of ACTES technologies for solar applications. 

Systems were sized for 10 locations in the United States. Three different building types 

(single family residences, multifamily residences, and apartment buildings) and four different community sizes (50-, 200-, 400-, and 1,000-unit sizes) were modeled. All designs 

used each of two collector types (flat plate and evacuated tube) at each of two different 

tilt angles. In all, 440 systems were sized. . 

Two linear relationships were derived which simplify system sizing. The average ambient 

temperature is used to determine average yearly collector efficiency. This parameter 

combined with estimates of space/DHW loads, storage/distribution losses, and total 

yearly insolation per square meter allows estimation of collector area. Storage size can 

be estimated from the winter net load which is based on space and DHW loads, storage 

and distribution losses, and collector solar heat gain for the winter months. 

These algorithms, which would be applicable to other types of annual storages such as 

aquifers, can be further refined.as results from the operation of ACTES solar systems 

become available. Calculations also can be refined with more detailed knowledge of a 

particular community design. 
TR-575 

TABLE OF CONTENTS 

1.0 Introduction ........................................... '. . . . . . . . . . . . . . 1 

1.1 

1.2 

1.3 

Scope of Study ................................................. . 

Advantages and Disadvantages of ACTES ••••••••••••••••••••••••••• 

1.2.1 

1.2.2 

Advantages .••••.••••••.••••••••.••.•.••••••••••••••.••.•• 

Disadvantages •••••.••.••• •. ' •.•.•.••••••.••••••••.••••••• 

Factors in ACTES Performance and Economics ••••••••••••••.••••••• 

1.3.1 

1.3.2 

1.3.3 

1.3.4 

1.3.5 

1.3.6 

1.3.7 

1.3.8 

1.3.9 

Stratification and Heat Loss •••••••.••••.••••.••••••••.••••• 

Transmission Loss •••••••••.•••••••• 

Soil Conductivity ••••••••••••••••••• 

Reflectors and Other Methods for Enhancing Collector 

Performance .................................. . 

Insolation Profile and Building Load •••••••••••••••••••••••••• 

Passive Designs .......................................... . 

Collector Storage Trade-off .•••••••••••.•••••••••••.••••••• 

Collec tor Design ......................................... . Ownership Options ........................•............... 

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2.0 Research Methods .....•....•..•....•............•. '. . • . . • . . . . . . . . • . . . . 11 

2.1 

2.2 

Variables ...................................................... . 

2.1.1 

2.1.2 

Unconstrained Variables ................................... . 

2.1.1.1 

2.1.1.2 

2.1.1.3 

2.1.1.4 2.1.1.5 2.1.1.6 

Community Size and Housing Type •.••••••••••••••••• 

Geographic Location .................. ............. . 

Collector Typ e ..........•..•...........••.......•• 

Collec tor Tilt .................................... . 

Soil Conductivity ................................. . 

Design Ambient Temperature 

2.1.1. 7 Insulation Thickness •••••••• 

Constrained Variables ••••••.•.•••••• 

2.1.2.1 Transmission Loss ................................ . 2.1.2.2 Heat Load Factor ..........•.........•..•........• 

2.1.2.3 Domestic Hot Water (DHW) Delivery Temperature ••••• 2.1.2.4 Maximum Design Tank Top Temperature •.••••••••••• 

2.1.2.5 Inlet Temperature to DHW System ••••••••••••••••••• 

2.1.2.6 Thermostat Setting •••••••••••••••••••••••••••••••• 

Sizing Procedure .............................. . 

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______________________________________________________ 

TABLE OF CONTENTS (concluded) 

3.3.4 Storage Volume/Collector Area versus Community Size •••••••• 25 

3.3.5 Solar Energy Stored versus Community Size. . • • . • • . • • • • • • . • • • • 25 

3.3.6 Storage Loss/Energy Stored versus Community Size. • • • • • • • • • • • 26 

3.4· Annual versus Diurnal (Daily) Storage. • • • . • • . . • • • • • • • • • • • • • • • • • • • • • 26 

4.0 Conclusion ......................................................... 47 

5.0 References ......................................................... 49 

Appendix A. Sim ulation Program Description. • . . • • • • • • . • . • • • • • . . • • • • . • • • • • • • 51 Appendix B. Graphs of Variables versus Community Size.. • . • • . • • • . • . . • • • . . . • • 53 
____________________________ 

LIST OF FIGURES 

Page 

1-1 Month-by-Month Insolation and Heat Loads.. •• •••• •• •• . •• •• •• •• • ••• • • ••• 2 

1-2a, b Month-by-Month Collector Gain, Heat Load, Efficiency 

and Temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3,4 

1-3 General Classification of Storage Technolgies • • • • • • .• • • • • • • • • • • • • • • • • • • • • 6 

1-4 Average Daily Horizontal Insolation by Month for Stockholm, 

Sweden, and Kenya ••••.••••..••••••••••••••..•••••••••••••••• '. • • • • . 9 

2-1 Insolation Map of the United States..................................... 12 2-2 Location of Cities in Study •••••••••••••••••••••••••••••• :... • • •••••••• 12 

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2-3 Relationship between Insolation, Latitude, and Degree Days 

for Cities in Study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

2-4 Collector Performance Curves. •• •• • .•• •• • • • • • • . • • • •• •• ••••• •• • •• •• • ••• 14 

2-5 Performance Isoquants for Annual Storage Systems 

in Wisconsin........................................... .... 18 

2-6 Hypothetical Plots of Cost versus System Size for Isoquants 

in Figure 2-5 for Two Different Collector and Storage Costs •••••••••.••• 18 

3;...1 Collector Area per Unit versus Annual Load per Unit Divided 

by Insolation per Square Meter for Single-Unit Buildings ••••••••.•••• • • • • 27 

3-2 Collector Area per Unit versus Annual Load per Unit Divided 

by Insolation per Square Meter for 10-Unit Buildings ••••••.••••••••••••• 28 

3-3 Collector Area per Unit versus Annual Load per Unit Divided 

by Insolation per Square Meter for 200-Unit Buildings. •• • . • . • • • •• • • • • •• • 29 

3-4 Average Operational Collector Efficiency versus Total 

Insolation per Square Meter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 
55'11*1 ____ ___________________ 

LIST OF FIGURES (Concluded) 

Page' 

3-9 Average Overall Collector Efficiency versus 57 - < T A> 

Divided by Total Insolation (Tilt = Latitude Plus 10) •••••••••••••••••••• 35 

3-10 Storage Volume per Unit versus Winter Net Load per 

Unit: Single-Unit Buildings ................... '. . . . . . . . . . . . . . . . . . . . . 36 

3-11 Storage Volume per Unit versus Winter Net Load per 

Unit: 1 a-Unit, Buildings ............................................ 37 

3-12 Storage Volume per Unit versus Winter Net Load per 

Unit: 200-Unit Buildings ........................................... 38 

3-13 Storage Volume per Unit versus Annual Load per Unit: 

Single-Unit Buildings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 

3-14 Storage Volume per Unit versus Annual Load per Unit: 

lO-Unit Buildings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 

3-15 Storage Volume per Unit versus Annual Load per Unit: 

200-Unit Buildings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 

3-16 Storage Volume per Unit versus Winter Building Load Plus 

Storage Losses per Unit: Single-Unit Buildings •••••••••••••••••••••••• 42 

3-17 Storage Volume per Unit versus Winter Building Load Plus 

Storage Losses per Unit: 10-Unit Buildings ••••••••••••••••••••••••••• 43 

3-18 Storage Volume per Unit versus Winter Building Load Plus 

Storage Losses per Unit: 200-Unit Buildings. • • • • • • • • . • • • • • • • • • • • • • • • • • 44 

3-19 Solar Yield without Annual Storage versus Winter Insolation 

to Annual Insolation Ratio. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 

3-20 Collector Size for a Diurnal Solar System (96% Solar Fraction) 

versus Winter Insolation to Annual Insolation Ratio.. • • • • . • • • • • • • • • • • • • • 46 
--,. __________________________ 

LIST OF TABLES 

Page 

2-1 Total Insolation and Degree Days by City. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 14 

3-1 Design Variables under Consideration .••••••••••••••••••••••••••••••••• 20 

3-2 Graphic Representations ............................................. 20 

3-3 Winter Collector Efficiencies. • • • • • . • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 23 

3-4 Unit Collector Area Requirem ents. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 24 

3-5 Storage Tank Volumes for 1,000-Unit Complexes ••••••••••••••••••.••••• 25 
TR-575 

SECTION 1.0 

INTRODUCTION 

Most active solar systems are optimized to provide approximately 50-70% of the space 

heating load; a full-:sized backup power source may be required to supplement the solar 

system. The capital cost of the solar system and the auxiliary supply must be more than 

offset by fuel savings so that this option may be economical. If electric heat is used as a 

backup, the most common form in present solar homes, auxiliary heat is occasionally 

required at peaks in the electric utility demand profile. In such cases, the real cost that 

the utility must pay for this backup power is likely to be greater than the market price. 

Solar systems designed "to supply 100% of space-heating loads using diurnal thermal storage are poor economic choices; the extra storage capacity and storage volume required 

to supply heating for those rare but extended periods of cloud cover would be underutilized at most times. Annual cycle thermal energy storage (ACTES) systems are an alterative method of utilizing solar energy to provide 100% of space-heating loads. 

An ACTES system employs a very large storage-typically enough to supply a month or 

more of space heat-and stores heat collected during the summer and fall to help supply 

the winter peak load. Such a system increases, rather than reduces, collector utilization 

while extending the proportion of heat provided to 100%. Because the cost of large-scale 

storage to store a given amount of energy can be substantially less than the cost of -collectors to supply the same amount of energy, incorporating an ACTES into building 

designs may allow economical use of 100% solar energy systems by permitting use of 

excess summer insolation to meet winter heating loads. Penetration of solar energy 

technologies into certain sectors of the buildings' heating and cooling markets may well 

be radically hastened by use of such long-term energy storages [l]. 

The need for seasonal storage is depicted in Figure 1-1, which shows m onth-to-m onth 

variation in load and insolation for both a northern city (Madison, Wisc.) and a southern 

city (Phoenix, Ariz.). The load shows a very sharp winter peak when insolation drops 

off. The load profile shows less of a peak for an apartment complex (HUB 200,. 2 hundred 

unit buildings) than for single family houses (SUB 50, 50 single-unit buildings) because the 

hot water load makes up a greater proportion of the total load for an apartment 

complex. Both load and insolation vary less throughout the year in Phoenix than in 

Madison, but even in Phoenix the discrepancy between load and insolation is large enough 

that ACTES systems may be feasible for large-enough communities. 

Figures 1-2a and 1-2b outline the simulated operation of a seasonal storage system for 
- TR-575 

20 Madison 20 Phoenix 

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'" TR-575 

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TR-575 

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Phoenix where efficiency remains above 20%. The low winter collector efficiency is 

another important reason for investigating seasonal storage, especially in northern cities. 

A wide variety of devices for storing thermal energy are presently available or are under 

development . [2]. A general classification of such technologies is presented in 

Figure Because of economics of scale in both cost and efficiency, it may be advantageous to build common, district storage systems to serve 50 or more housing units. 

Aquifers, large constructed tanks, and earth beds are potentially feasible· and attractive 

technologies. Aquifer storages which may allow storage of thermal 

energy are actively being developed in a program managed by Battelle Pacific Northwest 

Laboratories for the Office of Advanced Conservation Technologies in the U.S. 

Department of Energy (DOE). However, aquifer storages may not be feasible in certain 

locations. Large constructed tanks for use in ACTES have already been built at two 

locations in Canada by a team from the University of Toronto [3,4] and at one location in 

Sweden [5], and they are being designed in Canada [3,41, Sweden [5,6], and France [7]. 

Problems may be encountered with underground constructed tanks if the water table is 

high or if a high flow velocity is required for a solar system. 

1.1 SCOPE OF STUDY 

An analysis based on an hourly simulation of an ACTES solar system is used to (1) size 

systems in 10 locations, (2) identify critical design parameters, and (3) provide a basic 

conceptual approach for future stUdies and designs. 

The computer code was developed at the University of Toronto by Hooper and his associates [8]. The storage is a large, cylindrical, constructed water tank. Three different 

building types (single-family residences, multifamily residences, and apartment 

buildings), and four different community sizes (50:-, 200-, 400- and 1,000-unit sizes) were 

modelled in 10 geographic locations. In addition, systems having two collector types (flat 

plate and evacuated tube) at two different tilt angles were designed. Soil conductivity 

was varied for one of the configurations. An optimization method (see Section 2.0 for 

details) was used with this code to size the storage and the collector field for all 440 

configurations as well as to provide a daily record of parameters, such as 

efficiency storage-tank temperature and building load. 

This particular computer code was used because it was the only one available in North 

. America, it has been validated already in one demonstration project, Provident House [3], 

and it has been used to design a second larger facility, the Alymer community [4]. By 

using SERI support services and the University of Toronto code, an extensive and 

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Generic Category 

Category 

Containment 

Storage 

Durations 

Technologies 

Sensible Heat I 

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ShortAnnual term '-

fROCk 

Bins 

Aquifers t 

Solar Ponds 1 Earth 

Beds Large Tanks 

or Ponds 

Water 

Tanks Earth or 

Rock Beds 

Phase Change Materials 

Macro 

Encapsulation 

Organic Materials 

"Reversible Chemical 

Chemical 

Heat 

Pumps 

Figure 1-3. General Classification of Thermal Energy Storage Technologies. 
5 /.=, : TR-575 

cooling loads allow for the design of solar systems that meet 100% of all loads. The 

advantages and disadvantages, listed below, accrue from both the ACTES concept and 

the incorporation of district solar heating. 

1.2.1 Advantages 

• The collector area can be reduced because collector utilization is markedly 

improved. Use of well-designed homes or buildings may further reduce requirements for solar collectors. 

• Overall collection efficiency improves because collector stagnation in summer is 

all but eliminated. ' 

• As the storage size increases, the unit cost of storage decreases. 

• As the storage size increases, the unit heat losses decrease and storage efficiency increases because the surface area-to-volume ratio of the container 

decreases proportionately to the increase of the radius. 

• Statistical averaging of demand in a community increases system reliability. 

• Shared loads can decrease overall energy requirements. In fact, some welldesigned single-family passive-designed residences or large commercial buildings 

are actually, on occasion, energy sources. For communities in which none of the 

buildings are energy sources, shared loads may only decrease overall power- requirements. 

• Unit costs for energy-moving equipment may be less expensive on a large scale. 

• Operation and maintenance is an expense shared by community residents. 

• A community system may have the financing and tax advantages of a utility. 

1.2.2 Disadvantages 

• An energy distribution system with all the incumbent thermal losses and water 

freezing problems is required. 

• Energy losses from storage are larger over a longer period of time. 

• Large capital outlays are required for the storage component because ACTES 

systems are larger per unit of load than those for diurnal systems. 
TR-575 

1.3.1 stratification and Heat Loss 

For a given volume, the more closely .a storage tank approaches a spherical shape, the 

lower the heat losses are, if other factors such as tank insulation and soil conditions are 

held constant. Thus, a cylindrical tank with radius, r, equal height, h, (minimize the 

surface area to volume ratio, 2/h + llr, given that volume is constant, and h = r results) 

is optimal. However, in general the smaller the diameter to height ratio of a cylindrical 

tank, the more the fluid will be thermally stratified. With these considerations in mind 

and with the use of stratification enhancers such as tank baffles [91, an optimum design 

point will be found in trading off height and diameter. 

A recent Canadian study [101, which shows annual storage to be presently uneconomical, 

uses the worst possible tank shape-a very flat cylinder. In designing a system, a stratified, high-efficiency storage should be used. Generally, the larger the thermal utility, 

the lower the unit cost of storing energy. 

1.3.2 Transmission Loss 

Transmission in larger community systems is an important design consideration. 

These losses, as well as storage losses, determine the overall thermal efficiency of 

storage and directly affect the size of collector fields. An inefficient system is not likely to be economical. The type of collector, operating temperatures, soil temperatures 

and moisture content, distribution system configuration, and type and quantity of insulation determine the transmission losses. Such factors are more important in larger, 

more dispersed communities that require additional piping and controls. 

1.3.3 . Soil Conductivity 

The drier the soil surrounding the storage and distribution system, the lower the heat 

losses will be, if other factors are held constant. Soil moisture content varies from 

location to location. A factor in locating an ACTES system is, therefore, the soil characteristics. In those cases (e.g., in many parts of Florida) where the water table is shallow and interferes with either the storage tank or the distribution system, difficulties 

will be encountered. 

1.3.4 Reflectors and Other Methods for Enhancing Collector Performance 

In some climates, winter snow cover will increase solar gain especially when collectors 

are steeply inclined. Designs in new construction may increase system performance and 

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and Sweden. These conclusions depend on system size. A higher load will require larger 

storage to have lower associated unit-storage cost. 

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Annual (9.31 x 109j/m2/yr) Average 

Stockholm 

Annual (3.68 x 109 j/m2/yr) Average 

Figure 1-4. Average Daily Horizontal Insolation by Month for Stockholm and Kenya. 

The seasonal variation in heat load may also affect the desirability and the sizing of 

ACTES systems. As shown in Figure 1-2, buildings in which domestic hot water load is 

larger and space heat load is smaller have a flatter seasonal load profile and, therefore, 

require a smaller ACTES system. 

1.3.6 Passive Designs 
TR-575 

-------------------------------------

1.3.7 Collector Storage Trade-off 

When designing an ACTES system to meet a specified load or load percentage, a tradeoff exists between collector and storage size-increasing storage size to a certain point 

allows use of a smaller collector field. The exact trade-off between collector and storage is determined by the relative cost of each component. One uniform sizing algorithm 

was used for all systems presented here (see Section 2.2). 

1.3.8 Collector Design 

Flat-plate collectors (FPC) are designed for a medium-range temperature operation. 

Therefore, use of such .collectors would produce only moderate winter energy collection 

and necessitate use of either larger or more-efficient storages. However, FPC can be 

rotated or tilted at various angles that optimize performance at different times of the 

year. 

Evacuated-tube collectors (ETC) have higher efficiency at lower ambient temperatures. 

Therefore, fewer collectors are needed to provide a given amount of heat. Collectors 

could be placed on or near the actual storage site, allowing the facility to be utility 

managed and owned. The present disadvantage of ETC is higher initial cost, although in 

some applications ETC have lower life-cycle delivered energy costs than FPC. 

1.3.9 Ownership Options 

These systems can be owned by builders, community groups, utilities, or others. Operation and maintenance will be effected best by trained personnel. Larger systems may 

have to be managed by groups with more technical expertise. 

Solar collectors can be placed on individual dwellings or located at a central solar plant. 

Difficulties may arise in some instances. For example, if solar collectors are situated on 

individual buildings and owned by residents but the energy derived from the collectors is 

owned and managed at the central storage and control location by a utility, then residents may be legally able to disconnect their collectors from the main feed lines (by 

either turning a valve or planting a tree in front of their homes). Such an individual 

action would place a burden on other members of the solar community. 
______________________ ____________________________ 

SECTION 2.0 

RESEARCH METHODS 

This study is based on a code developed at the University of Toronto [3,41. A brief 

description of methods used in the computer program is provided in Appendix A. Section 

2.0 describes the input and output, the independent and dependent variables, and the 

procedure used in sizing the components of the ACTES solar system. In addition, 

preliminary designs are presented for the various configurations. These plans are used in 

more detail in the economic analysis in the second volume of this study. 

VARIABLES 

Variables are grouped into two categories. Unconstrained variables are major elements 

of the sensitivity analysis. Constrained variables are preselected. 

2.1.1 Unconstrained Variables 

The following parameters were varied: community size and housing type, geographic location, collector type, collector tilt angle, soil conductivity, design ambient temperature, and insulation thickness. 

2.1.1.1 Community Size and Housing Type 

Several community sizes and housing types are examined. Single-family detached homes, 

10-unit condominiums, and 200-unit apartment complexes provide a range of building 

types and are judged to be representative of current U.S. housing trends. Community 

sizes are varied from 50 to 200, 400, and 1,000 units. Thus, a total of 11 configuratIons 

(3 x 4 minus the excluded 50-unit apartment complex) are considered. 

The choice of building configuration is based on those in the recent OTA report on 

solar energy [11]. residences of 2,000 ft and 10-unit 3-bedroom condominiums with a 1,300 ft total area were modelled. The 2g0-unit apartment complex was 

10 st02ies consisting of 160 one-bedroom units of 850 ft and 40 two-bedroom units of 950 ft • 

2

1

1

2 Geographic Location 
TR-575 

-------------------------

Legend 

500 & Ov!:tr Mean Daily Solar 

_ Radiation (Langleys) Annual 

450-499 Mean Daily Solar 

Radiation (Langleys) Annual 

400-499 Mean Daily Solar 

Radiation (Langleys) Annual 

I1TTTTTI 350-399 Mean Daily Solar IllllllJ Radiation (Langleys) Annual 

m 300-349 Mean Daily Solar 

Radiation (Langleys) Annual 

Figure 2-1. Insolation Map of the United States. 

so· 
5=~11_1 

-

___ ---:-__________ ~ __________ .:T~R:=-::57~5 

1:l 

<D 

::J 

9000~ 

8700 -

-

co 8400 

~ 

II 8100 

1:l 

2 7800 

i= ..... 

~ 

7500 

<D co 

u <D 7200 co>- -

....... 

~~ 6900 

::J~ CfJ_ 6600 c: 

0 

c: 

0 

'':; 

~ 

0 

II) 

c: 

ALB 35.5° 

DOD 37.5° 

SAN 

35° 

47° 

GRE SIS 47° 

MED 

43° 

300 900 1500 2100 2700 3300 3900 4500 5100 5700 

Degree Centigrade Days 

Figure 2-3. Relationship between Insolation, Latitude, and Degree Days 

for Cities in Study 

5=~11_1 

-

___ ---:-__________ ~ __________ .:T~R:=-::57~5 

1:l 

<D 

::J 

9000~ 

8700 -

-

co 8400 

~ 

II 8100 

1:l 

2 7800 

i= ..... 

~ 

7500 

<D co 

u <D 7200 co>- -

....... 

~~ 6900 

::J~ CfJ_ 6600 c: 

0 

c: 

0 

'':; 

~ 

0 

II) 

c: 

ALB 35.5° 

DOD 37.5° 

SAN 

35° 

47° 

GRE SIS 47° 

MED 

43° 

300 900 1500 2100 2700 3300 3900 4500 5100 5700 

Degree Centigrade Days 

Figure 2-3. Relationship between Insolation, Latitude, and Degree Days 

for Cities in Study 
!.I 

_______________________ ----=T=R:..--5=7'""""5 

Table 2-1. TOTAL INSOLATION AND DEGREE DAYS BY CITY 

Total Insolation on Average 

Tilted SurIace = Lat Total Degree Ambient 

City (Mj/m /year) Centigrade Days Temperature f' C) 

Caribou 5239 5313 4.2 

Bismark 6205 4995 5.6 

Madison 5698 4191 8.0 

Great Falls 6262 4158 7.8 

Boston 5138 3256 10.5 

Dodge City 7503 3044 12.3 

Medford 6085 2991 11.4 

Albuquerque 8754 2587 13.3 

Santa Maria 7401 2032 13.2 

Phoenix 8719 897 22.1 

2.1.1.3 Colleetor Type 

Two collector types are examined in this study, evacuated tube collectors (ETC) and a 

medium-performance flat-plate collector (FPC). Efficiency curves used for these 

lectors are shown in Figure 2-4. 

6" 80 c: . 

. 

u 

;;: 

-

UJ 

... 

______ __ Evacuated 

"0 Tube Collector 

() 

.02 

Flat Plate 

Collector 

.04 .06 .08 .10 .12 

m2.oC 

T [watts] 
!.I 

_____________ -.,-________ --+TRa;-"".;SH-75 

2.1.1.5 Soil Conductivity 

Substantial variation in soil conductivity strongly affects the thermal efficiency of 

ACTES. Soil conductivities and thermal capacities were varied in one configuration to 

represent a range of conditions ranging from very dry to very damp soils. The following 

were used as baseline values judged to be representative of soil conditions in North 

America: 

• soil thermal conductivity-I.7307 W /mo C, 

• soil density-1762.0 kg/m3, and 

• soil thermal capacity-I. 0 kJ /kg 0 C. 

2.1.1.6 Design Ambient Temperature 

The design ambient temperature is a design device developed by ASHRAE [12] used to 

ensure that the space heating system is adequate to maintain comfort levels for all but 

• the most extreme weather conditions. A value was chosen for each of the 10 locations so 

that the outside ambient temperature would remain above the design ambient 

temperature for 99% of the time during a normal winter [12]. Use of this value allowed 

the storage capacity and collector field area to be chosen more accurately for the initial 

computer run. 

2.1.1.7 Insulation Thickness 

The storage tank insulation is distributed by the University of Toronto code to minimize 

tank heat losses. This distribution counteracts the tendancy of the upper of the 

partially stratified tank to have greater heat losses. The total amount of wall and floor 

insulation was limited to a maximum thickness of approximately. 9 in. (0.22 m). The limit 

was required because layers of greater thickness on the tank interior would not be stable 

structurally and could possibly creap at the elevated tank temperatures. 

The lid insulation thickness was chosen as a uniform 13 in. (0.333 m) for all sites. This 

value was chosen with respect to physical and structural considerations. 

2.1.2 Constrained Variables 

A variety of parameters were chosen which have either a fixed value or a value that 
______________ 

complexes were assumed to have losses of 10%, 5%, and 0% respectively added to the community heat-load factor (see below). The ACTES was assumed either to be integral 

to or adjacent to the apartment complex. The single-family community has substantially 

more piping than the multifamily grouping. 

2.1.2.2 Heat Load Factor 

The heat load factor was used to determine building energy load. When coupled with the 

hourly weather data, this factor provided a calculated building load. The heat 

load factor for a single-family residence of 2,000 ft feet was chosen to be 500 

Btu/degree hour, based on a recent SAl study [I3] and a personal communication with 

Steve Hogg of SERI. The value for the multifamily condominium based on the OTA study 

[11] was 202 Btu/degree hour per unit (this is an average since the units on the end of the 

building with more exposed surface area will have higher heat losses than the middle 

units). The heat load factor for the apartment complex was 25,748 Btu/degree hour, or 

130 Btu/degree hour per unit [11]. 

2.1.2.3 Domestic Hot Water (DHW) Delivery Temperature 

The DHW delivery temperature was chosen to be 120°F-lower than the normal 140°F 

but still in a perfectly functional range. This temperature was selected for two 

reasons. First, this lower temperature allows attainment of a more nearly 100% solar 

system. Other designs (for example, one has one ACTES tank and multiple DHW tanks 

which would be charged first[I41) would easily permit attainment of 100% solar 

systems. Second, the lower temperature maintains the philosophy of this study-use of 

renewable energy sources and conservation of energy. 

2.1.2.4 Maximum Design Tank Top Temperature 

The maximum-design tank-top temperature was chosen to be 175°F (79.4°C). This temperature is well within present limits of plastic liners for storage tanks, and it places less 

stress on tank insulation and on piping than higher temperatures WOUld. It is also the 

maximum design temperature of the Lyngby home in Denmark, an ACTES design that is 

currently operating [I 5] . -Ii. 

2.1.2.5 Inlet Temperature to DHW System 
______________________________________________________ 

2.2 SIZING PROCEDURE 

Once all the input parameters were chosen, the ACTES systems were thermally optimized, viz. minimum storage volume and collector areas which adequately supplied the 

load were chosen. However, as noted above, the trade-off between collector field area 

and storage size was not examined in detail. The sizing method used in this study 

selected the smallest systems that provided 100% space heating and that avoided 

dumping collected heat during the summer. No cost considerations were included except 

in the "fine tuning" of systems during which the decision was typically to increase either 

collector or storage size. In such cases, storage size was always chosen as the least-cost 

alternative for the following reasons. 

This method represents a natural sizing optimum because a larger storage tank would not 

provide any extra useable storage capacity. It is also consistent with standard 

assumptions about sizing annual storage systems by others [4,17]. System sizing trade 

off, in preliminary results from research underway at SERI, plots as an inflection point 

between two roughly linear regions in the performance isoquants (Figure 2-5). 

Hypothetical cost .curves along the isoquant are shown in Figure 2-6. Depending on the 

relative cost of collector and storage, the economic optimum may occur with daily (2-6 

days), intermediate (4-7 weeks), or seasonal storage (3-4 months)[l6] • 

Before actually running the simulation, a rough size estimate was made. A short computer program based on a correlation developed by James Cook at the University of 

Toronto was used. 

Next, using these estimates, a simulation was performed and results of the run were 

examined to determine whether the proposed configuration provided 100% solar space 

heating. Next, the tank temperature distribution was examined to determine (1) whether 

the maximum design tank temperature (79.4°C) had been reached; i.e., was the storage 

tank utilized efficiently or was the tank too large; and (2) whether the minimum design 

tank temperature had been reached (28° C); i.e., was the collector field too large. New 

estimates for collector area and storage size were made based on this information, and 

the simulation was rerun. 

The work was arranged by site. All 44 cases for each site were completed before sizing 

of the next site began. 
TR-575 i*1 ---------------------------

140 

130 

120 110 I 

100 I 

Daily Storage Optimum 

I 

I 

I Intermediate / Storage 

/' Optima 

/ ",/'" 

90 I / ,/ 

E / """ 

0 

0 0 

:::.. 

Q) 

N 

Ui 

'-

0 

ti 

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80 I 

I 

70 f" 

60 

50 I 

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40 

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/ 

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/ "" / ,/ 

/' ,/"" 

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-

30 /,/ ---

I / / /' 

-

20 / ,/,/ --

_--

10 _-- I /. _-

"'_ Sub 50 

-

1 0 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 

Storage Size (1000 m3) 

Sub 400 

• 

'-" . -" .:.;r",," .. 

Figure 2-5. Performance Isoquants for Annual Storage Systems in,Madison, Wisconsin 

Systems presented are designed to supply 100% of space heat requirements and 80% of hot water. 

(jj 

o 

U 

8 Sizing selected 

or study 

I 

I 

I 

I 

I 

(jj 

o 

U 
______________________________________________________ 

SECTION 3.0 

RESULTS 

An analysis of the baseline designs for ACTES solar systems generated from the University of Toronto computer code are pres.ented in this section. The data serve three 

purposes. First, sizes of storage and collector components are presented for 44 community designs in 10 geographic locations. Second, these results form the basis for a sensitivity anru.ysis that is used to identify the critical design parameters in such an ACTES 

solar system. * Third, the results enable us to present general guidelines for sizing 

ACTES systems in any location. 

3.1 PROCEDURE 

Results are organized as follows. Critical factors in sizing collector field area and storage tank volume are analyzed in Section 3.2. The sensitivity of design parameters to 

community size are investigated in Section 3.3. These considerations are used in a comparison of annual versus daily cycle storage/solar energy systems (Section 3.4). All 

figures for Section 3.0 are placed at the end of the section, pages 27-46, for better 

readability of text. 

Table 3-1 lists the design variables in this study. All explanatory graphs are presented 

either in the text or in Appendix B. Table 3-2 lists those relationships which were graphed. A number of proxies were used for location including total yearly insolation, yearly 

building load, and total degree days. In all the graphs, the following abbreviations are 

used: 

• SUB: Single-unit building; single residence; 

• TUB: lD-unit building; multifamily residence; and 

• HUB: 200-unit building; apartment complex. 

3.2 ANALYSIS OF SYSTEM SIZING 

In this section, patterns and regularities are examined in the simulated system sizes 

which point toward general system sizing ·algorithms. System sizing will be approached 

in two stages: collector area is sized to meet the yearly load, and storage is sized to 
__ ________________________________________________ 

Table 3-1. DESIGN VARIABLES UNDER CONSIDERATION 

Independent Variables: 

Dependent Variables: 

Total insolation 

Yearly degree days 

Total building or community load 

Community size 

Community typea 

Collecto.r type 

Collector inclination angle 

DHW load 

Collector area 

Solar energy collected Storage volume, radius and mass Solar energy stored Storage energy loss 

A verage overall collector efficiencyb 

Average operational collector efficiencyc 

Min/max tank temperature 

DHW performance 

aHUB, TUB, or SUB. 

bTotalload plus storage and distribution losses divided by total insolation. 

cCollected energy divided by total insolation. 

Table 3-2. GRAPmC REPRESENTATIONS 

Versus community size: 

Variable 

Collector area 

Solar energy collected 

Storage volum e Storage tank radius 

Storage volume per collector area 

Solar energy stored per solar energy collected 

Number of Graphs 

10 

10 

10 

10 

10 

10 

Units 
_______________________ 

3.2.1 Collector Sizing 

The key to collector sizing is the estimation of collector efficiency. The nonlinearity in 

the plots of collector area versus annual load per unit divided by the yearly insolation per 

square meter (Figures 3-1, 3-2, and 3-3) results from variations in average collector efficiency with climate. In colder climates, the insolation per square meter tends to decrease while the load per unit tends to increase (Figure 2-3). In these colder climates, 

average collector efficiencies of evacuated-tube collectors (ETC) are higher than those 

of flat-plate collectors (FPC) (Figure 2-4). The curves for FPCs, therefore, are above 

thooe for ETCs. In warmer climates, average efficiencies of FPCs are greater than those 

for ETCs. The relationships in Figures 3-1, 3-2, and 3-3 do not show this functional 

crossover. 

Collector operating efficiency depends directly upon two factors: the intensity of insolation and the thermal difference between collector operating temperature and outdoor 

ambient temperature. Indirectly, the efficiency may depend upon a variety of climatic 

parameters. Efficiency was found to be virtually constant among all building types for a 

given location. Since the average collector operating temperature varied little among 

locations, the key parameters are ambient temperature and insolation. Figures 3-4 and 

.3-5 present the variation in annual operating efficiency, defined as the heat collected 

divided by the incident insolation, versus insolation and ambient temperature for all 10 

locations. The range for each point in the efficiency graphs indicates the variation of efficiency at a particular location across the simulated community sizes. 

Figures 3-4 and 3-5 show that, while efficiency generally does increase with insolation 

and ambient temperature, the patterns were highly irregular. An index was sought that 

reflected the effect of both insolation and ambient temperature. Such an index was determined by the familiar equation for collector efficiency: 

where 

The parameter 

collector efficiency = heat gain - heat loss 

= Fr (Ta) - FrUl To - Ta 

I 

To = collector outlet temperature, 

T a = collector inlet temperature, and 

I = insolation per square meter. 
__________________________ 

T ...... R ..... -=S"-l7--..-S 

A system-sizing algorithm would make use of the relationship: 

collector efficiency = load 

incident insolation 

The yearly heat load, including storage losses as part of the load, is estimated. Collection efficiency is from Figure 3-6 or 3-7. Collector area may be found by the formula: 

collector area = load 

efficiency x insolation/m2 

where load equals total yearly space plus DHW load plus storage and distribution losses. 

One further problem should be noted in the sizing algorithm just outlined. The overall 

collector efficiency-equal to the building load plus the storage loss divided by the incident insolation-is not exactly the same as the operating efficiency (collector gain divided by insolation) used earlier. There are two differences between the former (overall 

efficiency) and the latter (operating efficiency). First, if heat is dumped in summer, this 

heat is not counted in the overall efficiency but is counted in the operating efficiency. 

This has a very small effect. Second, collector gain is greater than the heat load because 

most systems have been oversized to provide a margin of safety. In the simulation, 

storage temperature is initially set at 34°C and often ends at 37°C or 3SoC. The extra 

heat collected to raise the tank temperature from 34° C to 37° C is credited to the collector in the operating efficiency calculation but not in the overall efficiency. 

The operating efficiency has been used in order to be consistent with results from the 

overall simulation. Use of the overall efficiency introduces inaccuracy because the 

degree of oversizing was different for different locations. Figures 3-S and 3-9 represent 

overall efficiency versus the index. Again, the pattern is linear but less consistent than 

patterns in the previous graphs. 

For system sizing, we recommend using the operational efficiency graphs (Figures 3-6 

and 3-7) and then oversizing the collector by 10%. This oversizing provides a margin of 

safety against both inaccuracy in the sizing algorithm and severe weather conditions. 

N eedle$ to say, the efficiency curves are accurate only for those collectors that perform similarly. 

3.2.2 Storage Sizing 
________________________ ____________________________ 

Figures 3-10, 3-11, and 3-12 can be used to size storage volume. Load for the four 

winter months, including storage losses, can be estimated. Collector gain for these 

months also must be estimated. This is not an easy task. Table 3-3 presents winter efficiencies for both EPCs and ETCs in all 10 cities. Winter gains may be found by multiplying winter efficiency by collector area (based on the sizing algorithm in Section 

3.2.1) and by winter insolation. The difference between load and gain determines the 

winter net load from which the storage system may be sized. 

Table 3-3. WINTER COLLECTOR 

City 

ALB 

BIS 

BOS 

CAR 

DOD 

GRE 

MAD 

MED 

PHO 

STM 

EFFICIENCIES 

(November through February) 

FPC 

.27 .13 

.165 

.13 .24 .14 

.155 .17 

.35 

'.29 

ETC 

.32 

.24 

.25 .23 

.29 

.25 

.25 

.24 

.34 

.29 

Because winter collector efficiency is hard to estimate, it is desirable to explore other 

storage-sizing algorithms. Figures 3-13, 3-14, and 3-15 depict storage size versus total 

annual load for both flat-plate collectors and evacuated tube collectors. While there is-a 

pattern of increasing storage size per load, the pattern is not consistent enough to be 

useful in design. This is especially true because the SUB, TUB, and HUB plots do not lie 

along the same line (the line drawn through the points in the SUB plot was reproduced in 

the TUB and HUB plots for comparison). This is due to the fact that winter, not annual, 

load determines the need for storage, and the proportion of winter-to-annualloads is different for the three community types. Other irregularities can be noted. Evacuatedtube collectors take smaller storage sizes than flat-plate collectors. Cities with low 

winter insolation (such as Medford, Oreg.) require larger storage volumes. 

Graphs of storage versus winter load in Figures 3-16, 3-17, and 3-18 may be more suitable for rough designs. However, these would oversize the storage system when evac_________________________ ..... 

3.3.1 Conector Area versus Community Size 

Required collector area increases linearly with community size and with building load. 

Single-unit residences that have the largest unit loads require larger collector areas. In 

cities with severe climates, substantially less area is required for evacuated-tube than 

flat-plate collectors. Graphs of the slope of the collector area versus community size 

give the collector area required per unit. These figures are tabulated in Table 3-4. 

Table 3-4. UNIT COLLECTOR AREA REQumEMENTS 

SUB TUB HUB 

City (m ) (m ) (m ) (m ) (m ) (m ) 

Caribou 158.5 111.5 74.1 52.0 43.2 30.3 

Bismark 99.0 83.8 46.3 39.2 27.0 22.8 

Madison 97.5 78.6 46.3 37.3 26.9 21.7 

Great Falls 80.0 69.4 38.4 33.2 22.2 19.2 

Boston 88.0 69.5 46.6 34.5 25.0 19.8 

Dodge City 42.6 42.3 21.1 20.6 12.8 12.2 

Medford 53.9 51.3 26.5 25.2 15.2 14.5 

Albuquerque 29.4 29.5 14.5 14.8 8.4 8.7 

Santa Maria 27.7 21.1 . 14.1 13.8 8.0 7.9 

Phoenix 11.3 13.1 6.8 7.9 11.3 4.3 

As expected, there are substantial differences in the unit collector area needed for 

various climates, different building types, and FPCs versus ETCs. 

3.3.2 Solar Energy Conected versus Community Size 

The collected solar energy is a linear function of community size. There is little difference between energy collected by different types of collectors because collector 

areas have been adjusted to match loads. A more detailed discussion of the collection of 

solar energy follows in Section 3.4. 

3.3.3 Storage Volume versus Community Size 
______________________________________ ____________ 

volume, 10,786 

m3, and a radius of 31.8 m. In colder climates, ETCs offer a substantial 

savings in storage volume. In Cfibou, the tank volume for the community of 1,000 

single 1,IJ1its is reduced by 66,500 m , or approximately 15%. 

Table STORAGE TANK VOLUMES FOR I,OOo-UNIT COMPLEXES 

(Collector Tilt = Latitude) 

HUB 

OOit 

m3) TUB 004 

m3) SpB 00 

m3) 

City ETC FPC ETC FPC ETC FPC 

Caribou 8.970 10.786 16.327 16.529 37.089 43.741 

Bismark 9.575 10.986 16.630 18.444 38.702 43.439 

Madison 7.861 8.486 13.505 14.594 32.957 35.577 

Great Fans 7.861 8.466 13.505 14.715 32.960 35.578 

Boston 5.946 6.551 9.585 10.885 24.390 26.517 

Dodge City 5.757 5.758 9.333 10.394 21.212 24.242 

Medford 6.400 6.632 11.096 11.510 26.204 27.111 

Albuquerque 3.931 4.338 6.601 7.156 16.932 18.746 

Santa Maria 2.596 2.586 4.545 4.434 10.303 10.101 

Phoenix 1.613 1.612 2.721 2.469 6.349 6.249 

3.3.4 Storage Volume/Collector Area versus Community Size 

As community size increases, the ratio of storage volume to collector area increases in 

most cases, although the increase is more rapid at the smaller community sizes. In the 

more northern locations, the ratio is smallest for FPCs because larger areas are needed 

to deliver energy at the lower operating efficiencies. In all cases in warmer climates, 

the storage volume to collector area ratio is largest for SUB. This was true in almost all 

the colder locations except where the ETC efficiency was low enough to warrant use of 

large collector areas. For example, in Caribou both the HUB (ETC) and TUB (ETC) have 

higher ratios tpan

2SUB (FPC). The range of storage-volume-to-collector-area ratio is about 2 to 7 m /m • 
- TR-575 

-------------------------------

3.3.6 Storage Loss/Energy Stored versus Community Size 

Storage loss per energy stored (one minus the storage efficiency) decreases as the community size and, in turn, the storage volume increases. This is simply an expression of . 

the fact that losses decrease as surface area to volume of storage decreases. The three 

community types, each having distinctly different storage sizes, are grouped in order on 

all 10 graphs. Efficiency of storage increases as one goes from colder to warmer 

climates. The highest yearly efficiency is over 96% in Phoenix for 10 HUBs using FPCs. 

The lowest, approximately 84%, is for TUB-40 in Medford. 

3.4 ANNUAL VERSUS DIURNAL (DAILY) STORAGE 

A fundamental question in these considerations is how do ACTES solar systems compare 

to conventional oolar systems based on diurnal storages? Although a more thorough 

answer is presently under study which examines the economics of collector-storage 

trade-offs, we can draw some preliminary conclusions here. To this end, we compare the 

ACTES solar systems designed in this study with conventional solar systems for similar 

building types (SUB) in all 10 locations. Conventional systems are sized by the F-chart 

method, assuming 75 liters of storage per square meter of each solar collector. 

The percentage of oolar heat that could be delivered by conventional systems with the 

same collector field area as the one designed for the seasonal storage systems was calculated. Figure 3-19 presents these solar percentages for the 10 locations, graphed 

versus the ratio of winter-to-annual insolation. Three observations were apparent. 

• Without annual storage, about 65% of the heat load is provided by solar energy. 

Therefore, the annual storage adds 30% additional energy and reduces or 

eliminates the need for backup equipment. 

• Annual storage provides the greatest advantage in cities with poor winter insolation. Medford, Oreg., which receives a very small percentage of its annual insolation in winter, is the city where annual storage is by far the most advantageous. Annual storage tended to be less useful in warmer climates (Phoenix, 

Albuquerque). Although the difference between these cities and cities in colder 

climates (Boston and Madison) does not appear to be very striking from this technical analysis, the difference in delivered energy costs is more pronounced (see 

forthcoming report on economic analysis). 

• ETCs improve performance of an ACTES solar system as compared with FPCs 

because ETCs operate well over the relatively large temperature differences in 

seasonal storages. An ACTES system can collect and store heat at 60-70oC, but 

ti

l 

t

t

th

t lb

t

t

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-

i: 

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ca 

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.... 

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_________________________ T.:..:R=-.....:5:.:,7..::..5 -

160 

140 

120 

80 

60 

40 

20 

a Flat Plate Collector, Tilt = Latitude 

.6. Evacuated Tube Collector, Tilt = Latitude 

o Flat Plate Collector, Tilt = Latitude + 10° 

. • Evacuated Tube Collector, Tilt = Latitude + 10° 
!;::~II~, ________________________________________________________ 

T~R~-=5:=75 

-

CJ Flat Plate Collector, Tilt = Latitude 

• Evacuated Tube Collector, Tilt = Latitude 

70 

• Flat Plate Collector, Tilt = Latitude + 10° 

v Evacuated Tube Collector, Tilt = Latitude + 10° 

60 

50 

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t:. Evacuated Tube Collector, Tilt = Latitude 

• Flat Plate Collector, Tilt = Latitude + 10° 

T Evacuated Tube Collector, Tilt = Latitude + 10° 

2 3 4 

5 

• 

6 7 8 
S=~I 1.1 _______________________ -=-T::R-....,:5:..:..::75 

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~ 

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MAD MED BIS 

6000 

/' 

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- TR-575 

-----------------------------

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120 

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CJ Ti It = Latitude + 10, Flat Plate Collector 

• Tilt = Latitude, Evacuated Tube Collector 

• Tilt = Latitude + 10, Evacuated Tube Collector 

The diagonal line is the same as the diagonal lines in the 10- and 200-unit 

graphs, 

10 20 30 40 50 60 

Winter Net per Unit (109J) 

70 

Figure 3-10. Storage Volume per Unit versus Winter Net Load per Unit: 

Single Unit Buildings
______________________ _____________________________________ T_R_-_5_7 __ 5

100 

90 

80 

:::: 70 c: 

....... 

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CD 

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• Tilt = Latitude + 10, Evacuated Tube Collector • 

• 

co 

• 

The diagonal line is the same as the diagonal lines in1the single- and 200-unit 

graphs. 

2.5 5 7.5 10 12.5 15 

Winter Net per Unit (103J) 

o 

17.5 

Figure 3-11. Storage Volume per Unit versus Winter Net Load per Unit 

10-Unit Buildings. 

Winter net load equals building load pills storage and transmission Ipsses minus collector gain
- "TR-575 

200 

o Tilt = Latitude. Flat Plate Collector 

180 Cl Tilt = Latitude + 10, Flat Plate Collector 

• Tilt = Latitude. Evacuated Tube Collector 

160 -Tilt = Latitude + 10, Evacuated Tube COllector: • : 

- 140 c: 

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Q) 80 Ol 

CtI 

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40 

20 The diagonal line is the same as the diagonal lines in the single- and 

5 10 15 20 25 

Winter Net per Unit (10sJ) 

Figure 3-12. Storage Volume per Unit versus Winter Net Load per Unit: 

200-Unit Buiidings. 

30 

Winter net load equals building load plus storage and transmission losses mil'1us collector 

gain, for the months November through February. 
- TR-575 {tll --'-------------------------------.,...---

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400 

360 320 

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/' 

/' 

/' 

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/' 

/' 

/' The dotted line provides a common reference frame among the three storage vs. annual load graohs. /' ' 

20 40 60 80 120 140 

Annual Load per Unit (109 J) 

Figure 3-13. Storage Volume per Unit versus Annual Load per Unit: 

Single-Unit Buildings (Space and Water Heat) . 
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./ 

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20 

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30 40 

Annual Load per Unit (109J) 

/' 

50 60 70 

Note: The dotted line provides a common reference frame among the three storage vs. annual load graphs. 

Figure 3-14. Storage Volume per Unit versus Annual Load per Unit: 

10-Unit Buildings (Space and Water Heat) . 
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./ 

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5 20 25 30 35 

Annual Load per Unit (109J) 

Figure 3-15

Storage Volume per Unit versus Annual L

oad per Unit: 
- TR-575 

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320 

c: $. 280 

g 

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Winter Load per Unit (109J) 

80 

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100 

Figure 3-16

Storage Volume per Unit versus Winter Building 
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220 

200 

180 

160 

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60 

40 

20 

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10 

o • 

20 30 

Winter Load per Unit (109 J) 

50 

Figure 3-17. Storage Volume per Unit versus Winter Building Load 

Plus Storage Losses per Unit: 10-Unit Buildings
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100 

90 

80 

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10 

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• • 

• • a 

The diaaonalline is the same as in the other storage vs. 

winter loao graphs. 

5 10 15 20 

Winter Load per Unit (1 09J) 

• 

• 

25 

Figure 3-18. Storage Volume per Unit versus Winter Building 

L

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Santa Maria has been separated from the line plotting the 1t 

pattern because its Singularity is due to an oversized 1\ 

seasonal storage system. 

o Flat Plate Collector 

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GRE CAR MAD 

0

200 0

225 

0

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________________________________________________________ T_R __ 

SECTION. 4.0 

CONCLUSION 

Collector field area and storage volume have been sized for 440 community designs in 10 

geographic locations. Analysis of the data has allowed identification of those parameters 

that have first-order· effects on component sizing. Storage size is determined by the difference between "winter net" and collected energy. Collector area then is sized to fully 

charge storage. 

, 

Two linear relationships were derived which allow system sizing. The average ambient 

temperature is used to determine average yearly collector efficiency. This parameter 

combined with estimates of space/DHW loads, storage/distribution losses, and total yearly insolation per square meter allows estimation of collector area. Storage size can be 

estimated from the winter net load which is basedon space and DHW loads, storage/distribution losses, and collected solar heat for the winter months. . 

The algorithms, which would be applicable to other types of annual storages such as aquican be further refined as results from the operation of ACTES solar systems become available. Calculations also can be refined· with more detailed knowledge of a 

particular community design. 

These results provide information to allow DOE program managers to identify the critical design parameters for ACTES solar systems and to more carefully target future 

activities. In addition, these data will be of use to other researchers in examining the 

feasibility of such systems. 

In order to more accurately judge the relative merits of ACTES solar systems in different climates, a more detailed systems study and economic analysis is underway. Preliminary results indicate that as the DHW-to-space-heating-load ratio decreases and as 

community size decreases, system economics become less favorable. Modifications to 

the design presented here, such as incorporating a two-tank (annual storage for space 

heating; daily storage for DHW) storage system or using multiple tanks for annual storage 

of both heat and cold [14], may be economically promising technologies. In addition, the 

trade-off between storage and collectors [16] is being studied. 

Results from both the technical and economic analyses are being used as inputs to a more 

general analysis of the value of ACTES technologies in solar systems. Aquifers, large 

constructed tanks or pits, and solar ponds are being compared. 

/.- TR-575 

------------------------------

SECTION 5.0 

1. Metz, W. D. "Energy Storage and Solar Power: An Exaggerated Problem." Science. 

Vol. 200: 30 June 1978. 

2. Baylin, F. Low Temperature Thermal Energy Storage: A State-of-the-Art Survey. 

SERI/RR-54-l64. Golden', CO: Solar Energy Research Institute; July 1979. 

3. Hooper, F. C.; Attwater, C. R. Solar Space Heating Systems Using Annual Heat 

Storage. Progress Report. COQ-2939-l. Washington, D.C.: U.S. Department of 

Energy; April 1977; pp. 5T ... 61. 

4. Hooper, F. C.; et ale Solar Space Heating Systems Using Annual Heat Storage. Progress Report. COQ-2939-6. Washington, D.C.: U.S. Department of Energy; 

October 1978. 

5. Margon, Peter; Roseen, Rutger. "Central Solar Heat Stations and the Studivik Demonstration Plant." The 13th Intersociety Energy Conversion Engineering 

Conference of the Society of Automotive Engineers. September 1978; pp. 1614-

1619. . 

6. Swedish Energy Research and Development Commission. Directory of Research and 

Development Projects within the Swedish Governmental Programme for Energy 

Research and Development, 1975/76-1977/78. DFE Report #20. Swedish Energy 

Research and Development Commission; April 1979. 

7. Torren.ti, R. Seasonal Storage in Solar Heating Systems. International Solar Energy 

Society Conference; Atlanta, GA; May 1979. 

8. Hooper, F. C.; Attwater, C. R. "A Design Method for Heat Loss Calculations for InGround Heat Storage Tanks." Heat Transfer in Solar Energy Systems: Proceedings of ASME Winter Annual Meeting. Atlanta, GA; December 1979. Washington, 

D.C.: American Society of Mechanical Engineers; pp. 39-43. 

9. Lin, E. I. H.; Liu, K. W.; Sha, W. T. Validation, Application, and Extension of a Solar Design Tool. 211 Annual Systems Simulation and Economics 

Analysis Conference, San Diego, CA; 23-25 January, 1980. 

10. Dilworth, Secora, Meagher, and Associates, Ltd. The Potential Thermal Performance 

and Economic Viability of Solar Utilities in Canada. DSMA Report No. 1041/1008; 

Ottawa, Ontario: DSMA; 4 January 1979. . 

11

Office of Technology Assessment

U

S

Congress

Application of Solar Technology 
- TR-575 

REFERENCES (concluded) 

15. Esbensen, Torben V.; Korsgaard, Vagn. "Dimensioning of the Solar Heating System 

in the Zero Energy House in Denmark." Solar Energy. Vol. 19 (No.2): 

pp. 195-199. 

16. McGarity, A. E. "Optimum Collector-Storage Combinations Involving Annual Cycle 

Storage." Proceedings of Solar Energy Storage Options Conference, Volume 1. 

San Antonio, TX; 19-20 March 1979. CONF-790328-Pl. Washington, D.C.: U.S. 

Department of Energy. 

17. Mueller, R.O.; et ale "Optimal Design of Seasonal Storage for 100% Solar Space 

Heat in Buildings." Proceedings of AAAS Symposium; Efficient Comfort 

Conditioning: Heating and Cooling of Buildings. February 1978 • 

. 
- TR-575 

APPENDIX A 

SIMULATION PROGRAM DESCRIPTION 

This program simulates the performance of an ACTES space heating and domestic hot 

water system for a single building using hourly time increments. The major components 

in the system are: the liquid-type solar collectors with an optional. reflector arranged in 

a drain-down type of configuration; right-circular cylindrical constructed tank with optimally distributed insulation for the storage of solar heat water; double-walled heat exchanger to provide domestic hot water; and water-to-air heat exchanger to provide space 

heating. 

The crux of this program is its calculation of storage heat loss. This is performed by a 

lumped parameter effective-thermal-resistance (ETR) model. This model assumes that 

the thermal properties of soil are time, space, and temperature independent. 

The tank insulation distribution subroutine is an integral part of the ETR calculation. An 

insulation distribution is assumed such that tank-conductive heat loss for a given total 

volume of insulation is minimized. 

The domestic hot water (DHW) heat exchanger operates with a user-specified temperature drop across the heat exchanger. For this study, a value of 4.5°F (2.5°C) was used. 

At this time, there is no DHW heat-exchanger simulation model in the program. The load 

distribution of DHW is a function of time of day. 

________________________________________________________ 

APPENDIX B 

GRAPHS OF VARIABLES VERSUS COMMUNITY SIZE 

The important design variables plotted against community size with building type and 

solar collector type as parameters are presented below (also see Section 3.3). (In all 

graphs that follow collector tiltwas equal to latitude.) Collector area, solar energy collected, storage volume, the ratio of storage volume to collector area, solar energy 

stored, and the ratio of storage loss for energy stored are plotted. The data presented 

here should allow first order design of an ACTES solar system in many communities with 

climates similar to those chosen for this study. 
~~r-____________ :B~OS~T~O~N~,r1~A~ ____________ -

800.0 900.0 1000.0 

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In 

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COMMUNITY 51 Z[ 

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5f~NTA MAR I A, CA 

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COMMUNITy SIZ[ 

·00 GREAT fALLS,MT 

~~,--------------------------------------------------------, 

o 

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COMMUNITy SIZ[ 

DODGE CITY,KA 

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Document Control 11. SERI Report No. 

Page TR-721-575 

NTIS Accession No. 

4. Title and Subtitle 

Annual-Cycle Thermal Energy Storage for a Community 

Solar System: Details of a Sensitivity Analysis 

7. Author(s) 

Frank Bav1in Rosemarv Monte Sanford Sillman 

9. Performing Organization Name and Address 

Solar Energy Research Institute 

1617 Cole Boulevard 

Golden, Colorado 80401 

12. Sponsoring Organization Name and 

15. Supplementary Notes 

16. Abstract (Limit: 200 words) 

3. Recipient's Accession No; 

5. PUblication Date 

JulY 1980 

6. 

8. Performing Organization Rept. No. 

10. Project/Task/Work Unit No. 

5525 

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

(C) 

(G) 

13. Type of Report & Perioa Covered 

Technical Report 

14. 

This report presents results and conclusions of a simulation and sensitivity analysis of community-sized, annual-cycle thermal-energy-storage (ACTES) solar energy 

systems. The analysis which is based on an hourly simulation is used to (1) size 

systems in 10 locations, (2) identify critical design parameters, and (3) provide 

a basic conceptual approach for future studies and designs. This research is a 

forerunner to an economic analysis of this particular system (based on large constructed tanks) and a general analysis of the value of ACTES technologies for solar 

applications. A total of 440 systems were s:lzed for 10 locations. in the United 

States. Three different building types and four different community sizes were 

modeled. All designs used each of two collector types at each of two different tilt 

angles. 

Two linear relationships were derived which simplify system sizing. The average 

ambient temperature is used to determine average yearly collector efficiency. This 

parameter comblned with estimates of .space/DHW loads, storage/distribution losses, 

and total yearly insolation per square meter allows estimation of collector area. 

Storage size can be estimated from the winter net load which is based on space and 

DHW loads, storage and distribution losses, and collector solar heat gain for the winter months. 

17. Document Analysis 

a

Descriptors: Simulation ; Energy Storage ; Annual Energy Storage ; Design ;