\. 

SERI/TR-721-898 

UC CATEGORIES: UC-59,59a,59c 

ECONOMIC ANALYSIS OF COMMUNITY 

SOLAR HEATING SYSTEMS THAT USE 

ANNUAL CYCLE THERMAL ENERGY 

STORAGE 

F. BAYLIN 

R. MONTE 

S. SILLMAN 

SOLAR ENERGY RESEARCH INSTITUTE 

F. C. HOOPER 

J. D. MCCLENAHAN 

UNIVERSITY OF TORONTO, CANADA 

FEBRUARY 1981 

PREPARED UNDER TASK No. 1013.00 

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 
,,"', TR-898 I_I -------------------------------

FOREWORD 

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

(SERI) to examine all aspects of energy storage technologies with applications in solar 

energy systems. Storage subsystems are considered critical to many solar energy 

applications. 

This research examines the economics of applications of annual cycle thermal energy 

storage (ACTES) to solar space heating and domestic hot water systems. This effort is 

based on an earlier sensitivity analysis of such systems [1] and is a forerunner of two 

more thorough studies: an analysis of trade-offs between storage volume and collector 

field area; and a general study of the value of ACTES solar 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 their appreciation to numerous associates who contributed 

both information and critical reviews of this document, particularly Michael Davis, 

Michael Holtz, and Charles Wyman of SERI. 

Approved for 

SOLAR ENERGY RESEARCH INSfITUTE 
______________________________________ 

SUMMARY 

The economics of community-scale solar systems that incorporate a centralized annual cycle thermal energy storage (ACTES) coupled to a distribution system are examined in this document. Systems were sized for 

three housing configurations: single-unit dwellings, 10-unit, and 200-unit 

apartment complexes in 50-, 200-, 400-, and 1000-unit communities in 10 

geographic locations in the United States. Thermal energy is stored in 

large, constructed, underground tanks. Costs were assigned to each component of every system to allow calculation of total costs. Results are 

presented as normalized system costs per unit of heat delivered per building unit. 

These methods allow (1) identification of the relative importance of each 

system component in the overall cost, and (2) identification of the key 

variables that determine the optimum sizing of a district solar heating 

system. In more northerly locations, collectors are a larger component of 

cost. In southern locations, distribution networks are a larger proportion of 

total cost. Larger, more compact buildings are, in general, less expensive 

to heat. For the two smaller-scale building configurations, a broad minima 

in total costs compared with system size is often observed. 
___________________________________________________________________________ T_It __ -_8_9 __ 8 __ 

TABLE OF CONTENTS 

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

2.0 General Aspects of Thermal Distribution Systems •••••• 3 

3.0 Trade-Offs in System Design. . . . . . .. . . . . . . . . . . . . .. .. . . . . . . . . . . . . . . . .. . . . . . . . 5 

4.0 

5.0 

6.0 

7.0 

Methods • ............................................................. 

4.1 

4.2 

Component Sizing ............................................................................................. . 

Component Costing ...................................................................... a ....................... .. 

4.2.1 Collector Costs .................................................................... , ........ . 

4.2.2 Storage Tank Costs .............................................................. -a .............. .. 

4.2.3 Piping and Distribution System Costs/ •••••••••••.•• • l• •• \ ••••••• 

\ I 

Results .............................................................. .. 

Conclusions .................................................................................................................... 

Itef erences • ................................................................... 

9 9 

11 

12 

12 

16 19 

33 

35 
__________________________ -=-T.-:.R_-S.::....9:.....:.....S 

LIST OF TABLES 

4-1 Primary Loop Lengths ................................................ 4-2 Assumed Piping and Insulation Costs for the 

Page 

9 

Distribution System .................................................. 16 
I_I ______________________ ....::.T..;;:..::R.....,;-8:...,:.9....:,...8 

LIST OF FIGURES 

3-1 Designs for 100% Solar Space Heating for a Community of 

200 Single-Family Residences in Madison. • • • • • • • • • • • • . • . • • • • • • . • • • • . . . . • 6 

3-2 Trade-Off Between Collector Field Area and Storage Volume .•••.••.•••••• 8 

3-3 Trade-Off Between Insulation Thickness and Annual System Cost.. • •• • • •• • • 8 

4-1 General Schematic of Distribution System...... .••. .••• •• •• •. •. . .••••••. 10 

4-2 Cost of Uninsulated Buried Storage Tanks •••••••••.••••••••••••••••••••• 13 4-3 Comparison of Estimates of Storage Tank Costs. •• •••••••• ••••••••• •••••• 15 

5-1 Total System Cost per Unit Heat Delivery for Ten Cities... •••••••••• •• ••• 21 5-2 Cost per Unit Heat Delivered for Different Building ConfigurationsCaribou, Maine ...................................................... 22 

5-3 Cost per Unit Heat Delivered for Different Building ConfigurationsBoston, Massachusetts ................................................ 23 5-4 Cost per Unit Heat Delivered for Different Building ConfigurationsMadison, Wisconsin... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 

5-5 Cost per Unit Heat Delivered for Different Building ConfigurationsBismarck, North Dakota. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 

5-6 Cost per Unit Heat Delivered for Different Building ConfigurationsGreat Falls, Montana. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 5-7 Cost per Unit Heat Delivered for Different Building ConfigurationsMedford, Oregon.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 

5-8 Cost per Unit Heat Delivered for Different Building ConfigurationsDodge City, Karlsas.. • • . . . • • • • • • . • • . • • • • • • • • • . • • . • • • . • • • • • • • • • • • • • • • • • 28 

5-9 Cost per Unit Heat Delivered for Different Building Configurations/.."" TR-898 

SECTION 1.0 

INTRODUCTION. 

Community-scale solar energy systems that incorporate a centralized annual cycle thermal energy storage (ACTES) coupled to a distribution system may offer an attractive 

alternative to more conventional forms of heating or to solar systems based on diurnal 

storage. The economics on which such a decision should be based depend on component 

costs, system design, building densities, and climate, etc. We have attempted in this 

report to analyze the feasibility of this option by designing and sizing ACTES solar systems and by estimating system costs. Conclusions reached are necessarily limited by the 

sensitivity of the problem to basic assumptions about capital costs, interest rates, fuel 

costs, inflation rates, and details of system design. 

The economic analysis is based on a sensitivity analysis of a community-scale ACTES 

solar system [1,2,3,4]. In this analysis, three housing configurations-single-unit dwellings (SUB), 10-unit condominiums (TUB), and 200-unit apartment complexes (HUB) in 50-, 

200-, 400-, and 1000-unit sizes-were modeled in 10 geographic locations in the United 

States on the basis of previously developed methods [1]. Two collector types were used 

for each configuration: flat plate and evacuated tube-oriented at tilt angles equal to 

latitude or equal to latitude plus ten degrees. Collector field areas and required storage 

. volumes were obtained for all 44 configurations at each of the 10 locations, and general 

sizing algorithms were derived •. 

We subsequently have designed distribution systems for each of the SUB, TUB, and HUB 

configurations and have determined costs for 220 systems that have collectors oriented 

at tilt equal to latitude. This analysis has allowed us to identify the critical cost components of such systems, determine both economies and diseconomies of scale, and draw 

some preliminary conclusions about the economic feasibility of such systems. 

A number of technical options for seasonal storage of thermal energy are available, 

including large tanks constructed above or below grade, aquifers, and solar ponds. This 

study considers only large constructed tanks although the results derived can be generalized to include other storage options. 

Design of large community thermal utilities is complex. The general features of such 

thermal distribution systems are examined in Sec. 2.0. The decision to examine three 

specific designs for each community size is discussed. Trade-offs in system design are 

presented in Sec. 3.0. Details of cost assumptions are provided in Sec. 4.0. An economic 

analysis is outlined in Sec. 5.0. All costs are expressed in normalized form for two 

reasons

First

this allows more direct cross-comparisons

Second

these costs are vari.II II 

_ "",9 
I_I ______________________ ..... 8_"'98>L...._. 

SECTION 2.0 

GENERAL ASPECTS OF THERMAL SYSTEMS 

Most conventional residential district heating systems in current use in the United States 

, , supply steam to the heating load at above 100 psi (690 mpa) or hot water above 100° C. 

Annual storage solar heating systems using flat-plate collectors that appear most economical are low-temperature systems in which the stored water reaches a maximum 

temperature well below 100° C. Operation at these lower temperatures results in higher 

collector efficiencies and permits the use of less expensive materials such as plastic 

pipes and foamed insulation. However, it requires larger fluid flows and, consequently, 

larger pipes and pumps and larger heat transfer surface areas in the heating units. The 

economic optimum trade-off dictates operational temperatures of about 35° C minimum 

and 800 ,C maximum in most locations. 

There are many ways of arranging thermal storage for district heating. For instance, 

each building can have its own storage tank, or all may share a single tank, or some combination of these can be used. For the purpose of this study, each community design (i.e., 

50 single-family residences, SUB 50) shares a single, buried, hot water storage tank. 

Special requirements are imposed on a system that combines distributed heat sources and 

heat loads with a central storage. Figure 4-1 shows the essential elements for the control and operation of such a 

Basically, there is a two-pipe distribution and collector system in which one main carries 

water at a temperature.1T higher than in the other parallel main. GenerallY,.1T c would 

be on the order of 10° to C. During normal operation, a corresponding temperature 

difference will occur in the thermal storage tank, maintained by the density stratification effect. The absolute temperature level of stored water will vary over the year, 

from a maximum of about 75° C in October to a minimum of about 40° C in March. 

Each building served by the system will normally have both a solar collector bank, on the 

roof or elsewhere, and a heating system that uses locally thermostatically controlled fancoil units or similar warm water heaters. A control valve on the return water from the heating units will hold the discharge temperature.1T c below the warm main temperature. Room temperature control will be achieved by control of the air flow through the heater coils. The flow througth the collectors is initiated whenever the collector outlet 

temperature exceeds the warm main temperature by some predetermined amount .1T e of 

about 5° C. The temperature difference .1T c is subsequently maintained by a thermostatically controlled valve

The collector pump remains in service until 

1T e goes negative as 
TR-898 1111 ----------------------------------------------------------------

In practice, because of thermal capacitance in the piping, and practical control function 

considerations, the actual controls and pumping arrangements would be somewhat more 

complex. The arrangements for draindown of the collectors to prevent freezing, for 

instance, are not shown nor are the domestic hot water subsystems. These extra devices 

could add to the total system cost, although we expect our conservative assumptions will 

compensate for these extra costs. 
,1I, ______________________________________________________ 

SECTION 3.0 

TRADE-OFFS IN SYSTEM DESIGN 

Three different community types were examined in this study. Single-family residences, 

10-unit condominiums,and 200-uriit apartment buildings were chosen to represent the 

range of heating load characteristics encountered in community systems. Ten different 

locations in the United States were studied to reflect a wide variety of climates. Community sizes ranged from 10 to 1000 living units. Each system was designed to provide 

100% of the space heating load and approximately 85% of the service hot water load 

from the solar source in a typical meteorological year. 

The complete analysis of the economic trade-offs is quite complex. However, the sensitivity of system performance is readily expressed by three principal system variablesstorage capacity, collector area, and insulation thickness. Especially in the larger-sized 

thermal storages, very high storage efficiencies are possible with very modest capital 

outlays for thermal insulation. For such systems, the total solar acquisition cost is relatively insensitive to marginal changes in the insulation thickness, and the major design 

choice is of the particular combination of collector area and storage volume. 

Using an hourly simulation, it was possible to define the range of technically satisfactory 

collector area and thermal storage capacity sets that would meet the design load 

requirements. Figure 3-1 a typical example of such a set for a community of 

200 single family residences in Madison. The plot points represent combinations of collector area and storage capacity that provide 100% of the space heating load in a typical 

year and that fully utilize the heat stored during the summer season. 

For collectors with defined orientation and characteristics, an infinite number of combinations of collector area and storage size can satisfy the design criteria. The optimum 

configuration will be the combination at which the marginal costs of adding storage are 

equal to the marginal savings owing to reduction in collector area for a given performance. Previous analysis [1] has shown that systems in which the storage temperature 

cycles annually, normally reaching the design maximum of about 80° C and the minimum 

of about 35°C only once per year, inherently offer the best economics for northern locations [3]. The collector use factor is then unity, and the available capacity of the storage 

is fully utilized in the typical year. This type of system is said to be operating uncon-:-

strainedly [5]. For this study, all systems were designed for this mode of operation. 

Figure 3-1 shows the location of this unconstrained design point for the Madison 

community. 
______________________ -'-T_R_-..:...;S9'""'-S 

-

CD 

0 

'""" 

x 

'" .s 

ctI 

Q) 

... 

« 

0 

-

(,) 

"0 

0 

100 

80 

60 

40 

20 

For All Cases 

Maximum temperature was 79.5°C. 

Minimum temperature of 35° C was 

reached once each year. 

/ Design Chosen Representing Unconstrained Operation 

o w 100 

Storage Capacity (kg x 106 ) 
___________________________ T_R_,-_8_98_ 

In addition to these major variables, a considerable number of other factors must be considered and design choices made. To retain a comparable basis among the different 

installations, similar design concepts were compared. Certain minor design features 

might otherwise have been selected differently. For instance, in southern areas in which 

no freezing problem is encountered, the cost of freeze protection measures can be 

avoided. .--

The selection of a particular flat-plate solar collector to serve all applications is another 

arbitrary choice. In the colder climates, more of the operating hours are experienced 

when the collectors are functioning in a higher range of values of D.T/I (I is the instantaneous insolation) than they do for more southern locations. Generally for the northern 

zones the economic optimum choice would favor a collector of higher performance but 

greater expense. This trade-off between the first cost and the annual value of the collected energy is typical of the necessary design decisions (see in Fig. 3-2) •. If collector A 

is twice as costly on an installed unit area basis than collector B, its performance at the 

warm site being only about 10% better than that of collector B would not justify its 

selection, while at the cold site it would double the collected energy per unit area as 

compared to B and be well justi fied. 

All the classic trade-off decisions in the distribution systems are involved also, such as 

pipe size compared with pump capital and operating power costs. The case for pipe insulation thickness versus total annual costs is shown in Fig. 3-3. Here again, there would 

be small variations from site to site in the optimum thickness. These variations were 

suppressed to have systems fully comparable in other respects. 

Other factors that influence the final design choice include site-to-site variations in the cost and availability of electric power and of auxiliary fossil fuel, in labor efficiency and cost, in building materials costs, in soil conditions, in land costs, in the ratio of direct to 

diffuse radiation, and in wind velocities. Design decisions, optimal for all the sites 

examined, are not possible to make. The degree to which this affects the conclusions 

reached is unknown but probably is not so great as to invalidate the basic site-to-site 

trends that constituted the objective of the study. 
_______________________ T_R_-S_9_S 

>-

(.) c 

(1) 

(.) 

;;:: 

-

IJJ 

el(1) C .-:= 

e c 

(1) '= 

03: 

(1)- elct! 

(0 .s 

:> 0 

«a. 

I 

I 

el(1) c_ 

U5 

--" (1)- 0.0 

00 

(1)- elct! 

ctl_ 

(0 .s 

:> 0 

«a. 

t 

I 

I 

Figure 3-2. Collector Field Area vs. Storage Volume. 

-

en 

o 

o 

to 

::l 

c:: 

c 

Optimum 

Thickness 

Total Annual Cost 

Annual Cost 

of Insulation 
___________________________ T_R_-_8_9_8 

SECTION 4.0 

METHODS 

4.1 COMPONENT SIZING 

Collector and storage sizing for 10 cities and 11 system sizes have been described earlier 

[1,2,3]. Methods for sizing the piping and distribution system are presented here. 

Piping is separated into three components: the collector loop, the secondary loop, and 

the primary loop. The collector loop includes all piping, pumps, and controls within each 

building, the traditional "auxiliary costs" of a solar heating system. The secondary loop 

links each group of 50 living units, and the primary loop links the secondary loops with 

storage (see Fig. 4-1). The primary and secondary loops taken together form the distribution network. Not all building configurations have all the loops present; e.g., 50-unit 

systems have no primary loop; 100-unit buildings have no secondary loop. It was assumed 

that piping and distribution would be the same for evacuated-tube collectors as for flatplate collectors. 

Pipe diameters for collector loop were chosen to handle a mass flow rate of 

101.9 kg/h for each m collector area. Fluid velocity in the pipes was set at 1.5 m/s 

(5 ft/s). Flow rates for the primary and secondary loops were taken as the sum of the 

flow rates in the piping feeding into the loop. 

Collector loop length for SUBs was taken to be 22.8 m (75 ft) with 7.9 m (26 ft) vertical 

heads [6]. For TUBs and HUBs, the collector loop length was taken at 45.7 m (150 ft) and 

121.9 m (400 ft) with heads of 7.9 m (26 ft) and 30.5 m (100 ft), respectively. Secondary 

and primary loop piping lengths were selected based on an assumed building configuration. Secondary loop length was 887 m (2910 ft) for single-unit buildings and 594 m 

(1950 ft) for 10-unit buildings. Primary loop lengths are given in Table 4-1. 

Table 4-1. PRIMARY LOOP 

LENGTHS 

System Length 

Type (m) (ft) 

SUB 200 823.0 2700 SUB 400 1646.0 5400 SUB 1000 4115

0 13500 
-

o 

Central Thermal 

Storage 

6Tc 

Stratification 

>. 

L..o. 

as 0 

E 0 

·c -' 

a.. 

P1 

Cool Main (T,) 

Solar 

Collectors 

P2 

Typical Building 

T1 - Collector supply water temperature 

T2 - Collector temperature 

T3 - Collector return water temperature 

T4 - Heating return water temperature 

T5 - Heating supply water temperature 

CP1, CV1, CV2 - Controllers for pump and valves 

Secondary Loop or . 

Warm Main (T,+6Td Hundred Unit Apt. Bldg. 

- Primary loop feeds entire community (I.e., 200, 400, or 1000 living units) 

- Secondary loop feeds 50 living units (I.e., 50 single-family buildings). 

Figure 4-1. General Schematic of Distribution System. 

In 

III 

~ 

-
- TR-898 

on the system size and location. Note that, for the design assumption used here, primary 

and secondary loop head losses are greater in warm locations where the heating load is 

less because systems in these locations have smaller pipe diameters with the same linear 

flow rate. -

Separate pumps and controls were assumed for the loops. Pumps ranging in power from 

1.1 kW to 300 kW and in cost from $1,000 to $17,000 were selected from a catalog of the 

Bell and Gosset Company. Most of the primary loops required several pumps to meet the 

required flow rate and head loss. Because the costs of pumps and controls were small 

compared with the cost of piping, pumps were not selected with precision. 

The current report includes four systems-TUB 10 and SUB 1, 10, and 25-that were not 

sized in a previous SERI report [1]. These systems were added to this report to more 

closely examine the economies of scale for larger systems and to compare individual 

single- and 10-unit building heating systems with the district heating systems. 

Sizing of the SUB 1, 10, and 25 and TUB 10 systems was based on SUB and TUB 50 systems, with the smaller system sized in direct proportion to the number of units. This is 

justified for two reasons. First, the variation in collector and storage sizing relative to 

load among 50-unit through lOOO-unit systems was always less than 5% and typically less 

than 3%. Such a deviation in collector and storage size can safely be ignored. Second, it 

was judged that, for small systems, the decrease in distribution losses would compensate 

for the proportional increase in storage loss that is the cause of increased efficiency in 

larger systems. Consequently, the efficiency of smaller systems may be justifiably 

assumed to equal that of 50-unit systems. Practically, this sizing is of sufficient accuracy for 10- and 25-unit systems. Dimensions of the SUB 1 system are of questionable 

accuracy but are included here as a rough comparison. 

Lengths of distribution systems for the SUB 10 and SUB 25 systems were assumed to be 

equal to 20% and 50% of the SUB 50 length, respectively. Other details of the sizing of 

the distribution system for SUB 10 and SUB 25 were identical to the sizing of larger 

systems. 

4.2 COMPONENT COSTING 

Selection of costs for system components presented a major problem because the cost of 

the three major components (collector, storage, and distribution piping) are subject to 

major variations. The following section presents a summary of cost estimates for system 

components and gives the costs chosen for the economic analysis in this study. 
- TR-898 

-

architect1.ll'al design. We have avoided spectacularly low cost assumptions, although in 

some cases such low costs may be reasonable and worth investigating. All costs are in 

1979 dollars and include contractor overhead and profit. Selected costs are not considered a definitive statement but, rather, were chosen as sample costs for this economic 

analysis. 

4.2.1 Collector Costs 

Collector costs are subject to two uncertainties. First, costs for c1.ll'rently available 

solar collectors vary over a wide range. Second, mass production, an increasingly defined 

market place, and other forces may substantially decrease collector prices in the 

fut1.ll'e [7]. 

A variety S01.ll'ces report the cost of manufactured flat-plate collectors to be 

$1l0-$170/m , uninstalled [6,7,8,9]. The authors have confirmed this cost range in discussions with several manufacturers. It is possible, however, to find cost estimates both 

much higher and much lower than this range. t recent conference on site-built solar collectors included price estimates of $40-$85/m of collector area installed, for a collector 

of reasonable performance [l0]. 

Prices for evacuated-tube collectors vary from $150-$300/m2, depending largely on 

collector performance [6,9]. Although there have been claims that evacuated-tube collectors could be mass produced at much lower costs, no such claims have been 

substantiated. 

Installation of an assembled such as one that is mounted directly onto a slanted 

roof, can cost as little as $15-$30/m if installation is simple. More difficult conditions 

may push the installation cost up to $80/m2 or higher. Systems that require a separate 

support struct1.ll'e for the collectors are particularly expensive. 

01.ll' study uses collector plus installation costs of $140/m2 for flat-plate collectors and 

$200/m2 for evacuated-tube collectors. Assumed collector performance was low: the 

parameters Fr(at) and FrU l were set, at 0.661 and 6.104 W/m2 °c for flat- plate collectors and 0.397 and 1.17 W/m C for evacuated-tube collectors. 

4.2.2 Storage Tank Costs 

The most comprehensive study of the cost of large storage tanks was performed by the 

Office of Technology Assessment [6] (see Fig. 4-2). Although there is a considerable 

range in the reported costs, especially for small storage tanks, the costs may be fitted 

fairly well to a linear plot on the logarithmic graph. This plot corresponds to the 

ti
-00 

Q) 

0> 

as 

.... 

a 

iii 50 -.-

a 

., 

E 

"-

~ 

- :: 20 (/) CI.) a 

0-

==== Curve based on O.T.A. data (see Fig. 4-3) 

<:> Provident house, actual cost, including insulation. 

6) Aylmer house, projected cost 

e Aylmer house, actual cost 

o Argonne National Lab, concrete tanks (11) 

X Minturn, Oak Ridge National Labs (12) 

+ Midwest Bunker Silo Co., concrete tank (13) 

---- - King and Carlock, steel and fiberglass tanks (15), insulated 

9 Greene, California Energy Commission (14) 

I!J Margen et aI., Swedish lined pit storage (22) 

f/llllllJ Range of costs in Solar Products Catalog (9) 

4 6 810 20 200 1000 

Storage Tank Volume .(m3 ) 

10,000 

Figure 4-2. Comparison of Estimates of Storage Tank Costs (all costs In 1979 $) . 

100,000 

III 

III 

N 

-;;;;:~ 

II II 

-.;:-~ 
5 TR-898 ---------------------::------------

Figure 4-3 gives other estimates of storage costs in comparison to the previous equation. The Office of Technology Assessment (OTA) costs for large tanks are apparently 

consistent with costs reported by more recent studies [11] and with the estimates of 

researchers and manufacturers [12,13].* These costs, however, assume favorable soil 

conditions. When soil conditions are poor, particularly when groundwater is near the surface, the actual storage costs for large tanks may be two to three times as much as is 

shown in Fig. 4-3. An illustration of this was the Aylmer house storage tank, which cost 

more than twice as much as orginally projected. 

King, Carlock, and Shingleton report costs for small and intermediate size storage tanks 

that are nearly double the costs of the OTA report [15]. Possible reasons for this discrepancy are the inclusion of the cost of insulation and the use of more expensive materials in the King and Carlock study. The OTA report also shows a greater variation in the 

cost of small tanks than in the cost of large ones. 

King, Carlock, and estimate the cost of insulation for storage to be in 

the range of $10-$20/m of tank surface for R-IO to approximately $lOO/m for R-30 

[15] • Because the surface area-to-volume ratio decreases for large tanks, the cost of 

progressively smaller per unit volume for larger2storage tanks. For! 10,000-m tank, the cost of insulating to the R-ll level of $lO/m adds only $0.30/m 

volume. 

In addition, studies have suggested [6,16,17] that the igsulating effect of the surrounding 

earth makes storage insula,tion unnecessary for 500-m tanks or larger. There are, however, limitations to the use of the earth as an insulator: initial storage losses remain 

high until the surrounding earth is heated and poor soil conditions (notably, running 

groundwater) could negate the insulating value of earth. 

Since we are assuming favorable soil conditions for our study and since the cost of insula3' 

tion is negligible for large tank sizes, we will ignore insulation costs tanks of 1000 m 

or larger. For smaller tanks, we assume insulation at a cost of $40/m surface area. For 

the tank cost itself, we use the curve drawn from the OTA data (adjusted for inflation). 

In locations where land is available and inexpensive and soil conditions are suitable, it 

may make 5onomic sense to use a solar pond for storage. Edesess et al. [18] re:pOl't that 

for a 700-m surface area salt solar pond, incl¥ding the equivalent of storage 

volume, the total cost is approximately $30/m of surface area, or $2l/m of storage. 

Although such a pond would replace collectors as well as storage, the cost of land may 

make such a system impractical in an urban or suburban location. However, a saltless 

solar pond, consisting merely of a lined pond is 30 m in diameter and 10 m dee§, 

covered with insulation, would provide 700 m of storage at a cost of under $4/m • 

Although of uncertain eff ectiveness as solar collectors, saltless ponds seem very likely to 

provide cheap, large-scale storage of solar heat where suitable land is available. 
103~-----------------------------------------------------------------------------------, 

E Coated steel tank, installed underground, 20·yr warranty 

galvanic rrotection 

X Coated steel tank, installed underground 

181 Steel tank, installed 

VI Steel fixed roof oil tank, installed 

e Fiberglass, U.L. listed, installed underground 

<:> Prestressed concrete tank, installed 

A Precast concrete septic tank, ins!alled underground 

'Q),100 

OJ 

~ 

o 

.... 

(/) 

..... 

o 

E 

"'-

~ 

::- 20 (/) 

o ~ U 

10 

8 

6 

4 

2 

Excavation, Backfill, 

and Soil Compaction 

1L-___ ~ ___ ~~-L~ ___ _L ____________ _L ___ ~~---------~~---------------~~---------------~~ 1 m3 4 100 200 103 104 105 m3 

Storage Tank Volume (m3) 

Figure 4-3., Cost of Uninsulated Buried Storage Tanks (in 1976 $), reported by Ref. 6. 

The double line represents the curve the authors fitted to the plot, following the equation 

C = 300 Vs -0.27, with CMin = 17 $/m3. 

III 

III 

~ 

-;;;::~ 

II II 

<-~ 

0-3 

00 

~ 

'co 

100 
- TR-898 

. -------------------::----------

4.2.3 Piping and Distribution System Costs 

The distribution cost is the most difficult to estimate because costs vary widely with 

location and soil type. In our costing estimate, we make no attempt to deal with local 

variations; average values are selected as estimated prices. 

Piping costs were taken from Means' Building Construction Cost Data [19]. For the collector loop, schedule 40 galvanized black steel piping was used. Cost for a 2-in. diameter 

pipe was $26 per linear meter, including installation, plus $16 per linear meter for 2 in. 

of fiberglass insulation. 

Costing for the distribution loops (primary and secondary loops) was more complicated. 

The assumed cost was taken from Means' estimate for corrugated steel outdoor piping, 

the only outdoor piping included. Means' Cost Data also provides the cost of piping insulation for pipes up to 61 cm (24 in.) in diameter. Insulation costs for larger pipes were 

estimated from Means' cost for small pipes. Table 4-2 presents the costs used for this 

study. 

Table 4.2. ASSUMED PIPING AND INSULATION COSTS 

FOR THE DISTRmUTION SYSTEM 

Costs ($/Linear meter) 

Pipe 

Diameter Piping Insulation 

(in.) (m) (2" polyurethane) Excavation 

8 16.40 49.20 9.80 

12 24.90 65.60 9.80 

24 52.50 105.00 16.40 

36 105.00 131.20 29.50 

48 134.50 164.10 108.30 

60 203.40 190.90 118.10 

72 301.90 213.30 137.80 

Similar piping costs have been reported for low-temperature district heating systems 

using smooth steel outdoor piping instead of corrugated steel [20]. There have been several district heating systems that use piping costs which are higher by a factor of three 

or more [21]

but these have been systems equipped to use pressurized water above the 
________________ =:--_____ 

For the primary and secondary loops, the cost of pumps was insignificant compared to 

the cost of piping. Secondary loop costs ranged among the different cities from $100,000 

to $200,000 for a loop of 50 houses. Primary loop costs ranged from $100,000 to $3 million, depending on location and system size. Locational variation was caused by the difference in pipe diameters among the 10 cities. 

- TR-898 

. 

SECTION 5.0 

RESULTS 

Results of an economic analysis based on the costs cited in Sec. 4.0 are presented here. 

The goals of this presentation are to illustrate the relative importance of each system 

component in the overall cost and to identify the key variables that determine the optimum sizing of a district solar heating system. It should be noted that our conclusions 

show general trends only; they are not meant to provide hard costs. In particular, the 

differences among the 10 cities described here are based solely on climatic variations; 

the local cost variations, which can be equally significant, are ignored. 

Figure 5-1 shows the normalized total system cost per unit of heat delivered based on 

storage, collector, and piping costs cited in Sec. 4.0. Costs were normalized with respect 

to a TUB 1000 system with flat-plate collectors (note that all of Figs. 5-1 to 5-11 are 

normalized on· the same basis). Cities in Fig. 5-1 are listed in order of increasing 

insolation. Collector cost, the largest component of total cost, decreases with increasing' 

insolation as a result of increased collector efficiency. Storage and distribution costs, 

each of roughly equal magnitude, remain constant per unit heat delivered for all the 

northern cities. Storage costs are relatively smaller and distribution costs are relatively 

higher in warmer cities than in cooler ones. In Phoenix, which has a relatively small heat 

load, distribution costs are the major component. 

Evacuated-tube collector systems were found to be more expensive than flat-plate collector systems in all cities except Caribou, Maine. In Caribou and in Boston, costs for 

flat-plate and evacuated-tube systems were virtually equal. 

The cost breakdown by components, as presented in Fig. 5-1, varies somewhat for the 

two other building configurations. For single-unit buildings, auxiliary costs per unit of 

heat delivered are approximately three times as great as in TUBs. For HUBs, 

distribution costs are negligible. In addition, the storage component increases for smaller 

systems. 

Figures 5-2 through 5-11 show the cost variation among each building configuration and 

system size for the 10 cities. Examination of these charts will identify the key parameters that determine system optimization. 

Auxiliary costs (the collector loop) remain constant for each building type, regardless of 

system size. The auxiliary cost is only a significant part of the total cost for single-unit 
________________ 

::--

______ T_R_-S_9_S 

Although the distribution cost appears to level off for 200-mit systems, the cost 

increases again from 400 to 1,000 units. For larger district heating systems, increasing 

distribution losses eventually reduce system efficiency to the point at which the system 

economics suffer. Aside from assuming 10%, 5%, and 0% overall distribution losses for 

the SUB, TUB, and HUB systems, respectively, this variation in distribution losses was 

ignored in this work and in the earlier paper [l]. 

Results for the 10 cities show that costs drop consistently as system size increases, 

owing to economies of scale in storage. The optimum for all cities and building 

types occurred when storage reached 40,000 m , the size at which we assumed there 

would be no more economies of scale in storage costs. The optimum system sizing, 

therefore, depends almost entirely on the economies of scale in storage. The conclusions 

in this report are valid for constructed tanks and are subject to change because storage 

prices may vary from region to region. We also stress that new technologies such as 

saltless solar ponds, lake storage, or aquifers may lower storage costs dramatically. 

Perhaps the most interesting comparison is between a single-building system and the 

larger district heating system. For single-unit buildings, it is clear that individual annual 

energy storage is inferior to district storage systems. For 10-unit buildings, the case is 

less clear. For most cities, the analysis demonstrates that district heating systems are 

less expensive than individual systems. For Phoenix and Santa Maria, however, individual 

systems appear cheaper. In these cities, heat loads are low and subsequently the distribution system is a proportionately greater factor in the total cost, and the savings are 

more significant from elim,ination of the distribution system. These results suggest that 

individual systems for 10-unit buildings, which incorporate greater conservation, also 

may have an advantage over district heating systems in colder cities. 

The variations among building types in these results are not surprising. For larger, individual building units, the system cost per unit heat delivered decreases, providing an 

additional savings to that already realized because the building load is reduced. Figures 5-2 to 5-11 indicate the reasons that larger unit buildings result in reduced costs. 

For 100-unit buildings, the savings over 10-unit buildings stem mainly from reduction in 

the distribution system cost. For 10-unit buildings as opposed to SUBs, savings result 

because auxiliary costs are decreased; furthermore, system efficiency is increased 

because for larger buildings, in which the hot water load is a greater proportion of the 

annual load, the demand for heat is more level throughout the year than for single-unit 

buildings. 
:!:: 

c: 

=> 

Q) 

Co 

-

en 0 

(,) 

E 

Q) 

-

en 

>- en 

"0 

Q) 

(ij 

E 

0 

z 

0.75,--------------------------------, 

Total Cost 

(Ten Unit Buildings) 

Auxiliary 

0.50 

Collectors 

0.25 

Distribution System 

0 
1.1 __________________ ------T.=..:::R-898 -

c: 

::::l 

.... 

CD 

-

III 

0 

C) 

E 

CD 

-

III 

>-

en 

"0 

CD 

.!::! 

co 

E 

.... 

0 

z 

1.0 

0.75 

I Caribou, Maine 

I 

1 

1 

I 

i" 

I 

I I 

Auxiliary I \\ 

1 

Collector I I" · 

I 1 I I 

1 I 

I 

I " 

__ 

1 1 

Storage I 'l 

1 r, 

\1 \ 

'i \ 1 

\ Distribution I, \ 

1 \ 1\ 
________________ 

Q) 

0. 

-

rn 

o 

() 

E 

Q) 

1;) 0.50 

>. 

C/) 

"C 

Q) 

.!::! 

ca 

E 

o 

Z 

Collector 

Storage 

Boston, Massachusetts 

I 

I I 

1 

I 

I 

1 

, 1 \ 

1', 

I 

1 

I 

---. ./\ I 

'\ 

1\ \ I \ \ \J 

r\ 1\ 
________________ ------=:.T=R-=-8:..::....::;.98 

1.0 

0.75 

-

c: 

::> 

... 

CIl 

Q. 

iii 

0 

(J 

E 2 0.50 

CJ) 

>-

C/) 

'tl 

CIl 

ttl 

E 

... 

0 

z 

0.25 

0 

I Madison, Wisconsin 

I 

I 

I 

I 

I 

1 

L '1, 

"I I I \\ 

I 1, I \\ \\ 

Collector I 

I I I I I 

.. r \ I I \1 I 

Storage \ I 

I 1\ 

\ I 

\1 \1 1 

r\ 1\ 1\ 1\ \ \ 
'*1 

_______________ --,-_____ 

I Bismarck, North Dakota 

I I I 

, I 

, I , 

I I I I 

::: , 

c: I I I Q) t, Q. 

- 1 CJ) 0 

........ , U I E 

Q) 

-

CJ) I ''J >. I en . 

1J 

Q) Collector , .!:::! 

«i I I 'E 0 

z 

I , I 

0.25 I ',{ 

Storage I 1'\ 1 I .... .---- 1 \ I \1 

\, 

1\ 
______________ ---, ______ T_R-_8_98 

... 

Q) 

a. 

-

en 

o 

(,) 

E 

Q) i 0.50 

en 

"0 

Q) 

(ij 

E 

... 

o 

Z 

I Great Falls, Montana I I 

I I 

I I 

I I 

I I 

I I 

I I 

J- I 

I 

I 

Collector 

Storage 
- TR-898-

... 

Q) 

c.. 

-

(/) 

o 

U 

E 

Q) 

-

(/) 

>- en 

"0 

tU 

E 

... 

o 

Z 

0.75 

I 

1 1 

I 

1 

I 

I 

Medford, Oregon 

+ I 

I 

I I 

"1 1 

I " I 

I '{ 

Storage 1 1\ 

I \1 '\1 

Distribution ll' I' , \ , 
________________ ,....--______ TB-=8_9_8 

-

en 

o 

() 

E 

Q) 

-

en 

>. 

en 

"'0 

Q) 

.!:::! 

Cii 

E 

o 

Z 

10 25 

Storage 

I Dodge City, Kansas 

I I 

I 

I I 

I 

I I 

50 200 400 1000 I 200 400 1000 

TUB HUB 

System Size and Building Type 
_______________ ---:-_______ T..;,...R ___ -....:c.. 89.:.....:.,.8 

... 

CD 

Q. 

-

CJ) 

o 

U 

E 

CD 

-

CJ) >-

CI) 

"0 

CD 

.!:::! 

«i 

E 

... 

o 

Z 

I Santa Maria, California 

I 

1 

I 

I 

1 

I 

;-Total I 

------ , I , 

Auxiliary '1\ I ---.----

I \' 

Collector , 

1 I ' 1---' · 

__ 

I 

Storage I ",I \ I l, \ I \ I 

\! \1 

Distribution 1\ }, 

1\ 

1\ 
_______________ 

Q) 

Q. 

-

en 

o 

o 

E 

Q) 

-

en 

>- (/J 

'0 

Q) 

.!:::! 

(tj 

E 

o 

Z 

.1.0 

Storage 

System Size and Building Type 
________________ ------T-R.:-898 

-

c 

=> 

Q) 

Co 

-

'" o () 

E 

Q) 

iii 

>. 

(/) 

"C 

Q) 

.!::! 

(ij 

E 

o 

Z 

0.75 0.25 

Auxiliary 

Collector 

Storage 

Distribution 

Phoenix, Arizona 

\ 

\ 

\ 

\ t . 

1\ 

1'/ I \l , I . 1\ 

\ \ I \ \/ \ I \ 

, \ 

\ I \ 1 \ \ \ 

1 

I \ 1\ I \ \ \ I \ I \ ' 

_______________________ --=-TR::..:......:-8:..:9-=.8_ 

SECTION 6.0 

CONCLUSIONS 

The goals of this analysis, as stated in Sec. 5.0, are to identify the relative importance of 

each system component in the overall cost and to identify the key variables that determine the optimum sizing of a district solar heating system. Interpretation of the results 

of the study provides a partial answer to these queries: the constraint on the conclusions 

is uncertainty in component costs. All costs are expressed in normalized form for this 

reason. 

The most uncertain component of cost is the distribution system. Costs assumed here 

are substantially lower than those for steam or pressurized water distribution systems. 

They may possibly be reduced by use of low-cost plastic piping components. Also, costs 

of excavation vary from region to region. Costs are lower for new construction compared to retrofit construction. Finally, use of some innovative distribution systems may 

substantially lower costs. For example, an inexpensive distribution network may be constructed by laying pipes in basements and connecting through basement walls. 

Solar energy is cheaper in climates with higher insolation levels. However, annual cycle 

thermal energy storage (ACTES) may be more valuable in northern climates with lower 

insolation. Figure 5-1 demonstrates that the collectors are a substantially larger component of cost in northern cities. The extra expense for an ACTES is also a smaller proportion of collector costs. Once an investment is made for solar collectors in Boston and 

Albuquerque, then the additional investment in an ACTES is more profitable in Boston. 

The relative cost of the distribution system also increases as one proceeds southward. In 

Phoenix, a large distribution system may be prohibitively expensive and unnecessary. In 

cities with substantial winter insolation, well-designed buildings may render ACTES solar 

systems uneconomical. (A publication on the value of ACTES in different climates' for 

conventional homes and homes incorporating energy-conserving and passive features, The 

Trade-Off Between Collector Area and Building Conservation in Annual Storage sOlar 

Heating Systems by Sanford Sillman, is presently underway at SERI and will be published 

shortly.) 

The community type has important effects on the system cost per unit heat delivered. 

Larger, more compact buildings are, in general, less expensive to heat (see Figs. 5-2 

to 5-11). This drop-off in unit cost from SUBs to TUBs to HUBs is more pronounced in 

Phoenix than in Caribou, for example, because the distribution and auxiliary system costs 

that are reduced by the same proportion are a higher percentage of the total in Phoenix 

than in Caribou. In addition, a relatively flat optimum in total costs is apparent in all 
______________ ----::-:-______ 

would be from designing inexpensive ACTES solar systems in locations with higher heating loads. Systems may be very attractive that combine low-cost collectors with heat 

pumps to collect energy inexpensively on a large scale during the summer for use with an 

ACTES. The conclusions in this study are useful even though hard costs are not available; the trends in cost and the conclusions are still valid. We expect that the effect of 

changes in costs will not radically affect the validity of our conclusions. 
- TR-898 

SECTION 7.0 

REFERENCES 

1. Baylin, F.; Monte, R.; Sillman, S. 1980 (May). Annual Cycle Thermal Energy Storage for a Community Solar System: Details of Sensitivity Analysis. SERI/TP-355-

575. Golden, CO: Solar Energy Research Institute. 

2. Baylin, F.; Monte, R.; Sillman, S. 1980. "Sensitivity Analysis of a Community Solar 

System Using Annual Cycle Thermal Energy Storage." Proceedings of Second 

Miami International Conference on Attractive Energy Sources. Miami, FL: 10-13 

Dec. 1979. 

3. Hooper, F. C.; Cook, J. D. "Design of Annual Storage Solar Space Heating Systems." Proceedings of the Symposium for Solar Energy Fundamentals and Applications. 72nd Annual American Institute of Chemical Engineers Meeting, San 

Francisco, CA: 25-29 Nov. 1979. 

4. Hooper, F. C. et ale "Analysis of Community Solar Systems for Combined Space 

and Domestic Hot Water Heating Using Annual Cycle Thermal Energy Storage." 

Proceedings of the Second Annual Systems Simulation and Economics Analysis Conference. San Diego, CA: 23-25 Jan. 1980. 

5. Hooper, F. C.; Cook,' J. D. 1980. "Design of Annual Storage Space Heating Systems." Fundamentals and Applications of Solar Energy. American Institute of 

Chemical Engineers Symposium Series 198. Vol. 76: pp. 80-91. 

6. U.S. Congress, Office of Technology Assessment. 1978 (June). Application of Solar 

Technology to Today's Energy Needs. Vols. I and 2. Washington, DC: OT A; 

OTA-E-66. 

7. King, T. A.; Carlock, J. B. Construction Cost Factors Identified in Commercial 

Solar Energy Systems. Baltimore, MD: Mueller Associates, Solar 10822-79/60, 

1979. Also personal communication with J. B. Carlock, Jan. 1980. 

8. Morehouse, J. H. 1979 (Jan.). Direct Solar Heating Systems with High Performance 

Collectors: Thermal Performance and System Costs Estimates. McLean, VA: 

Science Applications, Inc. 

9. Solar Products Catalog, Harrisville, NH. Total Environmental Action, 1978. 
______________________________________ ____________ T_R __ -8_9_8 __ _ 

13. Midwest Bunker Silo Co., Charlotte, MI: Personal communication. Jan. 1980. 

14. Greene, B., California Energy Commission, Sacramento, CA: Personnal communication. Jan. 1980. 

15. King, T.; Carlock, J.; Shingleton, J. The Construction Cost of Thermal Storage for 

Solar Systems. Baltimore, MD: Mueller Associates, 1979. 

16. Shelton, J. 1975. "Underground Storage of Heat in Solar Heating Systems." Solar 

Energy. Vol. 17 (2): p. 138. --

17. Taylor, T. Low Cost Solar Energy. Ft. Lauderdale, FL; International Scientific 

Forum on an Acc eptable Nuclear Energy Future of the World, Nov. 1977. 

18. Edesess, M.; Benson, D.; Henderson, J.; Jayadev, T. 1979 (Aug.). Economic and 

Performance Comparisons of Salty and· Saltless Solar Ponds. SERI/TP-35-2l3. 

Golden, CO: Solar Energy Research Institute. 

19. Mean's Building Construction Cost Data. Duxbury, MA: KS Means Co., Jan. 1979. 

20. Personal communication with Charles Meyer, Santa Barbara, CA: G. E. Tempo, 

Inc., Feb. 1980. 

21. Morgen, P.; Peters, B.; Spangsberg, K. 1979 (May). "Swedish Central Solar Heating 

Plant for 2000 Dwellings." UNICHAL Conference (Study Energitechnik). Stockholm, 

Sweden. 

22. Morgen, P.; Rossen, R. "Solar Heat for Small District Heating Systems." International Total Energy Congress. Copenhagen, Denmark; Oct. 1979. 
Document Control 11. SERI Report No. 

Page TR-72l-898 

12. NTIS Accession No. 

4. Title and Subtitle 

Economic Analysis of Community Solar Heating Systems 

That Use Annual Cycle Thermal Energy Storage 

7. Author(s) r·. Bay.Ll.n, K. t1onte, :s. :Sl.llman, F. C. Hooper, 

J. D. McClenahan 

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 

16. Abstract (Limit: 200 words) 

3. Recipient's Accession No. 

5. Publication Date 

February 1981 

6. 

8. Performing Organization Rept. No. 

10. ProjecVTask/Work Unit No. 

1013.00 11. Contract (Cl or Grant. (G) No. 

(C) 

(Gl 

13. Type of Report & Period Covered 

Technical Report 

14. 

This report examines the economics of community-scale solar systems 'that incorporate a centralized annual cycle thermal energy storage (ACTES) coupled to a 

distribution system. Systems were sized for three housing configurations: 

single-unit 'dwellings, la-unit, and 200-unit apartment complexes in 50-, 200-, 

400-, and 1000-unit communities in 10 geographic locations in the United States. 

Thermal energy is stored in large, constructed, underground tanks. Costs were 

assigned to each component of every system in order to allow calculation of 

total costs. Results are presented as normalized system costs per unit of heat 

delivered per building unit. These methods allow (1) identification of the 

relative importance of each system component in the overall cost, and (2) identification of the key variables that determine the optimum sizing of a district 

solar heating system. In more northerly locations, collectors are a larger 

component of cost. In southern locations, distribution networks are a larger 

proportion of total cost. Larger, more compact buildings are, in general, less 

expensive to heat. For the two smaller-scale building configurations, a broad 

minima in total costs versus system size is often observed.