Net Zero?

Lily Pond House Solar Plant

It has been exactly a year since we moved to our Lily Pond House. So how did our passive and active solar design perform? How close did we get to our net zero goal where annual energy consumption is roughly equal to renewable energy generation?

Starting with the passive solar design with South facing wall of glass, every sunny day was a joy during winter. On those sunny days with freezing temperatures, we often had to open windows as the temperature reached about 78 degrees after the living room cement slab could no longer absorb the sun’s heat. If the cement slab was not covered with furniture and rugs, we could have stored more solar energy but life trumps energy efficiency. Radiant heating did not have to kick in about 3+ hours after sunset while the cement slab warmed the house releasing the stored heat.

One of the best decisions our HVAC contractor Jim Godbout Plumbing & Heating made at the suggestion of our solar contractor Revision Energy was to go with an all electric energy for the house. Daikin Altherma, air to water heat pump that provides low temperature hot water for radiant heating and radiators, proved to be an efficient workhouse. We didn’t have a single day laboring to heat the house thanks to Theodore and Theodore Architects high performance shell design with minimal thermal bridging. Having radiant heating in the living room and basement cement slab, and bathroom tiles also  helped the cause with heat retention.

For active solar, Revision Energy designed a ballasted system that does not require the installation to penetrate the roof, which is a plus. However, solar panels are then limited to 15 degree pitch because beyond that the wind load gets too high and the required ballast gets too heavy for roof support. Although the optimal angle for solar panels is about 40 degrees for our location, the total annual production is fairly insensitive to installation angle since we can bank excess production for 12 months by feeding the excess solar electricity  into the grid. This is because the lower installation angle is optimized for the summer months when the sun’s radiant energy is at its highest. The penalty for 15 degrees tilt is less than 8%. Low pitch installations also hold snow longer than steeper ones;however, the penalty for snow cover is about less than a 10% effect in southern Maine.

Revision Energy estimated our expected electricity consumption at 11 MWh per year:

Plug Loads: Design estimate was 450 kWh a month, or 5,400 kWh per year. Although it was on the low side for a 2,500 sq. ft. house, it was justified by all LED lighting and  energy efficient appliances.

Heat: Design estimate was 15,000 BTU/sq. ft. for the expected annual heat load. That translates into 38 MBTU/year for space heating, given the 2.5 Coefficient of Performance (COP) for Daikin Altherma heat pump  yields an expected electricity consumption of an additional 4,450 kWh per year.

Domestic Hot Water: Design estimate was 40 Gallons per Day (GPD) load with two occupants at 20 GPD each. That translates to an annual load of 10 MBTU/year. Given the 3.25 Energy Factor  performance of our Voltex Hybrid Electric Heat Pump  water heater, that translates to an additional electric load of 1,200 kWh per year.

11 MWh solar plant design called for 36 255 -Watt Canadian Solar photovoltaic panels. Unfortunately, our two roofs could only accommodate 27 panels. So we had to go with the more efficient 300 Watt LG NeON panels resulting in a 10 MWh per year power generation.

Best Month – May

Worst Month – January

Since roughly half of our total electric load is space heating, our electric consumption in winter is effectively supplied by our solar production in summer. Our best month was May with 50 kWh electricity production on sunny days while our worst month was January with 20 kWh electricity production on sunny days except when the panels were covered with snow, which was about 10 days. Since photovoltaic panels can capture scattered sunlight to create electricity, they generated electricity even on cloudy days.

Solar Electricity Production by Month

Revision Energy’s initial design employed Solectria PVI inverters, which convert the DC output of a photovoltaic (PV) solar panel into AC that can be fed into the electrical grid or used by house electrical network.  As the solar  crew was laying out the roof and moving panels around to accommodate the parapet, and roof obstacles (e.g. vent pipes), they felt there might be enough partial shading on this system so it would really benefit from a slightly more advanced inverter solution. At their suggestion, we went with a Solaredge system that combines a central string inverter in the basement with individual panel optimizers on the back of each individual panel to improve system performance, especially when part of the array is shaded. In contrast to a traditional inverter system where the partial shading of a panel typically results in the total loss of power from that panel in order to optimize the power of all panels in the same string, Solaredge recovers the partial power from the shaded panel. In addition, Solaredge provides module level monitoring and performance information, which is a pretty neat bonus.

Upper Roof Panels Performance

Solar system was designed to be a net generator in summer and net consumer in winter.  As the data shows, Lily Pond House Power Plant is a net generator roughly 5 months (May, June, July, August, September), a net consumer 5 months (November, December, January, February, March), and roughly balanced  in the transition months (April, October).

Generated vs. Consumed Electricity

So what is the bottom line? We consumed 11.5 MWh while the consumption estimate was 11MWh. So if we could generate 11 MWh per our original design, we would have a net zero home. We took a 10% hit due to roof constraints, and another 10% hit because 10 MWh solar design produced only 9 MWh, mostly due to parapet shading. At the end, we generated 9 MWh of solar electricity while we consumed 11.5MWh. For the 2.5 MWh shortfall, we paid $300, which we really can’t complain about given the environmental wellness, which encourages us to live in harmony with the Earth by taking action to protect it:

 

 

Mechanical Wizardry

Based on the nicest looking mechanical rooms in the portfolio of homes we visited, our Design Development conversation, and  our architect’s and builder’s recommendations, we went with Jim Godbout for our mechanical subcontractor. As a preference, we wanted radiant heating in order to avoid forced hot air with its space wasting duct work, noisy operation, dry air generation, and dust ball production that we grew to hate over the years. Current green builder consensus is to use ductless mini-splits to reduce the cost of radiant heating installation. Mini-splits eliminate the noise of conventional forced hot air systems by locating the noisy compressor and condenser outside the house, and do away with the  bulky ductwork by pumping refrigerant directly to wall mounted blowers – albeit ugly – inside the house. However, they still produce an uneven heat distribution as you don’t have to be a heat transfer specialist to see that radiant heating which makes the whole floor the heat source will produce a more even heat distribution than a mini-split with a point heat source. At the end of the day, we just couldn’t accept a forced hot air solution with mini splits after observing that every house with radiant heating we visited seemed more comfortable. Plus subfloor heating has been around for centuries from the hypocausts in Turkish baths to hot water pipes under floors in Frank Lloyd Wright’s buildings in 1930’s.

For Lily Pond House, Jim Godbout designed a radiant floor heating system everywhere except panel radiators in the bedrooms. In contrast to forced hot air, which is a convective system that uses air to distribute the heat, radiant heating system uses mostly energy waves to distribute heat from warmer (i.e. floor) to cooler (i.e. people) objects, is more efficient, and provides a more uniform temperature distribution with less dry air issues. For the living room, entrance, and basement, Crete-Heat foam insulation panels (3″ R-15) were installed in the cement slab to secure the Viega PEX radiant tubing in place. For first and second floor common areas and baths, radiant tubing were applied below sub floor with Low-E Tab insulation under radiant system. For the bedrooms, we went with high efficiency low temperature Biasi steel panel radiators.

Viega PEX radiant tubing over Crete-Heat insulation

Radiant heating needs low temperature water (85º-95º for radiant tubing, 105º-120º for radiator panels), and saves energy by having to heat to the lower temperature. Design Development called for propane fired Viessmann Vitodens condensing boiler (one of the highest efficiency (98%) boilers with excellent reliability) for delivering the low temperature water to the radiant PEX tubing and Viessmann Vitocell for domestic hot water. Before the Construction Documents got finalized, we decided to install solar panels on the second roof in order to complement the passive solar design towards a net-zero home. Our solar energy subcontractor Revision Energy came up 8.1 kilowatt-hours (kWh) photovoltaic array with 27 LG Mono X NeON  300 watt solar panels. Maine has a progressive solar energy policy with “net energy,” which is the difference between the energy used and the energy generated over a month. If Lily Pond House panels generate electricity in excess of our monthly usage, the unused kWh credits go into a “bank”. Our utility CMP invoices based on net energy for the month, taking into account any accumulated kilowatt-hour credits from the previous 12 months, which is better than the net billing in Massachusetts where the utility purchases the excess electricity at the wholesale price. Our solar panels are expected to generate roughly 10,000 kWh of clean electricity annually, and offset roughly 9,000 lbs. of CO2 emissions annually. Currently, our electric utility charges $0.15/kWh. Assuming that electricity costs stay constant over the next 25 years as we are also ignoring the cost of capital, we will recover our solar energy capital investment in the first half of the life span. In other words, we will have free electricity for the second half and beyond after the first half. Another way to think about this is that we are locking the price of electricity at $0.08 for the next 25 years, which translates to $18K of savings if we had purchased the same electricity from our utility at the current price. Currently, Maine does not provide the State incentives that, for instance, Massachusetts provides, which substantially reduces the period for recovery of capital.

Radiant heat transfer aluminum plates for PEX sub floor installation

In The Third Wave, futurologist Alvin Toffler coined the term “prosumer” when he predicted in 1980 that the role of producers and consumers would merge in the future.  Given our decision to become a prosumer of electrical energy that we’ll be able to generate abundantly in the summer and bank it for winter use, our mechanical subcontractor changed the heating design to Daikin Altherma – an air to water heat pump with backup electric coil. Altherma air-to-water heat pump uses a sustainable energy source – heat from the outside air. The heat extracted by the Altherma outdoor unit is transferred via refrigerated lines to the indoor Altherma heat exchanger, which uses the intake to heat and distribute the water to the radiant heating tubes in the floors, and low temperature radiators. Altherma delivers about 4 kWh of usable heat for every 1 kWh of electricity it uses, giving it a high coefficient of performance (COP) of 4. For the domestic hot water, the design changed to Voltex Hybrid Electric Heat Pump from A. O. Smith. Voltex reduces water heating cost by 2/3 by extracting the heat from the ambient air. In our application, the water heater is placed in the unconditioned basement so as not to rob off the heat from the conditioned space. In addition to the heat pump, this model also has heating elements that help the water heater recover quickly during periods of high demand.

In older New England homes, it is not uncommon that Air Changes per Hour (ACH) can record between 12 to 30. Maine Uniform Building and Energy Code is currently based on the 2009 International Residential Code (IRC), which calls for 7 ACH. The Department of 2012 Energy Air Leakage Guide  ratchets up the target to 3 ACH for our climate zone, which is yet to be adopted in Maine. Our architect specified 0.6 ACH – the Passive House standard as the target, which dictates fresh air ventilation. The ventilation design called for Venmar Energy Recovery Ventilator (ERV).  Using the energy consumed by a compact fluorescent bulb, ERV provides cost effective fresh air ventilation while recovering the heat exhausted out of the house. In the winter, the heat and humidity of the outdoor fresh air is transferred to the intake air stream, thus not drying out the inside air during the heating system. In the summer, the heat and humidity of the outdoor fresh air is transferred to the exhaust air stream, to keep the inside air humidity low. We shall see.