1999 HVAC Applications Part 8 docx

GROUNDWATER HEAT PUMPS A groundwater heat pump system GWHP removes ter from a well and delivers it to a heat pump or an intermediateheat exchanger to serve as a heat source or sink.. Opt

31.20 1999 ASHRAE Applications Handbook (SI)

are the two-pipe arrangement used with chain trenchers and thefour- or six-pipe arrangements placed in trenches made with a widebackhoe bucket

An overlapping spiral configuration shown in Figure 22, has alsobeen used with some success However, it requires special attentionduring the backfilling process to ensure soil fills all the pocketsformed by the overlapping pipe Large quantities of water must beadded to compact the soil around the overlapping pipes The back-filling must be performed in stages to guarantee complete fillingaround the pipes and good soil contact The high pipe density (up to

10 m of pipe per linear metre of trench) may cause problems in longed extreme weather conditions, either from soil dry out duringcooling or from freezing during heating

pro-The extra time needed to backfill and the extra pipe lengthrequired make spiral configurations nearly as expensive to install asstraight pipe configurations However, the reduced land area needed

Fig 20 Approximate Groundwater Temperatures (°C) in the United States

Fig 21 Horizontal Ground Loop Configurations

Fig 22 General Layout of a Spiral Earth Coil

Geothermal Energy 31.21

for the more compact design may permit their use on smaller

resi-dential lots The spiral pipe configuration laid flat in a horizontal pit

arrangement is used commonly in the northern midwest part of the

United States, where sandy soil causes trenches to collapse A large

open pit is excavated by a bulldozer, and then the overlapping pipes

laid flat on the bottom of the pit The bulldozer is also used to cover

the pipe; the pipes should not be run over with the bulldozer tread

Most horizontal loop installations place flow loops in a parallel

rather than a single (series) loop to reduce pumping power (Figure

23) Parallel loops may require slightly more pipe, but may use

smaller pipe and thus have smaller internal volumes requiring less

antifreeze (if needed) Also, the smaller pipe is typically much

cheaper for a given length, so total pipe cost is less for parallel loops

An added benefit is that parallel loops can be flushed out with a

smaller purge pump than would be required for a larger single-pipe

loop A disadvantage of parallel loops is the potential for unequal

flow in the loops and thus reduced heat exchange efficiency

The time required to install a horizontal loop is not much

differ-ent from that for a vertical system For the arrangemdiffer-ents described

above, a two-person crew can typically install the ground loop for

an average house in a single day

While not restricted to single-family residential applications,

horizontal loops are rarely used in larger commercial buildings due

to the land area that is required Even if the land adjacent to the

building were initially available, installation of a horizontal loop

could prevent any future construction above the loop field, tying up

a considerable investment in vacant land Placement of horizontalloops under parking lots may have a negative impact on the effec-tiveness of the ground loop due to the greater surface heat exchange.Soil characteristics are an important concern for any ground loopdesign With horizontal loops, the soil type can be more easily deter-mined because the excavated soil can be inspected and tested EPRI

et al (1989) compiled a list of criteria and simple test proceduresthat can be used to classify soil and rock adequately enough for hor-izontal ground loop design

Leaks in the heat-fused plastic pipe are rare when attention ispaid to pipe cleanliness and proper fusion techniques Should a leakoccur, it is usually best to try to isolate the leaking parallel loop andabandon it in place The time and effort required to find the source

of the leak usually far outweighs the cost of replacing the defectiveloop Because the loss of as little as 1 L of water from the systemwill cause it to shut down, leaks cannot be located by looking forwet soil, as is commonly done with water lines

GROUNDWATER HEAT PUMPS

A groundwater heat pump system (GWHP) removes ter from a well and delivers it to a heat pump (or an intermediateheat exchanger) to serve as a heat source or sink Both unitary orcentral plant designs are used In the unitary type, a large number ofsmall water-to-air heat pumps are distributed throughout the build-ing The central plant design uses one or a small number of large-capacity chillers supplying hot and chilled water to a two- or four-pipe distribution system

groundwa-Regardless of the type of equipment installed in the building, thespecific components for handling the groundwater are similar Theprimary items include (1) the wells (supply and, if required, injec-tion), (2) well pump, and (3) groundwater heat exchanger The spe-cifics of these items are discussed in the section on Direct-UseSystems In addition to those comments, the following consider-ations apply

Groundwater Flow Requirements

Generally, the greater the groundwater flow, the better the formance (COP or EER) of the heat pumps However, increasingheat pump performance can be compromised quickly by wellpump power at high groundwater flow rates For this reason,optimum groundwater flow should be based on electrical powerrequirements of the well pump, heat pumps, and circulatingpump Optimum groundwater flow (for minimum system energyconsumption) is a function of groundwater temperature, wellpump pressure, heat exchanger design, loop pump power andheat pump performance

per-For moderate-efficiency heat pumps (COP of 4), efficient looppump design (0.016 W/W), and a heat exchanger approach of 1.5

K, Figure 24 provides curves for two different groundwater peratures (21°C and 10°C) and two well pump pressures (300 and

tem-900 kPa)

Although the four curves show a clear optimum flow, sometimesoperating at a lower groundwater flow reduces the well/pump cap-ital cost and reduces the problem of fluid disposal These consider-ations are highly project specific, but do afford the designer somelatitude in flow selection

Well Pumps

Submersible pumps have not performed well in ture, direct-use projects However, in a normal groundwater temper-ature as encountered in heat pump applications, the submersiblepump is a cost-effective option The low temperature eliminates theneed to specify an industrial design for the motor/protector, therebygreatly reducing the first cost relative to direct-use Caution shouldstill be exercised for wells that are expected to produce moderate

higher-tempera-Fig 23 Parallel and Series Ground Loop Configurations

Geothermal Energy 31.23

2 Chiller capacity is controlled by the heating water (condenser)

loop temperature, and the groundwater flow through the chilled

water exchanger is controlled by chilled water temperature

For buildings with a significant heating load, the former may be

more attractive, while the latter may be appropriate for conventional

building in moderate-to-warm climates

SURFACE WATER HEAT PUMPS

Surface water bodies can be very good heat sources and sinks if

properly used In some cases, lakes can be the very best water

sup-ply for cooling A variety of water circulation designs are possible

and several of the more common are presented

In a closed-loop system, a water-to-air heat pump is linked to a

submerged coil Heat is exchanged to (cooling mode) or from

(heat-ing mode) the lake by the fluid (usually a water-antifreeze mixture)

circulating inside the coil The heat pump transfers heat to or from

the air in the building

In an open-loop system, water is pumped from the lake through

a heat exchanger and returned to the lake some distance from the

point at which it was removed The pump can be located either

slightly above or submerged below the lake water level For heat

pump operation in the heating mode, this type is restricted to

warmer climates; water temperature must remain above at least

5.5°C

Thermal stratification of water often keeps large quantities of

cold water undisturbed near the bottom of deep lakes This water is

cold enough to adequately cool buildings by simply being circulated

through heat exchangers A heat pump is not needed for cooling,

and energy use is substantially reduced Closed-loop coils may also

be used in colder lakes Heating can be provided by a separate

source or with heat pumps in the heating mode Precooling or

supplemental total cooling are also permitted when water

tempera-ture is between 10 and 15°C

Heat Transfer in Lakes

Heat is transferred to lakes by three primary modes: radiant

energy from the sun, convective heat transfer from the surrounding

air (when the air temperature is greater than the water temperature),

and conduction from the ground Solar radiation, which can exceed

950 W per square metre of lake area, is the dominant heating anism, but it occurs primarily in the upper portion of the lake unlessthe lake is very clear About 40% of the solar radiation is absorbed atthe surface (Pezent and Kavanaugh 1990) Approximately 93% ofthe remaining energy is absorbed at depths visible to the human eye.Convection transfers heat to the lake when the lake surface tem-perature is lower than the air temperature Wind speed increases therate at which heat is transferred to the lake, but maximum heat gain

mech-by convection is usually only 10 to 20% of maximum solar heatgain The conduction gain from the ground is even less than convec-tion gain (Pezent and Kavanaugh 1990)

Cooling of lakes is accomplished primarily by evaporative heattransfer at the surface Convective cooling or heating in warmermonths will contribute only a small percentage of the total because

of the relatively small temperature difference between the air andthe lake surface temperature Back radiation typically occurs atnight when the sky is clear, and can account for significant amount

of cooling The relatively warm water surface will radiate heat to thecooler sky For example, on a clear night, a cooling rate of up to 160W/m2 from a lake 14 K warmer than the sky The last major mode

of heat transfer, conduction to the ground, does not play a major role

in lake cooling (Pezent and Kavanaugh 1990)

To put these heat transfer rates in perspective, consider a 4000

m2 lake that is used in connection with a 35 kW heat pump In thecooling mode, the unit will reject approximately 44 kW to the lake.This is 11 W/m2, or approximately 1% of the maximum heat gainfrom solar radiation in the summer In the winter, a 35 kW heatpump would absorb only about 26 kW, or 6.5 W/m2, from the lake

Thermal Patterns in Lakes

The maximum density of water occurs at 4.0°C, not at the freezingpoint of 0°C This phenomenon, in combination with the normalmodes of heat transfer to and from lakes, produces temperature pro-files advantageous to efficient heat pump operation In the winter, thecoldest water is at the surface It tends to remain at the surface andfreeze The bottom of a deep lake stays 3 to 5 K warmer than the sur-face This condition is referred to as winter stagnation The warmerwater is a better heat source than the colder water at the surface

Fig 25 Central Plant Groundwater Systems

31.24 1999 ASHRAE Applications Handbook (SI)

As spring approaches, surface water warms until the temperature

approaches the maximum density point of 4.0°C The winter

strati-fication becomes unstable and circulation loops begin to develop

from top to bottom This condition of spring overturn (Peirce 1964)

causes the lake temperature to become fairly uniform

Later in the spring as the water temperatures rise above 7°C, the

circulation loops are in the upper portion of the lake This pattern

continues throughout the summer The upper portion of the lake

remains relatively warm, with evaporation cooling the lake and

solar radiation warming it The lower portion (hypolimnion) of the

lake remains cold because most radiation is absorbed in the upper

zone Circulation loops do not penetrate to the lower zone and

con-duction to the ground is quite small The result is that in deeper lakes

with small or medium inflows, the upper zone is 21 to 32°C, the

lower zone is 4 to 13°C, and the intermediate zone (thermocline) has

a sharp change in temperature within a small change in depth This

condition is referred to as summer stagnation

As fall begins, the water surface begins to cool by radiation and

evaporation With the approach of winter, the upper portion begins

to cool towards the freezing point and the lower levels approach the

maximum density temperature of 4.0°C An ideal temperature

ver-sus depth chart is shown in Figure 26 for each of the four seasons

(Peirce 1964)

Many lakes do exhibit near-ideal temperature profiles However, a

variety of circumstances can disrupt the profile These characteristics

include (1) high inflow/outflow rates, (2) insufficient depth for

strat-ification, (3) level fluctuation, (4) wind, and (5) lack of enough cold

weather to establish sufficient amounts of cold water necessary for

summer stratification Therefore, a thermal survey of the lake should

be conducted or existing surveys of similar lakes in similar

geo-graphic locations should be consulted

Closed-Loop Lake Water Heat Pump

The closed-loop lake water heat pump shown in Figure 17 has

several advantages over the open-loop One advantage is the

reduced fouling resulting from the circulation of clean water (or

water-antifreeze solution) through the heat pump A second tage is the reduced pumping power requirement This results fromthe absence of an elevation head from the lake surface to the heatpumps A third advantage of a closed-loop is that it is the only typerecommended if a lake temperature below 4°C is possible The out-let temperature of the fluid will be about 3 K below that of the inlet

advan-at a flow of 54 mL/s per kilowadvan-att Frosting will occur on the headvan-atexchanger surfaces when the bulk water temperature is in the 1 to3°C range

A closed-loop system has several disadvantages Performance ofthe heat pump lowers slightly because the circulation fluid temper-ature drops 2 to 7 K below the lake temperature A second disadvan-tage is the possibility of damage to coils located in public lakes.Thermally fused polyethylene loops are much more resistant todamage than copper, glued plastic (PVC), or tubing with band-clamped joints The third possible disadvantage is fouling on theoutside of the lake coil—particularly in murky lakes or where coilsare located on or near the lake bottom

Polyethylene (PE 3408) is recommended for all intake piping.All connections must be either thermally socket fused or butt fused.These plastic pipes should also have protection from UV radiation,especially when near the surface Polyvinyl chloride (PVC) pipeand plastic pipe with band-clamped joints is not recommended.The piping networks of closed-loop systems resemble thoseused in ground-coupled heat pump systems Both a large-diameterheader between the heat pump and lake coil and several parallelloops of piping in the lake are required The loops are spread out tolimit thermal interference, hot spots, and cold pockets While this

Fig 26 Idealized Diagram of Annual Cycle of

Thermal Stratification in Lakes

Fig 27 Closed Loop Lake Coil in Bundles

(Kavanaugh 1991)

Geothermal Energy 31.25

layout is preferred in terms of performance, installation is more

time consuming Many contractors simply unbind plastic pipe coils

and submerged them in a loose bundle Some compensation for

thermal interference is obtained by making the bundled coils longer

than the spread coils A diagram of this type of installation is shown

in Figure 27

Copper coils have also been used successfully Copper tubes

have a very high thermal conductivity, so coils only one-fourth to

one-third the length of plastic coils are required However, copper

pipe does not have the durability of PE 3408 or polybutylene, and if

the possibility of fouling exists, coils must be significantly longer

Antifreeze Requirements

Closed loop horizontal and surface water heat exchanger

sys-tems will often require an antifreeze be added to the circulating

water in locations with significant heating seasons Antifreeze maynot be needed in a comparable vertical borehole heat exchangersince the deep ground temperature will be essentially constant At adepth of 2 m, a typical value for horizontal heat exchangers, theground temperature varies by approximately ±5 K Even if the meanground temperature were 15°C in late winter, the natural groundtemperature would drop to 10°C The heat extraction process wouldlower the temperature even further around the heat exchanger pipes,probably by an additional 5 K or more Even with good heat transfer

to the circulating water, the entering water temperature (leaving theground heat exchanger) would be around 5°C Lakes which freeze

at the surface in the winter approach 4°C at the bottom, yieldingnearly the same margin of safety against freezing of the circulatingfluid An additional 5 K temperature difference is usually needed inthe heat pump’s refrigerant-to-water heat exchanger to transfer theheat to the refrigerant Having a refrigerant-to-water coil surfacetemperature below the freezing point of water risks the possibility ofgrowing a layer of ice on the water side of the heat exchanger In thebest case, icing of the coil would restrict and may eventually blockthe flow of water and cause a shutdown In the worst case, the icecould burst the tubing in the coil and require a major serviceexpense

Several factors must be considered when selecting an antifreezefor a ground loop heat exchanger The most important consider-ations are: (1) impact on system life cycle cost, (2) corrosivity, (3)leakage, (4) health risks, (5) fire risks, (6) environmental risks fromspills or disposal, and (7) risk of future use (will the antifreeze beacceptable over the life of the system) A study by Heinonen et al.(1997) of six antifreezes against these seven criteria is summarized

in Table 9 No single material satisfies all criteria Methanol and anol have good viscosity characteristics at low temperatures, yield-ing lower than average pumping power requirements However,they both pose a significant fire hazard when in concentrated forms.Methanol is also toxic, eliminating it from consideration in areasthat require non-toxic antifreeze to be used Propylene glycol had nomajor concerns, with only leakage and pumping power require-ments prompting minor concerns Potassium acetate, calcium mag-nesium acetate (CMA), and urea have favorable environmental andsafety performance; but they are all subject to significant leakageproblems, which has limited their use in the past

eth-REFERENCES

Anderson, K.E 1984 Water well handbook, Missouri Water Well and Pump

Contractors Association, Belle, MD.

Austin, J.C 1978 A low temperature geothermal space heating

demonstra-tion project Geothermal Resources Council Transacdemonstra-tions 2(2).

Bullard, E 1973 Basic theories (Geothermal energy; Review of research and development) UNESCO, Paris.

Caneta Research 1995 Commercial/institutional ground-source heat pump engineering manual ASHRAE, Atlanta.

CSA 1993 Design and construction of earth energy heat pump systems for

commercial and institutional buildings Standard C447-93 Canadian

Standards Association, Rexdale, ON.

Campbell, M.D and J.H Lehr 1973 Water well technology McGraw-Hill,

New York.

Carslaw, H.S and J.C Jaeger 1947 Heat conduction in solids Claremore

Press, Oxford.

Chandler, R.V 1987 Alabama streams, lakes, springs and ground waters for

use in heating and cooling Bulletin 129 Geological Survey of Alabama,

Tuscaloosa, AL.

Christen, J.E 1977 Central cooling—Absorption chillers Oak Ridge

National Laboratories, Oak Ridge, TN.

Combs, J., J.K Applegate, R.O Fournier, C.A Swanberg, and D Nielson.

1980 Exploration, confirmation and evaluation of the resource In cial Report No 7, Direct utilization of geothermal energy: Technical

Spe-handbook Geothermal Resources Council.

Cosner, S.R and J.A Apps 1978 A compilation of data on fluids from

geo-thermal resources in the United States DOE Report LBL-5936.

Lawrence Berkeley Laboratory, Berkeley, CA.

Table 9 Suitability of Selected GCHP Antifreeze Solutions

Category Methanol Ethanol

lene Glycol

Propy- sium Acetate CMA Urea

Potas-Life cycle cost *** *** ** 1 ** 1 ** 1 ***

Risk of future use * 16 ** 17 *** ** 18 ** 19 ** 19

Key: * Potential problems, caution in use required

** Minor potential for problems

*** Little or no potential for problems

Life cycle cost 1 Higher than average installation and energy

costs.

Corrosion 2 High black iron and cast iron corrosion rates.

3 High black iron and cast iron, copper and copper alloy corrosion rates.

4 Medium black iron, copper and copper alloy corrosion rates.

5 Medium black iron, high cast iron, and extremely high copper and copper alloy corrosion rates.

Leakage 6 Minor leakage observed.

7 Moderate leakage observed Extensive leakage reported in installed systems.

8 Moderate leakage observed.

9 Massive leakage observed.

Health risk 10 Protective measures required with use See

MSDS.

11 Prolonged exposure can cause headaches, sea, vomiting, dizziness, blindness, liver dam- age, and death Use of proper equipment and procedures reduces risk significantly.

nau-12 Confirmed human carcinogen.

Fire Risk 13 Pure fluid only Little risk when diluted with

water in antifreeze.

14 Very minor potential for pure fluid fire at vated temperatures.

ele-Environmental risk 15 Water pollution risk.

Risk of future use 16 Toxicity and fire concerns Prohibited in some

locations.

17 Toxicity, fire and environmental concerns.

18 Potential leakage concerns.

19 Not currently used as GSHP antifreeze solution May be difficult to obtain approval for use.

Source: Heinonen and Tapscott (1996)

31.26 1999 ASHRAE Applications Handbook (SI)

Culver, G.G and G.M Reistad 1978 Evaluation and design of downhole

heat exchangers for direct applications DOE Report No RLO-2429-7.

Di Pippo, R 1988 Industrial developments in geothermal power

produc-tion Geothermal Resources Council Bulletin 17(5).

Efrid, K.D and G.E Moeller 1978 Electrochemical characteristics of 304

and 316 stainless steels in fresh water as functions of chloride

concentra-tion and temperature Paper 87, Corrosion/78, Houston, TX.

EPRI 1989 Soil and rock classification for the design of ground-coupled

heat pump systems International Ground Source Heat Pump

Associa-tion, Stillwater, OK Electric Power Research Institute, National Rural

Electric Cooperative Association, Oklahoma State University.

Ellis, P 1989 Materials selection guidelines Geothermal Direct Use

Engi-neering and Design Guidebook Ch 8 Oregon Institute of Technology,

Geo-Heat Center, Klamath Falls, OR.

Ellis, P and C Smith 1983 Addendum to material selection guidelines for

geothermal energy utilization systems Radian Corporation, Austin, TX.

Ellis, P.F and M.F Conover 1981 Material selection guidelines for

geother-mal energy utilization systems DOE Report RA/27026-1, Radian

Cor-poration, Austin, TX.

EPA 1975 Manual of water well construction practices

EPA-570/9-75-001 U.S Environmental Protection Agency, Washington, D.C.

Eskilson, P 1987 Thermal analysis of heat extraction boreholes University

of Lund, Sweden.

Gudmundsson, J.S 1985 Direct uses of geothermal energy in 1984

Geo-thermal Resources Council Proceedings, 1985 International Symposium

on Geothermal Energy, International Volume, Davis, CA.

Hackett, G and J H Lehr 1985 Iron bacteria occurrence problems and

control methods in water wells National Water Well Association,

Wor-thington, OH.

Heinonen, E.W And R.E Tapscott 1996 Assessment of anti-freeze

solu-tions for ground-source heat pump systems New Mexico Engineering

Research Institute for ASHRAE RP-863 ASHRAE.

Heinonen, E.W., R.E Tapscott, M.W Wildin, and A.N Beall 1997

Assess-ment of anti-freeze solutions for ground-source heat pump systems.

ASHRAE Research Report 90BRP.

Ingersoll, L.R and A.C Zobel 1954 Heat conduction with engineering and

geological application, 2nd ed McGraw-Hill, New York.

Interagency Geothermal Coordinating Council Geothermal energy,

research, development and demonstration program DOE Report

RA-0050, IGCC-5 U.S Department of Energy, Washington, D.C.

Kavanaugh, S.P 1985 Simulation and experimental verification of a

verti-cal ground-coupled heat pump system Ph.D thesis Oklahoma State

University, Stillwater, OK.

Kavanaugh, S.P 1991 Ground and water source heat pumps Oklahoma

State University, Stillwater, OK.

Kavanaugh, S.P 1992 Ground-coupled heat pumps for commercial

build-ing ASHRAE Journal 34(9):30-37.

Kavanaugh, S.P and M.C Pezent 1990 Lake water applications of

water-to-air heat pumps ASHRAE Transactions 96(1):813-20.

Kavanaugh, S.P and K Rafferty 1997 Ground-source heat pumps—

Design of geothermal systems for commercial and institutional

build-ings ASHRAE, Atlanta.

Kindle, C.H and E.M Woodruff 1981 Techniques for geothermal liquid

sampling and analysis Battelle Pacific Northwest Laboratory, Richland,

WA.

Lienau, P.J 1979 Materials performance study of the OIT geothermal

heat-ing system Geo-Heat Utilization Center Quarterly Bulletin, Oregon

Institute of Technology, Klamath Falls, OR.

Lienau, P.J., G.G Culver and J.W Lund 1988 Geothermal direct use opments in the United States Oregon Institute of Technology, Geo-Heat Center, Klamath Falls, OR.

devel-Lund, J.W., P.J Lienau, G.G Culver and C.H Higbee, C.V 1976 Klamath

Falls geothermal heating district Geothermal Resources Council actions 3.

Trans-Lunis, B 1989 Environmental considerations Geothermal direct use neering and design guidebook, Ch 20 Oregon Institute of Technology,

engi-Geo-Heat Center, Klamath Falls, OR.

Mitchell, D.A 1980 Performance of typical HVAC materials in two

geo-thermal heating systems ASHRAE Transactions 86(1):763-68.

Muffler, L.J.P., ed 1979 Assessment of geothermal Resources of the United

States—1978 U.S Geological Survey Circular No 790.

Nichols, C.R 1978 Direct utilization of geothermal energy: DOE’s resource assessment program Direct Utilization of Geothermal Energy: A Sym- posium Geothermal Resources Council.

OSU 1988a Closed-loop/ground-source heat pump systems installation guide International Ground Source Heat Pump Association, Oklahoma

State University, Stillwater, OK.

OSU 1988b Closed loop ground source heat pump systems Oklahoma

State University, Stillwater, OK.

Peirce, L.B 1964 Reservoir temperatures in north central alabama

Geolog-ical Survey of Alabama Bulletin 8 Tuscaloosa, AL.

Pezent, M.C and S.P Kavanaugh 1990 Development and verification of a

thermal model of lakes used with water-source heat pumps ASHRAE Transactions 96(1).

Rafferty, K 1989a A materials and equipment review of selected U.S thermal district heating systems Oregon Institute of Technology, Geo- Heat Center, Klamath Falls, OR.

geo-Rafferty, K 1989b Absorption refrigeration Geothermal direct use neering and design guidebook, Ch 14 Oregon Institute of Technology,

engi-Geo-Heat Center, Klamath Falls, OR.

Reistad, G.M., G.G Culver, and M Fukuda 1979 Downhole heat ers for geothermal systems: Performance, economics and applicability.

exchang-ASHRAE Transactions 85(1):929-39.

Roscoe Moss Company 1985 The engineers manual for water well design.

Roscoe Moss Company, Los Angeles, CA.

Stiger, S., J Renner, and G Culver 1989 Well testing and reservoir

evalu-ation Geothermal and direct use engineering and design guidebook, Ch.

7 Oregon Institute of Technology, Geo-Heat Center, Klamath Falls, OR Svec, O J 1990 Spiral ground heat exchangers for heat pump applications Proceedings of 3rd IEA Heat Pump Conference Pergamon Press, Tokyo.

UOP 1975 Ground water and wells Johnson Division, UOP Inc., St Paul,

MN.

BIBLIOGRAPHY

Allen, E 1980 Preliminary inventory of western U.S cities with proximate hydrothermal potential Eliot Allen and Associates, Salem, OR Anderson, D.A and J.W Lund, eds 1980 Direct utilization of geothermal

energy: Technical handbook Geothermal Resources Council Special Report No 7.

Caneta Research 1995 Operating experiences with commercial

ground-source heat pumps ASHRAE Research Project 863.

CHAPTER 32

SOLAR ENERGY USE

Quality and Quantity of Solar Energy 32.1

Solar Energy Collection 32.6

Heat Storage 32.11

Water Heating 32.11

Components 32.14

Cooling by Solar Energy 32.16

Cooling by Nocturnal Radiation and Evaporation 32.16

Solar Heating and Cooling Systems 32.17 Sizing Solar Heating and Cooling Systems—

Energy Requirements 32.19 Installation Guidelines 32.23 Design, Installation, and Operation

Checklist 32.25 Photovoltaic Applications 32.26

HE major obstacles encountered in solar heating and cooling

Tare economic—the equipment needed to collect and store solar

energy is high in cost In some cases, the cost of the solar equipment

is greater than the resulting savings in fuel costs Some of the

prob-lems inherent in the nature of solar radiation include:

• It is relatively low in intensity, rarely exceeding 950 W/m2

Con-sequently, when large amounts of energy are needed, large

collec-tors must be used

• It is intermittent because of the variation in solar radiation

inten-sity from zero at sunrise to a maximum at noon and back to zero

at sunset Some means of energy storage must be provided at

night and during periods of low solar radiation

• It is subject to unpredictable interruptions because of clouds, rain,

snow, hail, or dust

Systems should make maximum use of the solar energy input by

effectively using the energy at the lowest temperatures possible

QUALITY AND QUANTITY OF SOLAR ENERGY

Solar Constant

Solar energy approaches the earth as electromagnetic radiation,

with wavelengths ranging from 0.1 µm (X rays) to 100 m (radio

waves) The earth maintains a thermal equilibrium between the

annual input of shortwave radiation (0.3 to 2.0 µm) from the sun and

the outward flux of longwave radiation (3.0 to 30 µm) Only a

lim-ited band need be considered in terrestrial applications, because

99% of the sun’s radiant energy has wavelengths between 0.28 and

4.96 µm The current value of the solar constant (which is defined

as the intensity of solar radiation on a surface normal to the sun’s

rays, just beyond the earth’s atmosphere at the average earth-sun

distance) is 1367 W/m2 The section on Determining Incident Solar

Flux in Chapter 29 of the 1997 ASHRAE Handbook—Fundamentals

has further information on this topic

Solar Angles

The axis about which the earth rotates is tilted at an angle of

23.45° to the plane of the earth’s orbital plane and the sun’s equator

The earth’s tilted axis results in a day-by-day variation of the angle

between the earth-sun line and the earth’s equatorial plane, called

the solar declination δ This angle varies with the date, as shown in

Table 1 for the year 1964 and in Table 2 for 1977 For other dates, the

declination may be estimated by the following equation:

The earth’s rotation causes the sun’s apparent motion (Figure 1).The position of the sun can be defined in terms of its altitude βabove the horizon (angle HOQ) and its azimuth φ, measured asangle HOS in the horizontal plane

At solar noon, the sun is exactly on the meridian, which containsthe south-north line Consequently, the solar azimuth φ is 0° The

noon altitude βN is given by the following equation as

(2)where LAT = latitude

Because the earth’s daily rotation and its annual orbit around thesun are regular and predictable, the solar altitude and azimuth may

be readily calculated for any desired time of day when the latitude,longitude, and date (declination) are specified Apparent solar time

(AST) must be used, expressed in terms of the hour angle H, where

(3)

Solar Time

Apparent solar time (AST) generally differs from local standardtime (LST) or daylight saving time (DST), and the difference can besignificant, particularly when DST is in effect Because the sun

The preparation of this chapter is assigned to TC 6.7, Solar Energy

Utiliza-tion.

δ = 23.45 sin 360[ °(284+N)⁄365]

Fig 1 Apparent Daily Path of the Sun Showing Solar

Altitude (β) and Solar Azimuth (φ)

Solar Energy Use 32.3

To determine θ, the surface azimuth ψ and the surface-solar

azimuth γ must be known The surface azimuth (angle POS in

Fig-ure 2) is the angle between the south-north line SO and the normal

PO to the intersection of the irradiated surface with the horizontal

plane, shown as line OM The surface-solar azimuth, angle HOP, is

designated by γ and is the angular difference between the solar

azimuth φ and the surface azimuth ψ For surfaces facing east of

south, γ = φ − ψ in the morning and γ = φ + ψ in the afternoon For

surfaces facing west of south, γ = φ + ψ in the morning and γ = φ −

ψ in the afternoon For south-facing surfaces, ψ = 0°, so γ = φ for all

conditions The angles δ, β, and φ are always positive

For a surface with a tilt angle Σ (measured from the horizontal),

the angle of incidence θ between the direct solar beam and the

nor-mal to the surface (angle QOP′ in Figure 2) is given by:

(8)For vertical surfaces, Σ = 90°, cos Σ = 0, and sin Σ = 1.0, so Equa-

tion (8) becomes

(9)For horizontal surfaces, Σ = 0°, sin Σ = 0, and cos Σ = 1.0, so

The surface faces south, so φ = γ From Equation (8),

ASHRAE Standard 93, Methods of Testing to Determine the

Thermal Performance of Solar Collectors, provides tabulated values

of q for horizontal and vertical surfaces and for south-facing faces tilted upward at angles equal to the latitude minus 10°, the lat-itude, the latitude plus 10°, and the latitude plus 20° These tablescover the latitudes from 24° to 64° north, in 8° intervals

of the spectrum below 0.40 µm contains 8.73% of the total, another38.15% is contained in the visible region between 0.40 and 0.70 µm,and the infrared region contains the remaining 53.12%

Solar Radiation at the Earth’s Surface

In passing through the earth’s atmosphere, some of the sun’s

direct radiation I D is scattered by nitrogen, oxygen, and othermolecules, which are small compared to the wavelengths of theradiation; and by aerosols, water droplets, dust, and other particleswith diameters comparable to the wavelengths (Gates 1966) This

Fig 2 Solar Angles with Respect to a Tilted Surface

cos θ = cos β cos γ sin Σ+sin β cos Σ

cos θ = cos β cos γ

32.4 1999 ASHRAE Applications Handbook (SI)

scattered radiation causes the sky to appear blue on clear days, and

some of it reaches the earth as diffuse radiation I

Attenuation of the solar rays is also caused by absorption, first by

the ozone in the outer atmosphere, which causes a sharp cutoff at

0.29 µm of the ultraviolet radiation reaching the earth’s surface In

the longer wavelengths, there are a series of absorption bands caused

by water vapor, carbon dioxide, and ozone The total amount of

attenuation at any given location is determined by (1) the length of

the atmospheric path through which the rays traverse and (2) the

composition of the atmosphere The path length is expressed in terms

of the air mass m, which is the ratio of the mass of atmosphere in the

actual earth-sun path to the mass that would exist if the sun were

directly overhead at sea level (m = 1.0) For all practical purposes, at

sea level, m = 1.0/sin β Beyond the earth’s atmosphere, m = 0.

Prior to 1967, solar radiation data was based on an assumed

solar constant of 1324 W/m2 and on a standard sea level atmosphere

containing the equivalent depth of 2.8 mm of ozone, 20 mm of

pre-cipitable moisture, and 300 dust particles per cubic centimeter

Threlkeld and Jordan (1958) considered the wide variation of water

vapor in the atmosphere above the United States at any given time,

and particularly the seasonal variation, which finds three times as

much moisture in the atmosphere in midsummer as in December,

January, and February The basic atmosphere was assumed to be at

sea level barometric pressure, with 2.5 mm of ozone, 200 dust

particles per cm3, and an actual precipitable moisture content that

varied throughout the year from 8 mm in midwinter to 28 mm in

mid-July Figure 4 shows the variation of the direct normal

irradi-ation with solar altitude, as estimated for clear atmospheres and for

an atmosphere with variable moisture content

Stephenson (1967) showed that the intensity of the direct normal

irradiation I DN at the earth’s surface on a clear day can be estimated

by the following equation:

(11)

where A, the apparent extraterrestrial irradiation at m = 0, and B, the

atmospheric extinction coefficient, are functions of the date and takeinto account the seasonal variation of the earth-sun distance and theair’s water vapor content

The values of the parameters A and B given in Table 1 were selected so that the resulting value of I DN would be in close agree-ment with the Threlkeld and Jordan (1958) values on average cloud-

less days The values of I DN given in Tables 15 through 21 in

Chapter 29 of the 1997 ASHRAE Handbook—Fundamentals, were

obtained by using Equation (11) and data from Table 1 The values

of the solar altitude β and the solar azimuth φ may be obtained fromEquations (5) and (6)

Because local values of atmospheric water content and elevation

can vary markedly from the sea level average, the concept of ness number was introduced to express the ratio between the actual

clear-clear-day direct irradiation intensity at a specific location and theintensity calculated for the standard atmosphere for the same loca-tion and date

Figure 5 shows the Threlkeld-Jordan map of winter and summerclearness numbers for the continental United States Irradiation val-ues should be adjusted by the clearness numbers applicable to eachparticular location

Design Values of Total Solar Irradiation

The total solar irradiation I tθ of a terrestrial surface of any tation and tilt with an incident angle θ is the sum of the direct com-

orien-ponent I DN cos θ plus the diffuse component I dθ coming from the

sky plus whatever amount of reflected shortwave radiation I r mayreach the surface from the earth or from adjacent surfaces:

(12)The diffuse component is difficult to estimate because of its non-directional nature and its wide variations Figure 4 shows typicalvalues of diffuse irradiation of horizontal and vertical surfaces Forclear days, Threlkeld (1963) has derived a dimensionless parameter

(designated as C in Table 1), which depends on the dust and

mois-ture content of the atmosphere and thus varies throughout the year:

(13)

where I dH is the diffuse radiation falling on a horizontal surfaceunder a cloudless sky

Fig 4 Variation with Solar Altitude and Time of Year for

Direct Normal Irradiation

Solar Energy Use 32.5

The following equation may be used to estimate the amount of

diffuse radiation I dθ that reaches a tilted or vertical surface:

ρg = reflectance of the foreground

I tH = total horizontal irradiation

(17)

The intensity of the reflected radiation that reaches any surface

depends on the nature of the reflecting surface and on the incident

angle between the sun’s direct beam and the reflecting surface

Many measurements made of the reflection (albedo) of the earth

under varying conditions show that clean, fresh snow has the

high-est reflectance (0.87) of any natural surface

Threlkeld (1963) gives values of reflectance for commonly

encountered surfaces at solar incident angles from 0 to 70°

Bitumi-nous paving generally reflects less than 10% of the total incident

solar irradiation; bituminous and gravel roofs reflect from 12 to

15%; concrete, depending on its age, reflects from 21 to 33%

Bright green grass reflects 20% at θ = 30° and 30% at θ = 65°

The maximum daily amount of solar irradiation that can be

received at any given location is that which falls on a flat plate with

its surface kept normal to the sun’s rays so it receives both direct and

diffuse radiation For fixed flat-plate collectors, the total amount of

clear day irradiation depends on the orientation and slope As shown

by Figure 6 for 40° north latitude, the total irradiation of horizontal

surfaces reaches its maximum in midsummer, while vertical

south-facing surfaces experience their maximum irradiation during the

winter These curves show the combined effects of the varying

length of days and changing solar altitudes

In general, flat-plate collectors are mounted at a fixed tilt angle

Σ(above the horizontal) to give the optimum amount of irradiation for

each purpose Collectors intended for winter heating benefit from

higher tilt angles than those used to operate cooling systems in summer

Solar water heaters, which should operate satisfactorily throughout the

year, require an angle that is a compromise between the optimal values

for summer and winter Figure 6 shows the monthly variation of total

day-long irradiation on the 21st day of each month at 40° north latitude

for flat surfaces with various tilt angles

Tables in ASHRAE Standard 93 give the total solar irradiation

for the 21st day of each month at latitudes 24° to 64° north on

sur-faces with the following orientations: normal to the sun’s rays

(direct normal data do not include diffuse irradiation); horizontal;

south-facing, tilted at (LAT−10), LAT, (LAT+10), (LAT+20), and

90° from the horizontal The day-long total irradiation for fixed

sur-faces is highest for those that face south, but a deviation in azimuth

of 15° to 20° causes only a small reduction

Solar Energy for Flat-Plate Collectors

The preceding data apply to clear days The irradiation for

average days may be estimated for any specific location by referring

to publications of the U.S Weather Service The Climatic Atlas of the United States (U.S GPO 1968) gives maps of monthly and an-

nual values of percentage of possible sunshine, total hours of shine, mean solar radiation, mean sky cover, wind speed, and winddirection

sun-The total daily horizontal irradiation data reported by the U.S.Weather Bureau for approximately 100 stations prior to 1964 showthat the percentage of total clear-day irradiation is approximately alinear function of the percentage of possible sunshine The irradia-tion is not zero for days when the percentage of possible sunshine isreported as zero, because substantial amounts of energy reach theearth in the form of diffuse radiation Instead, the following rela-tionship exists:

(18)

where a and b are constants for any specified month at any given

location See also Jordan and Liu (1977) and Duffie and Beckman(1974)

Longwave Atmospheric Radiation

In addition to the shortwave (0.3 to 2.0 µm) radiation it receivesfrom the sun, the earth receives longwave radiation (4 to 100 µm,with maximum intensity near 10 µm) from the atmosphere In turn,

a surface on the earth emits longwave radiation q Rs in accordancewith the Stefan-Boltzmann law:

(19)

where

e s= surface emittance

σ = Stefan-Boltzmann constant, 5.67 × 10 − 8 W/(m 2 ⋅Κ 4)

T s= absolute temperature of the surface, K

For most nonmetallic surfaces, the longwave hemispheric tance is high, ranging from 0.84 for glass and dry sand to 0.95 forblack built-up roofing For highly polished metals and certain selec-

emit-tive surfaces, e may be as low as 0.05 to 0.20

-100 = a+b (% possible sunshine)

q Rs = e sσT s4

Solar Energy Use 32.7

A flat-plate collector generally consists of the following

compo-nents (see Figure 8):

• Glazing One or more sheets of glass or other diathermanous

(radiation-transmitting) material

• Tubes, fins, or passages To conduct or direct the heat transfer

fluid from the inlet to the outlet

• Absorber plates Flat, corrugated, or grooved plates, to which the

tubes, fins, or passages are attached The plate may be integral

with the tubes

• Headers or manifolds To admit and discharge the fluid.

• Insulation To minimize heat loss from the back and sides of the

collector

• Container or casing To surround the aforementioned

compo-nents and keep them free from dust, moisture, etc

Flat-plate collectors have been built in a wide variety of designsfrom many different materials (Figure 9) They have been used to heatfluids such as water, water plus an antifreeze additive, or air Theirmajor purpose is to collect as much solar energy as possible at thelowest possible total cost The collector should also have a long effec-tive life, despite the adverse effects of the sun’s ultraviolet radiation;corrosion or clogging because of acidity, alkalinity, or hardness of theheat transfer fluid; freezing or air-binding in the case of water, or dep-osition of dust or moisture in the case of air; and breakage of the glaz-ing because of thermal expansion, hail, vandalism, or other causes.These problems can be minimized by the use of tempered glass

Glazing Materials

Glass has been widely used to glaze flat plate solar collectorsbecause it can transmit as much as 90% of the incoming shortwavesolar irradiation while transmitting virtually none of the longwaveradiation emitted outward by the absorber plate Glass with lowiron content has a relatively high transmittance for solar radiation(approximately 0.85 to 0.90 at normal incidence), but its transmit-tance is essentially zero for the longwave thermal radiation (5.0 to

50 µm) emitted by sun-heated surfaces

Plastic films and sheets also possess high shortwave tance, but because most usable varieties also have transmissionbands in the middle of the thermal radiation spectrum, they mayhave longwave transmittances as high as 0.40

transmit-Plastics are also generally limited in the temperatures they cansustain without deteriorating or undergoing dimensional changes.Only a few finds of plastics can withstand the sun’s ultraviolet radi-ation for long periods However, they are not broken by hail andother stones and, in the form of thin films, they are completely flex-ible and have low mass

The glass generally used in solar collectors may be either strength (2.2 to 2.5 mm thick) or double-strength (2.9 to 3.4 mm

single-Fig 8 Exploded Cross Section Through Double-Glazed

Solar Water Heater

Fig 9 Various Types of Solar Collectors

32.8 1999 ASHRAE Applications Handbook (SI)

thick) The commercially available grades of window and

green-house glass have normal incidence transmittances of about 0.87 and

0.85, respectively For direct radiation, the transmittance varies

markedly with the angle of incidence, as shown in Table 4, which

gives transmittances for single- and glazing using

double-strength clear window glass

The 4% reflectance from each glass-air interface is the most

important factor in reducing transmission, although a gain of about

3% in transmittance can be obtained by using water-white glass

Antireflective coatings and surface texture can also improve

trans-mission significantly The effect of dirt and dust on collector glazing

may be quite small, and the cleansing effect of an occasional rainfall

is usually adequate to maintain the transmittance within 2 to 4% of

its maximum

The glazing should admit as much solar irradiation as possible

and reduce the upward loss of heat as much as possible Although

glass is virtually opaque to the longwave radiation emitted by

col-lector plates, absorption of that radiation causes an increase in the

glass temperature and a loss of heat to the surrounding atmosphere

by radiation and convection This type of heat loss can be reduced

by using an infrared reflective coating on the underside of the glass;

however, such coatings are expensive and reduce the effective solar

transmittance of the glass by as much as 10%

In addition to serving as a heat trap by admitting shortwave solar

radiation and retaining longwave thermal radiation, the glazing also

reduces heat loss by convection The insulating effect of the glazing is

enhanced by the use of several sheets of glass, or glass plus plastic The

loss from the back of the plate rarely exceeds 10% of the upward loss

Collector Plates

The collector plate absorbs as much of the irradiation as possible

through the glazing, while losing as little heat as possible upward to

the atmosphere and downward through the back of the casing The

collector plates transfer the retained heat to the transport fluid The

absorptance of the collector surface for shortwave solar radiation

depends on the nature and color of the coating and on the incident

angle, as shown in Table 4 for a typical flat black paint

By suitable electrolytic or chemical treatments, selective

sur-faces can be produced with high values of solar radiation

absorp-tance α and low values of longwave emittance e s Essentially,

typical selective surfaces consist of a thin upper layer, which is

highly absorbent to shortwave solar radiation but relatively

trans-parent to longwave thermal radiation, deposited on a substrate that

has a high reflectance and a low emittance for longwave radiation

Selective surfaces are particularly important when the collector

sur-face temperature is much higher than the ambient air temperature

For fluid-heating collectors, passages must be integral with or

firmly bonded to the absorber plate A major problem is obtaining a

good thermal bond between tubes and absorber plates without

incurring excessive costs for labor or materials Materials most quently used for collector plates are copper, aluminum, and steel.UV-resistant plastic extrusions are used for low-temperature appli-cation If the entire collector area is in contact with the heat transferfluid, the thermal conductance of the material is not important.Whillier (1964) concluded that steel tubes are as effective as cop-per if the bond conductance between tube and plate is good Poten-tial corrosion problems should be considered for any metals Bondconductance can range from a high of 5700 W/(m2·K) for a securelysoldered or brazed tube to a low of 17 W/(m2· K) for a poorlyclamped or badly soldered tube Plates of copper, aluminum, orstainless steel with integral tubes are among the most effective typesavailable Figure 9 shows a few of the solar water and air heaters thathave been used with varying degrees of success

Because of the apparent movement of the sun across the sky,conventional concentrating collectors must follow the sun’s dailymotion There are two methods by which the sun’s motion can bereadily tracked The altazimuth method requires the tracking device

to turn in both altitude and azimuth; when performed properly, thismethod enables the concentrator to follow the sun exactly Parabo-loidal solar furnaces, Figure 10B, generally use this system Thepolar, or equatorial, mounting points the axis of rotation at the NorthStar, tilted upward at the angle of the local latitude By rotating thecollector 15° per hour, it follows the sun perfectly (on March 21 andSeptember 21) If the collector surface or aperture must be kept nor-mal to the solar rays, a second motion is needed to correct for thechange in the solar declination This motion is not essential for mostsolar collectors

The maximum variation in the angle of incidence for a collector

on a polar mount will be ±23.5° on June 21 and December 21; theincident angle correction would then be cos 23.5° = 0.917.Horizontal reflective parabolic troughs, oriented east and west,

as shown in Figure 10C, require continuous adjustment to sate for the changes in the sun’s declination There is inevitablysome morning and afternoon shading of the reflecting surface if theconcentrator has opaque end panels The necessity of moving theconcentrator to accommodate the changing solar declination can bereduced by moving the absorber or by using a trough with two sec-tions of a parabola facing each other, as shown in Figure 10D

compen-Known as a compound parabolic concentrator (CPC), this design

can accept incoming radiation over a relatively wide range of gles By using multiple internal reflections, any radiation that is ac-cepted finds its way to the absorber surface located at the bottom ofthe apparatus By filling the collector shape with a highly transpar-ent material having an index of refraction greater than 1.4, the ac-ceptance angle can be increased By shaping the surfaces of thearray properly, total internal reflection is made to occur at the me-dium-air interfaces, which results in a high concentration efficiency

an-Known as a dielectric compound parabolic concentrator

(DCPC), this device has been applied to the photovoltaic generation

of electricity (Cole et al 1977)

The parabolic trough of Figure 10C can be simulated by manyflat strips, each adjusted at the proper angle so that all reflect onto acommon target By supporting the strips on ribs with parabolic con-tours, a relatively efficient concentrator can be produced with lesstooling than the complete reflective trough

Another concept applied this segmental idea to flat and cal lenses A modification is shown in Figure 10F, in which a linearFresnel lens, curved to shorten its focal distance, can concentrate arelatively large area of radiation onto an elongated receiver Using

cylindri-Table 4 Variation with Incident Angle

of Transmittance for Single and Double Glazing and

Absorptance for Flat Black Paint

Incident

Angle, Deg

Transmittance

Absorptance for Flat Black Paint Single Glazing Double Glazing

32.10 1999 ASHRAE Applications Handbook (SI)

in Figure 12 Manufacturers of such surfaces should be asked forvalues applicable to their products, or test results that give the nec-essary information should be consulted

Example 5 A flat-plate collector is operating in Denver, latitude = 40°

north, on July 21 at noon solar time The atmospheric temperature is assumed to be 30°C, and the average temperature of the absorber plate

is 60°C The collector is single-glazed with flat black paint on the absorber The collector faces south, and the tilt angle is 30° from the horizontal Find the rate of heat collection and the collector efficiency Neglect the losses from the back and sides of the collector.

Solution: From Table 2, δ = 20.6°.

From Equation (2),

From Equation (3), H = 0; therefore from Equation (6), sin φ = 0 and thus, φ = 0° Because the collector faces south, ψ = 0°, and γ = φ Thus γ = 0° Then Equation (8) gives

From Table 1, A = 1085 W/m2, B = 0.207, and C = 0.136 Using

Equation (11),

Combining Equations (14) and (15) gives

Assuming I r = 0, Equation (12) gives a total solar irradiation on the collector of

From Figure 11, for n = 1, τ = 0.87 and α = 0.96.

From Figure 12, for an absorber plate temperature of 60°C and an

air temperature of 30°C, U L = 7.3 W/(m2·K).

Then from Equation (24),

The collector efficiency η is

The general expression for collector efficiency is

(26)For incident angles below about 35°, the product τ times α is essen-tially constant and Equation (26) is linear with respect to the param-

eter (t p− t at )/I tθ, as long as U L remains constant

ASHRAE (1977) suggested that an additional term, the collector

heat removal factor F R, be introduced to permit the use of the fluidinlet temperature in Equations (24) and (26):

(27)(28)

where F R equals the ratio of the heat actually delivered by the

col-lector to the heat that would be delivered if the absorber were at t fi

F R is found from the results of a test performed in accordance with

ASHRAE Standard 93.

Fig 11 Variation of Absorptance and Transmittance with

Incident Angle

Fig 12 Variation of Upward Heat Loss Coefficient U L with

Collector Plate Temperature and Ambient Air Temperatures

for Single-, Double-, and Triple-Glazed Collectors