Electricity

The goal of the tiny house's electrical system is to supply the power needed to maintain a comfortable lifestyle in the Chicago area, relying solely on clean and renewable energy. Thus, an off-grid, stand-alone photovoltaic (PV) system with battery bank harvests and stores electricity for the home. All major household loads are extremely efficient, and12VDC powered in order to avoid loss when converting to 120VAC.  The system is optimized for maximum power generation and sized according to calculated estimates of daily power consumption and available solar energy. A 1120W array is flush mounted to the 140 square foot roof.

 

System Components

Below is a schematic of the entire PV system:

Figure by Alejandro Sklar

The system’s various components, pictured above, and their functions are as follows:

Photovoltaic Array

The array is comprised of four large format, 72-cell, MEMC  PV panels. The panels are each rated at 280W, and are 14.3% efficient.

The array is flush mounted to the house's roof using the IronRidge, flush-mount racking system.  Quickmount flashings are used for waterproof attachment to the roof rafters. Ideally, to maximize the power from the sun's rays, the panels would be tilted at 42 degrees (Chicago's latitude). However,  because of height constraints due to highway restrictions, the roof is only tilted 25 degrees towards the South.

                                                                         

Data from PV Sun angles: http://www.yourhome.gov.au

    
    OUTBACK Power System's PV-12 outdoor rated combiner box is used for combining PV strings, and providing main PV disconnect circuit breakers.

12-Volt Battery Bank and Maximum Power Point Tracking (MPPT) Charge Controllers

Off-grid systems must store electrical power in batteries because the amount of electricity generated by PV systems varies from day to day, and power demand often increases during nighttime hours (when no solar energy is available). The batteries then distribute electricity to the home when necessary. Also, electricity generated by PV panels is inherently DC and can therefore directly charge a battery bank.

Many efficient household appliances can be powered using 12VDC, therefore a the house contains a 12V battery bank . 12-Volt batteries usually operate best within a “cutoff” region between 10 to 15 Volts. Operating at a voltage outside of this region decreases battery performance and lifespan.

The bank consists of three, 4D, 225Ahr, gel-sealed batteries that are not adversely affected by constant charging and discharging, unlike traditional lead-acid batteries. The battery bank resides inside an IronRidge outdoor rated battery enclosure, which sits on the trailer hitch.


Maximum Power Point Tracking (MPPT) Charge Controllers

The PV panels operate best at a specific voltage and current, or “maximum power point” (MPP), which can range up to 60 Volts. Thus, small MPPT microcontrollers are wired in series with the modules to regulate the panel voltage.

However, because 60 Volts is well above the battery bank’s “cutoff” region, the MPPT charge controllers simultaneously regulate their output voltage to efficiently charge the batteries. 

The system contains two of OUTBACK Power Systems' FLEXmax80 MPPT charge controllers.


Off-Grid Inverter

Most common American appliances require 120-Volt, 60-Hertz alternating current (AC) electricity; however, the PV modules generate direct current (DC) electricity. Although none of the vital household loads require AC power, one OUTBACK Power Systems FX2800W off-grid inverter is included in the system to allow for the use of a few household items. FLEXware250 safety modules are used on both the AC and DC sides of the inverter, providing proper GFDI and surge protection.

This inverter uses a pure sine-wave to modulate the input signal, producing the exact AC signal used by the American power grid. This will increase the performance and overall lifespan of the home’s AC loads.

A drawback of any inverter is that 10% of the power passing through the inverter is lost during the conversion. Also, the inverter itself consumes a small amount of electricity during operation;  therefore minimizing the use of AC loads is vital.

 

Control Panel

A well-designed PV system informs the homeowner of the status of each component, and displays the exact amount of charge stored in the battery bank. Depending on conditions, the homeowner may also want to interact with the system to modify the operation of certain circuit elements.

OUTBACK Power Systems' products can all connect and communicate with a MATE control panel through an ethernet connection to a multi-port HUB.

One should note that OUTBACK sells pre-wired product packages (the FLEXpower ONE and TWO) that combine one FX inverter, one FLEXmax charge controller, a FLEXware250 DC and AC, and the HUB/MATE control panel into a single, space-saving module that is simple and easy to install.


OUTBACK FLEXpower ONE includes:
FX Inverter (left), FLEXmax MPPT Charge Controller (bottom right), MATE (middle right), HUB (top right)


AC/DC Circuit Breaker Panels

Electrical sub-panels are required for routing power to both AC and DC loads. Midnite Solar sells a "BabyBox" panel with minimal footprint, that can be used for wiring up to 4 AC or DC circuit breakers.

Midnite Solar "BabyBox" Circuit Breaker Panel


Vital Household Loads

   
The key to generating enough electricity in such a small space is the use of efficient electrical loads. As previously mentioned, one way of doing so is to use 12VDC appliances.

    Lighting

    
Although the house is naturally lit during the day by its window, at night, 12VDC LEDs are used. LEDs are inherently low-voltage and DC powered, and are about twice as efficient as CFLs and ten times as efficient as traditional incandescent bulbs.


   
The LEDs used in the house are 7W, E27 high power white bulbs for use with recessed lighting housings. The bulb screws into a medium Edison screw, and boosts a whopping 90 lumens/watt efficacy. It can be purchased at tmart.com



    Refrigeration

    Traditional refrigerators can easily consume over 2000 W-hrs per day; however, companies like Sundanzer and NOVAKOOL offer a variety of 12VDC refrigerators and freezers that consume as little as 100 W-hrs per day.

    For this project, where space and energy are both key issues, the NOVAKOOL R-4500 refrigerator was chosen. This model consumes 480 W-hrs per day. Exterior refrigerator dimensions are 20.25"W x 22"D x 28.75"H.

   
    
    
Water Circulation & Purification
   
    Harvested water must be purified with UV radiation, and pumped to the house's two faucets. Also, from time to time, water in the storage pillow must be aerated. The house's water system requires one FLOJET sprayer pump (48W, 12VDC), one Vitria TrojanUVMax UV Sterilizer (13W, 12VDC), and one PowerBubble Water Aerator (4W, 12VDC).

 

   
    
Ventilation


    Wiring Schematic




Figure by Alejandro Sklar

Safety Measures

To ensure proper protection from electrical hazards during operation and maintenance, the following safety measures are implemented that adhere to NFPA 70 national electric codes.


Circuit Breakers

To protect equipment against high current surges, circuit breakers are connected to all DC and AC main lines. These must be rated to withstand 125% of the line’s current.

Thus, 10 Amp breakers must be connected to the positive terminal of each panel. For increased safety, 10 Amp fuses are also connected to the negative terminals.

Additionally, each branch in the DC combiner box utilizes 63Amp DIN breakers, and each inverter connects to its own 30Amp AC breaker panel.

 

Lightning Arrestors

PV systems are prone to lighting strikes, during which immensely high voltages and currents flood the circuit. To protect equipment from these strikes, the system contains the following three components:

·      DC lightning surge arrestors between the PV array and charge controllers

·      AC lightning surge arrestors between the inverters and control panel

·      AC lightning surge capacitor between the inverters and control panel

All three components are made by Delta; however, the AC surge protection is already taken care of by the OUTBACK Power Systems FLEXware250 AC.

 

Grounding

To help dissipate electrical charge that can build up at any moment, especially during a lightning strike, the entire electrical system, along with any exposed metal surfaces, must be properly grounded to a large conducting surface. Thus the ground bars are connected to the metal trailer bed. When parked, an 8ft copper grounding rod is extended down from the trailer bed and dug into the ground.

In the case of lightning strike or ground fault, current spreads across the trailer bed and conducts into the ground, but the house and any inhabitants remain unharmed.

 

Disconnect switches

To allow for safe maintenance, disconnect switches must be installed between the various system components. Because circuit breakers function as disconnect switches, this has already been taken care of in the system.


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