Powering the LI-8100A and LI-8150 with a Solar Power Supply

Long-term automated simultaneous measurements at multiple locations within the same study site are often required to adequately characterize high spatial and temporal variability of the soil CO₂ fluxes. The multiplexed LI-8100/8150 is a fully automated system dedicated to making such measurements using up to 16 soil chambers. Increase in the global flux network coverage creates a need for unattended measurements in remote locations where grid power is not available. Use of the “off-grid” power technology should be considered at remote sites to provide power for the LI-8100/8150 operation. One of the most developed of such technologies is Photovoltaics, PV (use of solar cells to convert sunlight to electricity). PV systems consist of several key elements (e.g., solar panels, batteries, charge controllers, and cables), supporting structure (panel mounting, battery enclosure, etc.), optional equipment (combiner box, disconnects, battery meter, inverter for AC power), and require careful calculations and proper placement to function at maximum efficiency.

LI-8100/8150 Power Requirements

The first important item to consider before designing the PV system is the power requirements of the multiplexed LI-8100/8150 system. Table 1‑1 shows normal and peak requirements for configurations with various numbers of chambers.

In regular sampling mode, when no chambers are moving, the multiplexed LI-8100/8150 system would require between 22.5 and 35.0 Watts of energy. This mode lasts for several minutes at a time, depending on the specific user-programmed configuration. This is the period when one chamber is closed for sampling. After the sampling is completed, this chamber opens, while another closes, so that up to two chambers could be moving at the same time. During this process, which lasts for about 90 seconds, power requirements increase to 47.5 Watts. This is a part of the normal operation of the system, because movement of chambers could be relatively frequent (depending on the user-programmed configuration), and may constitute up to 50% of time (90-second chamber exchange followed by 90-second sampling).

Table 1‑1. Power requirements of the LI-8100/8150 system at minimum operating temperature.

Number of chambers	Sampling: no chambers move	Sampling: up to 2 chambers move at a time	Warm-up: up to 4 chambers move at a time	Number of chambers	Sampling: no chambers move	Sampling: up to 2 chambers move at a time
	Amps@12.5VDC	Watts	Amps@12.5VDC	Watts	Amps@12.5VDC	Watts
1*	1.8	22.5	2.3	28.8	2.3	28.8
2	2.8	35	3.8	47.5	3.8	47.5
4	2.8	35	3.8	47.5	4.8	60.0
8	2.8	35	3.8	47.5	4.8	60.0
16	2.8	35	3.8	47.5	4.8	60.0
* LI-8100 operating in stand-alone mode (no LI-8150 attached).

Peak requirements in Table 1‑1 are for the warm-up or re-start periods, when up to 4 chambers move at the same time, requiring up to 60 Watts. Such periods are rare, but should be taken into account when designing the PV system.

Designing a PC System

The main challenge for the off-grid PV system is to ensure that it can keep up with regular power demands, and can provide enough power during infrequent peak periods. Keys to the design of such a system are:

1. Computing power demands at the study site,
2. Evaluating how many batteries are needed to ensure operation at night and on overcast days, and
3. Determining what solar panel array is required to satisfy power demands. Table 1‑2 shows an example of step-by-step computations of these items for various numbers of chambers in the multiplexed LI-8100/8150 system.

Power demands

To compute power demands, a number of items are evaluated including regimen wattage, hours of operating in each regimen and resulting watt-hours per day, and finally, total Amp-hour requirements for the LI-8100/8150 system (steps 1 through 13 in Table 1‑2). It is important to note that steps 1-3 are specific characteristics of the LI-8100/8150 system, and should not be changed. Steps 4-6, however, are experiment-specific, and would depend on the user-programmed configuration for sampling interval and frequency. Examples here assume that sampling interval would be very small, so that while one chamber opens after sampling, another closes for sampling. If, however, the LI-8100/8150 will be configured to sample once an hour per chamber, power demands would be significantly less than in the examples here.

Watts-Hours per day are computed simply by multiplying regimen wattage by regimen duration (steps 7-9) and totaled in step 10. Controller and battery losses are conservatively assumed to be 20% (step 11), but will depend on the specific PV system: a more efficient system may have less of such losses. Finally, Amp-Hour requirement is computed by dividing the wattage by the LI-8100/8150 system voltage (step 13). This number (Amp-Hours per day) is important for both battery bank and solar panel calculations.

Battery Bank Calculations

Batteries are needed to provide power to the system during the night and throughout periods with overcast sky. The size of the bank will increase with the number of days the LI-8100/8150 needs to run without solar panel input. Examples here assume 7 such days for one chamber configuration, and 5 days for multiple chamber configurations. Steps 14-24 compute the number of 12 V deep-cycle batteries needed to provide power to the LI-8100/8150 system for the specified number of days. Because batteries should never be fully discharged, these examples conservatively assume that 50% of battery discharge is a maximum (step 16). This number may be lower, depending on the specific battery.

Table 1‑2. Step-by-step PV design calculation for LI-8100/8150 multiplexed system with different number of chambers during a nearly continuous sampling. Microsoft Excel worksheet with this calculation is available at: www.licor.com/8100-pvdesign

Step #	Calculation type	Action	Units	Number of chambers	Step #	Calculation type	Action	Units
1	2	4	8	16	1	2	4	8
Regiment Wattage
1	Sampling: no movement	from Table 1‑1	Watts	22.5	35	35	35	35
2	Sampling: up to 2 chambers move	from Table 1‑1	Watts	28.8	47.5	47.5	47.5	47.5
3	Warm-up: up to 4 chambers move	from Table 1‑1	Watts	28.8	47.5	60	60	60
Hours per day
4	Sampling: no movement	depends on user	Hrs d-1	12	12	12	12	12
5	Sampling: up to 2 chambers move	depends on user	Hrs d-1	11.9	11.9	11.9	11.9	11.9
6	Warm-up: up to 4 chambers move	depends on user	Hrs d-1	0.1	0.1	0.1	0.1	0.1
Watt-Hours per day
7	Sampling: no movement	multiply [1] by [4]	Watt-Hrs d-1	270	420	420	420	420
8	Sampling: up to 2 chambers move	multiply [2] by [5]	Watt-Hrs d -1	343	565	565	565	565
9	Warm-up: up to 4 chambers move	multiply [3] by [6]	Watt-Hrs d -1	3	5	6	6	6
Amp-hour calculation
10	Total	sum [7-9]	Watt-Hrs d -1	616	990	991	991	991
11	Corrected for battery losses	add 20% to [10]	Watt-Hrs d -1	739	1188	1190	1190	1190
12	System voltage	given	Volts	12.5	12.5	12.5	12.5	12.5
13	Amp-hours per day	divide [11] by [12]	Amp-Hrs d -1	59	95	95	95	95
Battery bank calculation
14	Number of days to support	choose	days	7	5	5	5	5
15	Amp-hour storage	multiply [13] and [14]	Amp-Hrs	414	475	476	476	476
16	Depth of discharge	choose, 0.5 is safe	fraction	0.5	0.5	0.5	0.5	0.5
17	Storage corrected for discharge	divide [15] by [16]	Amp-Hrs	827	950	952	952	952
18	Worst-weather multiplier*	depends on user	fraction	1.55	1.55	1.55	1.55	1.55
19	Total battery capacity required	multiply [17] and [18]	Amp-Hrs	1282	1473	1475	1475	1475
20	Battery rating	choose battery	Amp-Hrs	255	255	255	255	255
21	Batteries wired in parallel	divide [19] by [20]	number	5.0	5.8	5.8	5.8	5.8
22	Batteries wired in series (for 12 V)	divide [12] by 12 V	number	1.04	1.04	1.04	1.04	1.04
23	Total number of batteries needed	multiply [21] and [22]	number	5.2	6.0	6.0	6.0	6.0
24	Total number of batteries, rounded	round upward	number	6	6	6	6	6
Solar Panel Array calculation
25	Sun hours per day (Lincoln, NE)	depends on user	Hrs	4.85	4.85	4.85	4.85	4.85
26	Amps required from solar panels	divide [13] by [25]	Amps	12.185	19.596	19.621	19.621	19.621
27	Peak amperage of solar panel	choose panel	Amps	6.5	6.5	6.5	6.5	6.5
28	Number of solar panels in parallel	divide [26] by [27]	number	1.9	3.0	3.0	3.0	3.0
29	Number of panels in series (12 V)	it is 1 for 12 V	number	1	1	1	1	1
30	Total number of solar panels	multiply [28] and [29]	number	1.9	3.0	3.0	3.0	3.0
31	Total number of panels, rounded	round upward	number	2	3	3	3	3
System output and charge controller
32	Power Rating of the panel	same panel as in [27]	Watts	110	110	110	110	110
33	Output of the off-grid system	multiply [31] by [32]	Watts	220	330	330	330	330
34	Voltage of the solar array	12 V as in [29]	Volts	12	12	12	12	12
35	Controller Amp rating	divide [33] by [34]	Amps	18.3	27.5	27.5	27.5	27.5
red numbers - user-defined variables; green numbers - recommended constants

Cold weather reduces the battery output, and needs to be accounted for as well. The multiplier of 1.55 used here is for an average wintertime ambient temperature of -7 °C. If measurements are done only in summer, and temperatures are above 20 °C, there may be no need for a multiplier. The number of batteries will also depend on the battery rating. Higher rated batteries can provide more power for a longer period of time (in this example, a 255 Amp-Hour battery is suggested).

For example, after calculations are finalized and the number of batteries is rounded to the highest integer, the LI-8100/8150 system, with 2-16 chambers, configured to sample nearly-continuously in the region with averaged winter temperature of -7 °C, would require a bank of 6 high-rated, deep-cycle batteries to operate for 5 days without power input from the solar panels (7 days for 1 chamber configuration).

Solar panel array calculations. Solar panels are the only elements in the PV system that actually produce electricity. Steps 25-31 in Table 1‑2 establish how many solar panels are needed to power the LI-8100/8150 system. The number of average sun hours per day is important to determine the number of panels, and depends on the geographic location (example here uses Lincoln, Nebraska). The number of panels will also depend on the efficiency of each panel in producing current that is described by peak amperage (here 6.5 Amp peak panels are used).

Finally, after panel calculations are finalized, and the number of panels is rounded up to the next highest integer, the LI-8100/8150 system with 2-16 chambers, configured to sample nearly-continuously in the region of Lincoln, Nebraska, would require 3 solar panels with peak Amperage of 6.5 Amps (2 panels for 1 chamber configuration).

Charge controller. Charge controllers are not absolutely necessary for the PV system to function, but are very useful for protecting batteries from overcharging, and for preventing the batteries from discharging into the solar array. These substantially prolong battery life, and in the long run, make usage of the PV system less expensive, because the battery bank is often the most expensive part of the system. Advanced charge controllers may also prevent batteries from over-discharging, system overloads, and can show status of the battery and electric current flow.

Selection of the charge controller depends on the Amp rating of the PV system, which in turn depends on the system output, and rating of the panels. The PV systems in these examples generally will produce 220-330 Watts of energy (steps 32-33), which should be enough to run the LI-8100/8150 equipment and charge the battery bank for the described sensor configurations. This would require a charge controller capable of handling 18.3 to 27.5 Amps of current, respectively (step 35). Most controllers have actual ratings 25% larger than specified in order to handle unexpected short-term surges of current.

Other Important Considerations

Solar panel orientation. Solar panels should generally face toward the solar south in the northern hemisphere and toward solar north in the southern hemisphere.

The angle of inclination of the panels should be similar to the latitude of the study site plus-minus 15 degrees, depending on the season (lower in winter, and higher in summer). However, at many sites located within 50 degrees from the equator, the panel inclination angle could be permanently kept equal to the site's latitude, with no adjustment for seasonal changes. Some modern solar arrays also may come with solar trackers, making panels follow the sun to maximize the PV system efficiency.

It is important to note that for some panels, shading may be detrimental to the entire system. Shading of just one (out of several dozen) PV cell in the module may lead to a total production loss of 50%. Other panels have bridging diodes, which minimize such losses. It is crucial, therefore, to make sure one is familiar with the type of PV panels being used, and to avoid shading the panels regardless of their type.

Safety considerations. Deep-cycle batteries must be protected from the environment in a dry, well-ventilated enclosure for a number of reasons, including minimizing explosion hazard (especially for open-cell batteries, where hydrogen can be released). If a battery bank is large, a special safe battery fan may be considered inside the enclosure.

Cables and connections need to be properly chosen to handle the load in the particular PV system. All elements in the system should be carefully connected, because DC power is harder on the connections, can arc easier than AC power, and because DC circuits often operate at higher currents than residential wiring.

Installation by a professional electrician, particularly one who specializes in PV systems, is the most reliable choice, especially if equipment has been insured, or if there are specific regulations on the electrical setup and operation. However, with proper precautions, simple self-made PV systems can be successfully designed and implemented in the field to provide solar power for years of operation.

Table 1‑3. Main essential and optional components of the PV system.

Essential	Optional
Solar panels	Combiner Box
Mounting structure	DC Panel
Cable to Battery	Exterior disconnect for DC
Deep-cycle batteries	Cable-exterior disconnect
Ventilated battery box	Battery Meter
Battery interconnect cable	Inverter (for AC, if needed)
Disconnect for Battery	Cable to inverter (for AC)
Charge controller	Solar tracker
Fuses	Back-up generator
Ground	Lightning rod

Additional information on PV system design. Key essential and optional components of the PV system are listed in Table 1‑3.

There are also numerous online and journal publications on the subject, as well as manufacturer-provided manuals for specific solar panels and commercial PV systems. A few useful examples of such literature are listed below.

A brief overall review of PV system principles is provided online by Solar4Power Advanced Energy Group (http://www.solar4power.com/solar-power-basics.html). Subsequent pages of this website also provide detailed discussion on the load, solar panel, and battery bank calculations. Another good source for general information on PV systems is the “System Design” section of the Colorado Solar Electric Company website: http://www.cosolar.com/system_design/systems_home.htm. The page explains the differences between on-grid and off-grid systems, AC and DC types of PV systems and load estimates, and provides do-it-yourself instructions on assembling PV systems. A very detailed, step-by-step PV system worksheet calculator can be found in the guide produced on-line by SunWize Technology Company. This guide also contains look-up tables for the number of solar hours for a given geographic region, and for weather-related corrections on system efficiency. A number of companies conduct PV system assessment for specific cases free of charge with an equipment quote. Pricing of the commercial PV systems suitable for use with the LI-8100/8150 could range from about $1500 USD for simple configurations and minimal requirements, to several thousand dollars for complete high-capacity PV systems with large battery banks (2006 estimate).

Additional Resources

American Solar Energy Society (http://www.ases.org)

“A Consumer's Guide to Buying a Solar Electric System”, 1999. National Renewable Energy Laboratory (http://www.nrel.gov/ncpv/pdfs/26591.pdf)

Chicago Solar Partnership (http://www.chicagosolarpartnership.com/teaching_tools/index.htm)

“A Guide to Photovoltaic (PV) System Design and Installation”, 2001. Endecon Engineering for California Energy Commission, Sacramento, California (http://abcsolar.com/pdf/2001-09-04_500-01-020.pdf)

Energy Efficiency and Renewable Energy Clearinghouse (http://www.eren.doe.gov)

“From Panel to Plug: Designing the "Works" of Your Renewable Energy System”, 2004. Real Goods Renewables Staff. Real Goods Trading Corporation. (http://www.realgoods.com)

National Center for Photovoltaics (http://www.nrel.gov/ncpv/)

“Photovoltaic Systems Performance and Reliability: Myths, Facts, and Concerns”, 1997. M. Thomas and H. Post. Sandia National Laboratories (www.sandia.gov/pv/hot/Pvq_496.htm)

“Renew the Public Lands: Photovoltaic Technology in the Bureau of Land Management”, 1996. T. Duncan, H. Post and M. Thomas. Sandia National Laboratories. (http://www.sandia.gov/pv/lib/rnwpblc.pdf)