National Energy Systems are Wasteful: About 80% of the fossil energy taken out of the ground never reaches the end user. Then the user feeds the energy into devices with efficiencies as little as 5%, such as incandescent light bulbs. Such wasteful, national energy systems developed over the decades, because the fossil energy was, and still is, low-cost. Even district CHP plants with thermal and electrical distribution systems, popular in Denmark, etc., have low efficiencies, if all energy losses and costs are taken into account, because the CHP plants may be efficient, but the distribution systems and connected buildings are not.
Going forward, economies of scale or not, with the prospect of fossil fuels being depleted, and with variable, intermittent RE costs being 2 – 4 times fossil, that type of national energy system would become increasingly less feasible, except, may be, for the richest nations, such as Germany. However, with its continued expensive RE build-outs under the ENERGIEWENDE program, and its continued BAU lifestyles, even Germany likely will soon be less competitive going forward; its GDP growth, 0.45%/yr. for the 2009 – 2013 period, 5 yrs., has become near zero in 2014.
As all technologies are fully developed and proven, more energy could be locally generated and locally consumed in energy-efficient buildings, all “under one roof”, as shown by the alternatives in the article. There would be massive resistance from special interests to go into that direction, as they have grown big by exploiting the fossil fuel-addicted society for at least the past 100 years.
Building Energy Efficiency: The energy efficiency of buildings did not become an issue until after the 4-fold increase of crude oil prices in 1973. The owners of mostly energy-hog buildings, seeing major increases in their heating and cooling costs, consulted with engineers to make energy surveys of buildings, which, after implementation of the recommendations, usually resulted in at least 50% decreases of energy consumption.
Such efficiency improvements regarding houses did not take place until much later, and then only on a case by case basis, because politicians were, and still are, very slow to upgrade building codes. For them it is so much easier to be for heavily subsidized, highly visible, renewable energy, than for lightly subsidized, invisible, energy efficiency.
Because CO2 emissions are one of the factors affecting global warming and climate change, it would be desirable to have buildings meet the goal of “net-zero-energy and near-zero CO2 emissions”.
Alternatives for Houses: Below are two energy, CO2 emission, and cost reduction alternatives for houses; one goes only part way towards the goal, the other goes much further.
The first alternative is having a standard, code-designed house to which is added a grid-connected PV solar system with sufficient capacity to charge a plug-in vehicle. This alternative would achieve CO2 emission reductions, but would be a long way off from the desirable goal of “net-zero-energy and near-zero CO2 emissions”.
The second alternative is having a very energy-efficient house to which is added a PV solar system with sufficient capacity to charge a plug-in vehicle, plus an electrical energy storage system and a thermal energy storage system. This alternative would achieve “net-zero-energy and near-zero CO2 emissions”.
Standard House, With Grid-connected PV Solar System and Plug-in Vehicle
This alternative is used worldwide, especially in Germany. Its main attraction is using the generators on the grid to supply steady, 24/7/365 energy when PV solar energy is insufficient or absent, at least 80% of the hours of the year.
In effect, the grid connection is a valuable, free (to the homeowner) energy service mostly paid for by the other ratepayers. To add to that free service, politicians often bestow high feed-in rates for any excess PV solar energy that cannot be used by the homeowner.
An average standard house uses about 6,000 kWh/yr and one plug-in vehicle consumes about 12,000 mi x 0.30 kWh/mi = 3,600 kWh/yr. In New England, the PV solar system capacity would need to be about 10 kW to produce 10 kW x 8,760 hr/yr x capacity factor 0.14 = 12,264 kWh/yr; this CF likely is optimistic, see Note 7. It would produce energy during the day and feed any excess into the grid, to be withdrawn at night to charge one or two plug-in vehicles.
Investment and Energy Cost Savings: The cost of the PV system would be about $40,000 less subsidies. Bills for electricity and gasoline would be minimal, but bills for space heating and domestic hot water, DHW, about $3,000/yr. (about $4,000 before tax), would remain.
Energy Efficient House, Off the Grid, With PV Solar System and Plug-in Vehicle
This alternative is becoming increasingly attractive, as the prices of PV solar systems have decreased and subsidies are generous. As battery systems become more widely used for electrical energy storage, their prices will decrease as well. Homeowners should receive the same 30% subsidy for the battery systems, as now applies to PV solar systems. The off-the-grid mode is distributed mode energy production and consumption under one roof. Heretofore, it was not economically feasible, now it is.
The off-the-grid mode can readily be applied to Passivhaus-type, freestanding houses, or Passivhaus-type housing developments; the latter would be have, say 20 pre-fabricated units to a building using centralized systems. The building would have a PV solar system on the roof, or have a parking area with a roof covered with PV solar panels. Energy use per household would be significantly less than for a Passivhaus-type, freestanding house. Here is how this would work for a freestanding house.
Investments and Energy Cost Savings: An absorbed glass mat, AGM, battery system costs about $300/100 Ah. A 2,000 Ah system, wired for 48 V, sufficient for about 6 days, would cost about $6,000; see Note 4. A PV solar system costs about $4,000/kW of panels. An 8 kW system would cost about $32,000 less subsidies.
On the grid, in a standard, code-designed house, no PV solar system, bills for electricity $1,200, space heating + DHW $3,000, and gasoline $1,500, would total about $5,700/yr. (about $7,500 before tax). Off the grid, in an energy-efficient house, they would be minimal.
Off-The-Grid: My starting point is a relatively NEW, freestanding house, similar to a Passivhaus, NOT grid-connected, with properly angled rooflines, proper solar orientation and passive solar features, and using about 80% less energy per square foot for heating, cooling, and electricity than a relatively NEW, standard, code-designed house.
In winter it would be challenging, as several days may pass with near-zero electrical and thermal energy generation. About a week’s consumption of electrical energy and domestic hot water storage would be required in less sunny areas, such as New England.
For living off the grid, in a near-zero-CO2 mode, the house would need to be equipped with:
– A roof-mounted, PV solar system + an AGM battery system with 12 V batteries, wired for 12, 24, or 48 V output, with charge/discharge controller + a hot water storage tank with DC electric heater + a system with DC pump and water-to-air heat exchanger.
– A gasoline-powered, 2 – 4 kW AC generator with 50-gallon fuel tank to periodically charge the batteries to about 90%, in case of insufficient PV solar energy during winter, due to fog, ice, snow, clouds, etc.
– Any excess electricity would bypass the already-full batteries and go to the electric heater in the DHW tank. Any excess thermal energy would be exhausted from the DHW tank to the outdoors.
– A whole house duct system to supply and return warm and cool air, with an air-to-air heat exchanger to take in fresh, filtered air and exhaust stale air at a minimum of 0.5 air changes per hour, ACH, per HVAC code.
– For space cooling, a small capacity, high-efficiency AC unit would be required on only the warmest days, as the house will warm up very slowly.
– For space heating, a DC electric heater, about 1.5 kW (about the same capacity as a hairdryer) for a 2,000 sq ft house, in the air supply duct, would be required on only the coldest days.
– A plug-in EV, such as a Nissan, or plug-in hybrid, such as a Chevy-Volt, would be charged with DC energy from the house batteries by bypassing the vehicle AC to DC converter, provided the house batteries have adequate remaining storage energy, kWh, for other electricity usages. During some winter days, this may not be feasible, as not enough PV solar energy would be available; public chargers would be needed.
Household Energy Management: To determine the capacity of the energy systems, list all the energy users on a spreadsheet, how much they use (amp-hours/day) and what time periods they are on and off. The sum will give the hour-to-hour energy consumption per day, or per week. Subtract the hour-to-hour PV energy generation to yield the hour-to-hour surplus (charges the batteries) or deficit (discharges the batteries). Energy consuming items can be scheduled on and off to manage the energy flows. If there is a prolonged period of no sun, the engine-generator supplies the energy. Having as many DC devices as possible reduces DC to AC conversion losses.
SUMMARY
The above alternatives clearly show to provide off-the-grid, standard (mostly energy-hog) houses with PV solar systems, and electrical and thermal storage systems, they would need to be of such large capacity the costs would be prohibitive, if “net zero-energy and near-zero CO2 emissions” is the goal.
As a result of better building practices and materials much more energy-efficient houses can be constructed. Such houses, equipped with efficient mechanical and electrical systems, and the lower cost PV solar and battery systems, enable more and more homeowners to “live off the grid”, plus charge one or two plug-in EV or hybrid vehicles.
PV systems have at least 25-year useful service lives, and battery systems, if property operated, have at least 10 to 15 year useful service lives. The homeowners will be enjoying annual cost savings for heating, cooling, electricity and gasoline for decades that are sure to increase year after year, plus they have the satisfaction of minimizing their CO2 emissions “footprint”.
NOTES:
1) If an EV travels 12,000 m/yr. at 0.30 kWh/mile, about 3,600 kWh/yr. would be required, equivalent to the production of a 3 kW PV solar system in New England. Gasoline cost avoided = 12,000 mi/yr. x 1 gal/28 mi x $3.50/gal = $1,500/yr.
2) Because PV solar systems have become much less costly, it would be less complicated and lower in O&M costs to increase the capacity of the PV solar system to also provide electricity for DHW, instead of having an $8,000 roof-mounted solar thermal system for DHW; no tube leaks, freeze-ups, less moving parts, etc. With a properly insulated, large capacity DHW tank, say 250+ gallons, there would be enough DHW for 5 – 7 days.
3) A maximum of about 70% of battery nameplate rating is available. To prolong the useful service life well beyond 8 years, batteries should typically be charged to a maximum of 90% and discharged to not less than 70%; shallow cycling. Very rarely should they be discharged to a minimum of 20%; deep cycling reduces life. Also, life is prolonged if charging and especially discharging is slow; a few amps for many hours is much better than many amps for a few hours. Depth-of-Discharge, DOD, factor = 100/(90 – 20) = 1.4. If 50% DOD, then DOD factor = 100/(90 – 40) = 2.0, battery capacity would be 2.0/1.4 x 2100 = 3,000 Ah, and cost would be $9,000.
4) Battery charging loss is about 10% and discharging loss is about 10%, i.e., input 100 kWh, store 90 kWh, output 81 kWh. Inverter DC to AC efficiency, low at low outputs, increases to about 90% at rated output (at which it almost never operates); i.e., using multiple inverters and minimizing DC to AC conversion by using DC devices (fans, pumps, heaters, etc.) avoids losses.
Example of required battery capacity = 10 kWh/d x 6 d x 1.4 DOD factor x 1.2 loss factor = 100.8 kWh, or (1000 x 100.8) Wh/48 V system = 2,100 Ah.
House low energy usage = 0.5 kW x 1 h x 1/0.5 inverter eff x 1/0.9, battery loss = 1.11 kWh from battery, or (1000 x 1.11) Wh/48 V = 23.2 Ah
House high energy usage = 2.0 kW x 1 h x 1/0.8 x 1/0.9 = 2.78 kWh from battery, or (1000 x 2.78) Wh/48 V = 57.9 Ah
PV solar energy to battery; overcast winter day = 8 kW x 4 h x 0.16 CF x 0.9 battery loss = 4.61 kWh; generator is needed for a few hours.
PV solar energy to battery; sunny summer day = 8 kW x 6 h x 0.70 CF x 0.9 battery loss = 30.24 kWh; excess energy; heat swimming pool, sauna, hot tub?
Here is another example of required battery system capacity using Ah.
10,000 AC Wh average daily consumption/0.9 Inverter efficiency = 11,111 Wh/d.
11,111 Wh/d/48 V DC system = 231 Ah/d.
231 Ah/d x 1.11 battery temperature factor x 6 days autonomy x 1.4 DOD factor = 2,153 Ah.
2,153 Ah/100 Ah individual battery capacity = 22 batteries, round off to 24; batteries with greater Ah may be used.
48V system voltage/12V battery voltage = 4 batteries in series.
6 parallel strings x 4 batteries = 24 batteries.
A battery bank of 24 100 Ah batteries will provide ample energy storage to meet the daily requirements, inverter loss, cold temperature inefficiency and days of autonomy while keeping the DOD at less than 70%. This example also assumes there will be zero PV solar energy for 6 days, which is unlikely.
5) As space heating and cooling would be required for just a few days of the year, an air-source heat pump would be overkill and too expensive in this case.
6) The PV solar system needs to be oversized to ensure adequate electrical and thermal energy during winter when the monthly minimum winter irradiance is about 1/4 – 1/6 of the monthly maximum summer irradiance. See monthly output from 2 monitored solar systems in Munich; 1/6 appears about right in South Germany.
7) Whereas, the daily or weekly maximum solar output of Germany may be up to 60% of installed capacity, kW, during a very sunny period, it may be near zero, due to fog, ice, snow, clouds, etc. As a result, Germany’s mix of PV solar systems (old and new, dusty or not, partially shaded or not, facing true south or not, correctly angled or not) has a low nationwide capacity factor of about 0.10. This compares with a New England CF of about 0.12; the theoretical CFs are about 0.12 for Germany, about 0.143 for New England.
source: http://theenergycollective.com/willem-post/2162036/comparison-grid-connected-and-grid-houses
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