TRANSLATING ASHRAE FOR ARCHITECTS:SIZING SOLAR HOT WATER SYSTEMS AND

SPACING ROOFTOP COLLECTORS IN PRELIMINARY DESIGN

Mark DeKaySchool of ArchitectureWashington UniversitySt. Louis, MO 63130

ABSTRACT

This paper describes a simple graphic method, derivedfrom ASHRAE techniques but easier, which allows archi-tects to size solar Domestic Hot Water (DHW) systems inpreliminary design before any details are known about thebuilding. A simple table of hot water loads on a per unitarea basis by occupancy type allows loads to be sized in arough and approximate, but quick way. The second part ofthe paper gives a quick method, again based on ASHRAEmethods, but quicker, for spacing rows of photovoltaics orsolar heat collectors on flat or sawtooth roofs such that onecollector does not shade another.

1. INTRODUCTION

Roofs should be large enough, sloped appropriately, andoriented to accommodate collection of sun for heating do-mestic water. So how does an architect determine thesefactors in preliminary design? A quick review of basic en-ergy texts used by architects reveals no quick and easy an-swers. ASHRAE methods (1) are cumbersome for the de-signer when quick, approximate information is needed.

Solar Collectors for heating service hot water in buildingscan be located on buildings or on the ground adjacent to thebuilding. Solar hot water systems have many benefits, in-cluding reduced pollution and attractive economics.

In the Solar Village 3 project (Figure 1) in Athens, Greece,Alexandros N. Tombazis and Associates used solar watercollectors mounted on racks to provide heated water whilealso shading roof terraces of multi-story housing (2). Col-lectors are tilted, facing south. The complex uses both ac-tive and passive solar heating and cooling systems in a vari-ety of types. Heated water is used for domestic use and insome cases, for some of the space heating requirements.Banks of collectors alternate with open rails, to give both ashady and a sunny side to the roof terraces, expanding op-tions for seasonal use and views.

2. RULES of THUMB

Collectors should face the equator whenever possible. De-viations from an equatorial orientation result in less heatbeing collected from the sun, especially at high latitudes inwinter.

For a winter bias, use a collector tilt of latitude plus 15 de-grees. For maximizing summer production, use a tilt of lat-itude minus 15 degrees. For maximizing annual produc-tion, use a tilt equal to the site's latitude.

Fig. 1 South Side of two story residences. Solar Village 3.

Flate Plate EvacuatedCity Lat Ave Good Ave Good

Imp (SI) Imp (SI) Imp (SI) Imp (SI)Albuquerque, NM 35 271 23.77 308 27.02 283 24.82 340 29.82Annette, AK 55 102 8.95 123 10.79 120 10.53 154 13.51Apalachicola, FL 30 189 16.58 235 20.61 211 18.51 258 22.63Astoria, OR 46 121 10.61 148 12.98 152 13.33 186 16.32Atlanta, GA 33 181 15.88 227 19.91 281 24.65 246 21.58Bethel, AK 61 113 9.91 138 12.11 131 11.49 160 14.04Binghampton, NY 42 191 16.75 141 12.37 145 12.72 174 15.26Bismark, ND 47 191 16.75 223 19.56 210 18.42 274 24.04Boise, ID 43 225 19.74 276 24.21 * * 292 25.61Boston, MA 42 145 12.72 185 16.23 172 15.09 211 18.51Brownsville, TX 26 167 14.65 199 17.46 209 18.33 259 22.72Caribou, ME 47 155 13.60 196 17.19 176 15.44 211 18.51Charleston, SC 33 168 14.74 201 17.63 196 17.19 240 21.05Chicago, IL 42 156 13.68 201 17.63 186 16.32 225 19.74Cleveland, OH 41 131 11.49 158 13.86 154 13.51 197 17.28Columbus, OH 40 137 12.02 164 14.39 161 14.12 202 17.72Detroit, MI 42 143 12.54 187 16.40 174 15.26 208 18.25Dodge City, KS 38 234 20.53 286 25.09 247 21.67 301 26.40Elpaso, TX 32 266 23.33 306 26.84 282 24.74 338 29.65Fairbanks, AK 65 134 11.75 163 14.30 140 12.28 171 15.00Ft Worth/Dallas, TX 33 177 15.53 208 18.25 208 18.25 251 22.02Fesno, CA 37 233 20.44 284 24.91 252 22.11 303 26.58Grand Junuction, CO 39 257 22.54 308 27.02 270 23.68 324 28.42Great Falls, MT 47 188 16.49 220 19.30 211 18.51 251 22.02Greensboro, NC 36 173 15.18 204 17.89 201 17.63 243 21.32Hilo, HI 19 172 15.09 215 18.86 197 17.28 242 21.23Honoluulu, HI 21 206 18.07 258 22.63 230 20.18 283 24.82Indianapolis, IN 39 147 12.89 191 16.75 178 15.61 219 19.21Lake Charles, LA 30 155 13.60 204 17.89 192 16.84 235 20.61Las Vegas, NV 36 262 22.98 300 26.32 281 24.65 337 29.56Lexington, KY 38 152 13.33 199 17.46 186 16.32 224 19.65Little Rock, AR 34 192 16.84 240 21.05 212 18.60 254 22.28Los Angeles, CA 34 222 19.47 276 24.21 242 21.23 290 25.44Madison, WI 43 167 14.65 199 17.46 187 16.40 219 19.21Medford, OR 42 172 15.09 202 17.72 201 17.63 243 21.32Miami, FL 26 181 15.88 225 19.74 213 18.68 261 22.89

TABLE 1: ANNUAL ENERGY PRODUCTION OF SOLAR WATER HEATING COLLECTORS FACING SOUTH1000 Btu/ft2-yr (kJ/m2-yr) U. S. Cities

For overcast dominated sky conditions, a tilt lower than thelatitude is recommended, since under overcast conditions,the top of the skydome is three times brighter than thehorizon.

As a rough rule-of-thumb, in sunny locations, collectors forresidences should be about 20 square feet (2 m2) of collec-tor area for each of the first two occupants and 8 squarefeet (0.7 m2) for each additional occupants. In higher lati-tudes, allow 12 -14 additional square feet (1.1 to 1.3square meters) per person (3).

3. SIZING DHW COLLECTORS

The sizing of a solar hot water system is based on a numberof factors, including:• The efficiency of the collectors• The radiation available at the site• The demand for hot water in the building• The temperature of the hot water desired, in relation to

the incoming water supply temperatureThe following section gives a quick method for sizing solardomestic hot water collectors for buildings.

Flate Plate EvacuatedCity Lat Ave Good Ave Good

Imp (SI) Imp (SI) Imp (SI) Imp (SI)Minneapolis/St Paul, MN 45 173 15.18 205 17.98 192 16.84 230 20.18Nashville, TN 36 156 13.68 183 16.05 181 15.88 224 19.65New York. NY 41 134 11.75 162 14.21 165 14.47 201 17.63Oklahoma C, OK 35 203 17.81 235 20.61 228 20.00 279 24.47Omaha, NE 41 184 16.14 218 19.12 187 16.40 250 21.93Pittsburgh, PA 40 127 11.14 157 13.77 156 13.68 191 16.75Portland, ME 43 143 12.54 181 15.88 167 14.65 207 18.16Rapid C, SD 44 204 17.89 240 21.05 225 19.74 231 20.26Sacremento, CA 38 226 19.82 281 24.65 246 21.58 295 25.88St Louis, MO 39 174 15.26 228 20.00 207 18.16 248 21.75Salt Lake C, UT 41 234 20.53 268 23.51 255 22.37 312 27.37San Antonio, TX 29 183 16.05 215 18.86 214 18.77 261 22.89Sault Ste Marie, MI 46 136 11.93 222 19.47 162 14.21 199 17.46Seattle, WA 47 123 10.79 150 13.16 155 13.60 189 16.58Springfield, MO 37 167 14.65 227 19.91 210 18.42 252 22.11Spokane, WA 47 161 14.12 189 16.58 185 16.23 233 20.44Tallahassee, FL 30 187 16.40 233 20.44 209 18.33 255 22.37Tampa, FL 28 195 17.11 250 21.93 222 19.47 272 23.86Tuscon, AZ 32 269 23.60 330 28.95 287 25.18 347 30.44Washington, DC 38 152 13.33 195 17.11 181 15.88 219 19.21

Source: Base data adapted from ASHRAE (1)

3.1 Estimate Collector Performance

Begin by determining from Table 1 the annual collectorperformance that you are likely to achieve in your location(1). The energy collected by a solar hot water heating sys-tem depends on a delicate balance between the amount ofcollectable solar radiation, efficiency of the collectors, andthe amount of energy lost from the storage and distributioncomponents.

Two basic choices of solar heat collectors for hot watersystems are available, "flat plate" and "evacuated." Flatplate collectors are lower in cost and widely available.Evacuated collectors offer better performance and adapt-ability at a higher cost, and are often preferred where thereis an extreme difference between the water temperature inthe collector and the outside air, or in climates where inso-lation levels are low. Evacuated collectors have adjustabletubes to accommodate a variety of roof slopes.

Table 1 lists annual energy production from flat plate andevacuated liquid-type solar collectors in various cities. Atrue south orientation is assumed, with collector tilt equalto latitude (1). If a city near your site is not listed, selectone that has a similar latitude and climate. Remember thatradiation availability can vary substantially from locationto location. Two values are given for each collector type ineach location, representing the range of commerciallyavailable efficiencies, one for an "average" collector andone for a "good" collector. Performance which is better orworse than that shown on the table is possible, dependingon the specific characteristics of the collectors used.

3.2 Determine the Required Temperature Rise

Then, using the chart in Figure 3, determine the tempera-ture rise of hot water that the collection system must pro-vide. Enter the chart on the horizontal axis using the wellwater temperature for your location from the map in Figure2 or local water temperature data found in (1). Move verti-cally to the diagonal line for the supply temperature of yourbuilding's hot water. Read the Temperature Rise on thevertical axis.

3.3 Size Your Collectors

Using both your annual collector performance (from sec-

Fig. 2 Well Water Temperature in the U.S. (4)

tion 3.1 above) and the temperature rise required (fromsection 3.2 above), use the linked nomographs in Figure 5to size your collectors. The diagram at the lower leftshows two different paths through the charts. Enter eitherthe residential (right side) or commercial (left side) chart.For residential buildings, begin at the bottom of the chartwith the number of residents, move up to the appropriatediagonal line for your building's rate of conservation. Atthe intersection , move left along the Annual DHW flowline to the center chart and intersect the diagonal line foryour required temperature rise of the hot water. Movedown to the bottom chart, intersecting the line for your An-nual Collector Performance for your site (from Table 1).Finally, read right for collector size. Values on the chart'svertical axis represent a 100% solar fraction, meaning pro-vision of all the building's hot water energy. In practice,this is rarely economical. The three columns to the rightgive sizes for lesser goals. In most temperate locations,even those with severe overcast winters, a 50% solar frac-tion is not unreasonable, and 80% is feasible. In sunny lo-cations, solar fractions above 80% are possible, while athigh latitudes, 50% annual fractions are still possible, withthe understanding that midwinter performance will likelybe poor.Fig. 3 Chart for estimating DHW temperature rise

Fig. 4 Roof mounted PV's. Science Park, Gelsenkirchen, Germany, 1995. Kiessler + Partner, architects.

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4. SPACING COLLECTORS

Photovoltaic panels or solar thermal collectors for servicehot water can be placed in east-west rows and spaced in thenorth-south direction. Collectors may also be placed on orintegrated with the solar face of complex roof forms, suchas sawtooth shaped roofs. In both cases, it is important toleave enough space between the rows of collectors, in orderto avoid reducing their efficiency by having one row cast ashadow on the row behind.

The Science Park in Gelsenkirchen, Germany, a researchand development building for 'soft' technologies, has 1520m2 of photovoltaic surface in 900 modules installed alongthe flat roofs of its 300 meter length and on its nine officepavilions. The long axis of the building runs north-south,giving each office pavilion south exposure. The PV's, seton south facing rack mounts, provide 190,000 kWh/yr andare spaced to avoid shading each other (6).

Collector spacing to avoid shading is a function of the sizeof the collector, the site's latitude, the collector's tilt, andthe collector's orientation (1). For due south (north in thesouthern hemisphere) collector orientations, the minimumspacing distance is most often determined for 10 AM on thewinter solstice (Dec 21 in the NH, Jun 21 in the SH). Fororientations east of south (or E of N in SH) the time usedwill be earlier than 10 AM; for orientations west of south,later than 2 PM. Fixed collectors should be oriented within30 degrees of south (or north for the SH). Collector orien-tation away from the due solar direction is not recommend-ed for high latitudes, because of the short winter sun path.

To determine the minimum spacing distance between rowsof collectors, use Table 2 to find the ratio between collectorspacing distance and collector length (D/L). Then use thevalue of L for your collector or roof scheme to solve for D.To find D/L, enter the table on the row for your latitude andcollector tilt. Move across to the column for your collectororientation and read D/L.

D = L x ratio from table

Some applications may not require significant winter col-lection, in which case spacing can be reduced using thehigher sun angles of different dates. In high latitude loca-tions, very low winter sun angles will create the need forlarge distances between collector rows. Since winter sol-stice insolation will generally be low, a more reasonabledate may be chosen as a shading criteria. For latitudes,tilts, or orientations not shown or for spacing based on sunangles other than winter solstice, see (1).

5. REFERENCES

(1) ASHRAE, Active Solar Heating Systems Design Manu-al, Atlanta: ASHRAE, 1988.

Values of D/L (dimensionless)Orientation from S(N)

Lat . T i l t 3 0 2 0 1 0 0l a t - 1 5 1 .9 1 .5 1 .4 1 .3

2 8 lat 2 .8 2 .0 1 .7 1 .5la t+15 3.5 2 .4 1 .9 1 .7l a t - 1 5 2 .4 1 .8 1 .6 1 .4

3 2 lat 3 .5 2 .4 1 .9 1 .7la t+15 4.3 2 .8 2 .2 1 .9l a t - 1 5 3 .2 2 .2 1 .8 1 .6

3 6 lat 4 .6 2 .9 2 .3 1 .9la t+15 5.6 3 .4 2 .5 2 .1l a t - 1 5 4 .8 2 .8 2 .2 1 .9

4 0 lat 6 .7 3 .6 2 .7 2 .2la t+15 8.1 4 .2 3 .0 2 .5l a t - 1 5 8 .7 3 .8 2 .7 2 .2

4 4 lat 4 .8 3 .3 2 .7la t+15 5.6 3 .7 2 .9l a t - 1 5 5 .7 3 .6 2 .8

4 8 lat 7 .3 4 .4 3 .4la t+15 8.4 4 .9 3 .7l a t - 1 5 5 .2 3 .8

5 2 lat 6 .4 4 .5la t+15 7.2 4 .9l a t - 1 5 9 .9 5 .8

5 6 lat 6 .9la t+15 7.6

TABLE 2. COLLECTOR SPACING RATIOS

Fig. 6 Section Through Parallel Rows of Collectors

(2) Cofaig, E. O., J. A. Olley, and J. O. Lewis, The Climat-ic Dwelling, London: James and James, pp. 147-154.(3) EREN, Fact Sheet: Solar Water Heating, ENERGY,DOE/GO-10096-050, FS 119, March 1996.(4) Bowen, A., Clark, E., and Labs, K., Passive Cooling,Boulder: ASES, 1981.(5) TVA, Energy Nomographs, A Graphic CalculationTechnique for the Design of Energy Efficient Buildings,Chattanooga: Tennessee Valley Authority. Sept., 1985.(6) Herzog, Thomas, ed., Solar Energy in Architecture andUrban Planning, New York: Prestel, 1996.