by
Patrick L. Bruckhart, Patrick J. Burns and Timothy A. Dierauf
pbruck@lamar.colostate.edu, pburns@ACNS.colostate.edu, and tdierauf@lamar.colostate.edu
Solar Energy Applications Laboratory (SEAL)
Colorado State University
Fort Collins, CO 80523
A area (m2)
cp specific heat (kJ/kg-C)
E energy (kJ)m mass (kg)
m mass flow rate (kg/hr)
P pressure (N/m2)
amb ambient (outdoor)
aux auxiliary tank
by bypass flow, from mains into the mixing valve
col collector
draw draw
in into the component or system
hor on the horizontal
hx heat exchanger
mains water mains
NC natural convection (water loop)
out out of the component or system
p primary, solar tank
pump solar collector loop pump
R room (indoor, room in which the storage tanks exist)
tilt on the tilted surface
w wind
1. Introduction
This paper provides additional detail to accompany the data sets taken as part of our Solar Domestic Hot Water activity, sponsored by the Department of Energy. Descriptions of the physical systems tested are given in Section 2. The details of the measurements are given in Section 3.The testing procedures are presented in Section 4. The file formats and availability are given in Section 5.
2. Physical Systems
The two systems include: (1) an Integral Collector Storage (ICS) system which uses a pump to circulate water between the solar storage (collectors) and the auxiliary storage tank, and (2) a Natural Convection Heat Exchanger (NCHX) system, utilizing glycol in the collector loop and water in the natural convection heat exchanger loop with the solar storage tank. Table 1 provides the system characteristics.
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Table 1. System Characteristics |
||
|
System Characteristic |
ICS |
NCHX |
|
Manufacturer |
Nippon Electric Glass (NEG) |
Thermodynamics |
|
Model |
PK-20 |
Solar Boiler |
|
Net Collector Area, m2 (ft2) |
3.00 (32.3) |
5.67 (61.0) |
|
Primary Solar Tank Volume, m3 (gal.) |
- |
0.21 (56.4) |
|
Aux. Tank Volume, m3 (gal.) |
0.14 (36) |
0.14 (36) |
|
Nominal Collector Flow Rate, m3/s (ft3/s) |
5.9 x 10-5 (2.1 x 10-3) |
1.8 x 10-5 (6.4 x 10-4) |
|
Pump Motor Size, W (hp) |
7.5 (1/100) |
248.6 (1/3) |
|
Controller DT - On/Off, °C |
12/4 |
7/2 |
The collectors are mounted facing due south at a 45° slope on the collector test bed located about 6.1 m (20 ft.) south of the basement of Solar House 1 at Colorado State University. Note that the aperture area is given for the ICS system. A TRNSYS IAM file is available from the authors for the ICS system. From there, piping is run into the basement of Solar House I, where all storage tanks (except the ICS collectors), pumps, and the external, natural convection heat exchanger are located. Also in the basement of Solar House I is the HP System 10 data acquisition system and the 80386 computer used for data logging. Labtech Notebook is used to control the acquisition, manipulation and storage of data.
The plumbing to the solar collectors begins in the basement, travels through a small annex used to house a desiccant wheel (a run of 3.51 m, or 11.5 ft), and then continues outdoors to the collectors (a run of 7.16 m, or 23.5 ft). All systems are connected in parallel to both the water main supply line and the mixing valve to the hot water draw line (
Figure 1). Except for the 0.0127 m (1/2 in.) sweated copper reducers/expanders near the pumps and the supply and return for the NCHX system (described later), all piping is 0.0187 m (3/4 in.) sweated copper. All lines are well insulated thermally. To minimize pressure drop, all valves are 0.0187 m (3/4 in.) or 0.0127 m (1/2 in.) (these 1/2 inch valves are used to isolate only the pumps) through-flow ball valves.2.1 ICS System
The ICS system consists of two collector modules, each with four evacuated tubes. Each evacuated tube contains 0.019 m3 (5 gallons) of integral storage. The inner tube which holds the water is coated with a selective surface of black chrome with an absorptance of 0.93. The aperture area is 3.00 m2 (32.3 ft2). The two modules are connected in parallel. The heat loss coefficient of these collectors is sufficiently low such that they are able to maintain temperatures above freezing for several days of inclement weather and no freeze protection is required for the modules. However, the pipes to and from the collector are subject to freezing, and consequently, rupture, and are therefore are drained during periods of freezing weather.
The ICS system plumbing is shown in
Figure 2. An induction pump, rated at 7.46 W (1/100 hp) circulates the water between the collector storage and auxiliary tank at a nominal rate of 5.9 x 10-5 m3/s (0.94 gpm). The pump was initially installed by the manufacturer to prevent overheating of the collectors. During a draw, the pump is inactivated due to the water mains supply pressure, when water mains supply flows out to the collectors. Since a draw is about 0.079 m3 (21 gallons) in volume, essentially half of the water from each collector module is displaced with main supply water. The displaced collector water flows into the bottom of a standard auxiliary tank. During a draw, energy is removed from the top of the auxiliary tank.The pump is operated by a differential controller with the upper dead band set at 12 °C and the lower dead band set at 4°C. The thermistors that measure the temperature difference are located on the outlet of the collector and on the bottom of the auxiliary storage tank.
2.2 Natural Convection Heat Exchanger (NCHX) System
The NCHX system consists of two standard 1.2 m x 2.4 m (4 ft. x 8 ft.) micro-flow collectors connected in series. The solar absorber consists of two aluminum fins metallurgically bonded to a copper tube and constructed in a serpentine fashion. The absorber plate is an anodic cobalt selective surface with an absorptance of 0.92. The cover is manufactured of low-iron, tempered glass. The supply tubing to the collector is 0.0095 m (3/8 in.) diameter and the return tubing is 0.0064 m (1/4 in.) diameter. Each tube is 7.62 m (25 ft.) in length.
A system schematic is presented in
Figure 3. This system utilizes 40% propylene glycol/60% water solution pumped from the heat exchanger to the collectors at a nominal rate of 1.8 x 10-5 m3/s (0.3 gpm). The pump, an impeller type rated at 248.7 W (1/3 hp), is operated by a differential controller. The upper dead band is set at 7 °C and the lower dead band is set at 2 °C. The thermistors that measure the controlling temperature difference are located on the outlet of the collectors and inside the heat exchanger shell.Upon returning from the collectors, the glycol enters the coil of a high performance heat exchanger, located adjacent to the solar storage tank. The overall heat transfer coefficient for the heat exchanger is 380 W/m2K (67 Btu/hr ft2 °F). The heat exchanger is a shell and coil type with four copper coils enclosed in a copper shell. Each coil is a copper tube 30.48 m (100 ft.) in length and 0.0064 m (1/4 in.) in diameter, with a heat exchanger area of 0.61 m2 (6.5 ft2). This tubing constitutes four concentric coils with an overall height of 0.41 m (16 in). The glycol flows in parallel through the four coils. This, together with the small diameter tubing to and from the collector, accounts for the pump being of much larger size than for the ICS system.
Water, inside the shell, circulates from the heat exchanger to the solar storage tank via natural convection, driven by the temperature difference between the hot glycol and the colder solar storage tank water. During a draw, mains supply flows into the bottom of the solar storage tank while the hot water at the top of the solar storage tank is transferred to the bottom of a 0.14 m3 (36 gallon) auxiliary tank. Energy is removed from the top of the auxiliary tank.
3. Instrumentation and Measurements
Instantaneous measurements were logged every 15 seconds. Two types of data files are provided: (1) data averaged over five minute intervals, and (2) data, not averaged over time, at 15 second intervals. The former data files are useful in assessing overall system performance, and for comparison with simulation (i.e. TRNSYS). The latter data files are useful when one is interested in the details of the draws, and are indeed needed to calculate the energy draws accurately. Measurements are classified into two categories: (1) environmental (ambient weather station and indoor room temperature) measurements, and (2) systems measurements. These are described separately by section below. Each data record is time stamped with with the current or averaged Mountain Standard Time.
3.1 Environmental Data
The weather station consists of two pyranometers used to measure horizontal and tilt radiation, a wind speed anemometer, a wind direction indicator, and a shielded, outdoor ambient temperature sensor. The horizontal pyranometer is located on the roof of Solar House 1, with an unobstructed view of the sky. The remainder of the weather station, including the tilt pyranometer (tilted at 45o, the collector slope), is located adjacent to the solar collectors, at a nominal height of two meters. Thus, the total tilt radiation, outdoor ambient temperature, wind speed and direction are indicative of the microclimate which the solar collectors experience. Also included along with the weather data are the shielded, indoor temperature readings, TR, located in the room adjacent to the storage tanks.
3.2 Common Systems Measurements
Figure 1 shows the common piping diagram for the supply and drain. Note that the supply (mains) water is provided to the NEG collectors directly, the primary, solar tank of the NCHX system, and to the common mixing valve. Note that the common, constant-temperature mixing valve was not used to control the draw temperature for these tests. Instead, the draw was unmixed allowing the temperature to float until draw temperature and mains temperature are within 1 oC. It should also be noted that the draw flow meter failed during testing. Mains flow should be used to determine the draw flow.
3.3 Individual Systems Data
The electrical power consumed by the circulating pumps and the auxiliary heaters are measured for both systems. The pressure differences across the pumps are also measured for both systems. Volume flow rates are measured at the locations shown on Figures 2 and 3. However, there is one exception to this; the mass flow rate is measured by the Micromotion flow meter for the natural convection loop. For consistency, the mass flow rates measured here are converted to volume flow rates using an equation of state for the density of water. An equation of state for density is necessary to compute mass flow rates. Temperatures are measured as indicated on the system diagrams, in the piping and tanks. Each tank is divided into eight nodes of equal volume and height. Eight temperature measurements are taken at the centroid of each node.
It should be noted that negative flows are possible for the natural convection loop of the NCHX system.
3.4 Conversions
All readings are converted to the engineering units shown in Section 5, and small values are set to zero. Raw data files and calibrations are available from the authors upon request.
4. Test Procedures
4.1 Test Protocols
The systems are allowed to operate normally, according to the controllers supplied with the systems (the on/off differentials are provided in Table 1). The systems were tested to meet the intent of the OG-300 protocol as follows:
Cold Start
a. Turn on pump and purge collector, solar storage tank, and auxiliary tank until draw temperature and mains temperature are within 1 oC, except for the thermodynamics system collector loop. The temperature difference for this will be reasonably small.
b. Commence data acquisition at approximately 9 AM solar time. Instantaneous data, sampled at 15 sec intervals, will be logged.
c. Circulating pumps will be energized by the differential controllers. To establish a definite initial state during quiescent conditions, data will be logged for 10 minutes before the systems tests are started.
d. Collect data until at least a 10 oC temperature rise in the average temperature of the water tank(s) is achieved.
e. Cover collectors, and draw all energy out of system through the auxiliary tank until draw temperature and mains temperature are within 1 oC. Draw will be between 2 and 3 GPM. Note that the energy will be purged from the Thermodynamics system first. During this time the NEG system will be allowed to operate normally. The NEG system will be covered and purged after the Thermodynamics system has been purged.
f. One data set will be collected during a "sunny" day, one during a "cloudy" day, and one during a "partly cloudy" day. It should be noted that the desired temperature rise was unatianable for the cloudy days.
Warm Start
a. Turn on the heating element in the auxiliary tank. Pump through collector, solar storage tank, and auxiliary tank until all tank and collector temperatures are within 1 oC, except for the Thermodynamics system collector loop. The temperature difference for this will be reasonably small.
b. Shut off heating elements. Commence data acquisition at approximately 9 AM solar time. Instantaneous data, sampled at a 15 sec intervals, will be logged.
c. Circulating pumps will be energized by the differential controllers. To establish a definite initial state during quiescent conditions, data will be logged for 10 minutes before the systems are turned on.
d. Collect data until a temperature rise of at least a 10 oC in the average temperature of the water tank(s) is achieved.
e. Cover collectors, and draw all energy out of system through the auxiliary tank until draw temperature and mains temperature are within 1 oC. Draw will be between 2 and 3 GPM. Note that the energy will be purged from the Thermodynamics system first. During this time the NEG system will be allowed to operate normally. The NEG system will be covered and purged after the thermodynamics system has been purged.
f. One data set will be collected during a "sunny" day, one during a "cloudy" day, and one during a "partly cloudy" day. It should be noted that the desired temperature rise was unattainable for the cloudy days.
5. Data Files
5.1 Availability
The data are stored in comma delimited, free format ascii files. Carriage returns separate each time stamp record. Use the following links to download the various sets:
Cloudy
Cloudy
5.2 Fields and Column Headings
The files can be read using Fortran format statements, or as free-form files. Before using the files for the first time, it is suggested to view the files with a text editor, or using the Unix utilities head and tail.
5.3 Format of Ambient Weather Data File - Files ambient.15s and ambient 5m
Field Units Description, Variable name
1 (hr) Time in hours and fractions of hours, i.e. 3.50 is 3:30 AM
2 (C) Inside ambient room temperature, TR
3 (C) Outside ambient temperature, Tamb
4 (kJ/hr-m2) Total irradiation on the horizontal, Ihor
5 (kJ/hr-m2) Total irradiation on the tilt (45o), Itilt
6 (m/hr) Wind speed, Vw
7 (degrees) Wind direction, qw
5.4 Format of ICS System Data File - Files ics.15s and ics.5m
Field Units Quantity
1 (hr) Time in hours and fractions of hours, i.e. 3.50 is 3:30 AM
2 (C) Water mains temperature, Tmains
3 (C) Auxiliary tank inlet temperature to the mixing valve, Tin,mix,aux
4 (C) Auxiliary tank inlet temperature (from collector), Tin,aux
5 (C) Collector indoor inlet temperature, Tin,col,R
6 (C) Pump inlet temperature, Tin,pump
7 (C) Collector outdoor inlet temperature, Tin,col
8 (C) Outside collector outlet temperature, Tout,col
9 (C) Node 8 (bottom) auxiliary tank temperature, Taux,8
10 (C) Node 7 auxiliary tank temperature, Taux,7
11 (C) Node 6 auxiliary tank temperature, Taux,6
12 (C) Node 5 auxiliary tank temperature, Taux,5
13 (C) Node 4 auxiliary tank temperature, Taux,4
14 (C) Node 3 auxiliary tank temperature, Taux,3
15 (C) Node 2 auxiliary tank temperature, Taux,2
16 (C) Node 1 (top) auxiliary tank temperature, Taux,1
17 (C) Mixed draw temperature, Tdraw
18 (m3/hr) Collector volume flow rate, Vcol
19 (m3/hr) Water mains supply volume flow rate, Vmains
20 (N/m2) Pressure difference across the pump, DPpump
21 (W) Electrical power input to the pump, Epump
22 (W) Electrical power input to the auxiliary tank, Eaux
5.5 Format of NCHX System Data File - Files nchx.15s and nchx.5m
Field Units Quantity
1 (hr) Time in hours and fractions of hours, i.e. 3.50 is 3:30 AM
2 (C) Water mains temperature, Tmains
3 (C) Auxiliary tank outlet temperature to the mixing valve, Tin,mix,aux
4 (C) Pump outlet (glycol) temperature, Tout,pump
5 (C) Heat exchanger glycol inlet temperature, Tin,hx,col
6 (C) Heat exchanger outlet temperature (water to tank), Tout,hx,p
7 (C) Heat exchanger inlet temperature (water from tank), Tin,hx,p
8 (C) Primary tank outlet temperature, Tout,p
9 (C) Outside collector inlet temperature, Tin,col
10 (C) Outside collector outlet temperature, Tout,col
11 (C) Mixed draw temperature, Tdraw
12 (C) Node 8 (bottom) primary tank temperature, Tp,8
13 (C) Node 7 primary tank temperature, Tp,7
14 (C) Node 6 primary tank temperature, Tp,6
15 (C) Node 5 primary tank temperature, Tp,5
16 (C) Node 4 primary tank temperature, Tp,4
17 (C) Node 3 primary tank temperature, Tp,3
18 (C) Node 2 primary tank temperature, Tp,2
19 (C) Node 1 (top) primary tank temperature, Tp,1
20 (C) Node 8 (bottom) auxiliary tank temperature, Taux,8
21 (C) Node 7 auxiliary tank temperature, Taux,7
22 (C) Node 6 auxiliary tank temperature, Taux,6
23 (C) Node 5 auxiliary tank temperature, Taux,5
24 (C) Node 4 auxiliary tank temperature, Taux,4
25 (C) Node 3 auxiliary tank temperature, Taux,3
26 (C) Node 2 auxiliary tank temperature, Taux,2
27 (C) Node 1 (top) auxiliary tank temperature, Taux,1
28 (m3/hr) Collector volume flow rate (glycol), Vcol
29 (m3/hr) Natural convection loop volume flow rate, Vloop
30 (m3/hr) Water mains supply volume flow rate, Vmains
31 (N/m2) Pressure difference across the pump, DPpump
32 (kJ/hr) Electrical power input to the pump, Epump
33 (kJ/hr) Electrical power input to the auxiliary tank, Eaux
