Living off the grid with comfort using Solar and Air to Water Heat Pump
Living Off the Grid with Comfort
Combining multiple renewable and semi-renewable technologies allows you not only to live off the grid but to have the same comfort you could have had when living in a grid tied home.
Proper design to accomodate owner's project requirements is a determinant factor for project's success and longevity.
Introduction
Public perception of living off the grid is that, this type of living does not offer the same comfort level as of a similar house connected to public utility grid. This might have been true in the past, in the early days of renewable energies development and when semi-renewable energies were not as advanced as today.
In such a case, you weren't able to decide when to fulfill basic daily needs (such as showering, washing the dishes, doing laundry, etc....) whenever you want, which reduced occupants comfort level and made off the grid living a temporary living (weekends, holidays, etc...).
This blog will show, how a combination of renewable solar energy (Thermal, Hybrid and Photovoltaic as a primary energy source) and semi-renewable energy (Air to Water Heat Pump or Geothermal Heat Pump as a secondary heat source) allows living off the grid without sacrificing personal comfort.
Case Analysis
The easiest way to present an off the grid design is to use data from an existing projects. We have recently finished a project in the Ottawa (ON) area. Property is 1.5 hour drive from the city of Ottawa and land where project is located has no electricty and no natural gas supply lines.
Due to the high estimated cost, given by Ontario Hydro, to supply property with a 200 Amp panel, customer contacted us to see if going off the grid will be cheaper. After few weeks of design iteration, customer decided to go off the grid since the off the grid cost was less than the infrastrcture cost associated with tying-in the future house to the utility grid.
Off the grid house has the following caractersitics:
| Item | Description | Value |
|---|---|---|
| 1 | House Type | Slab on Grade Bangalow, Area 2400 ft² (223 m²), Tilted Roof with tilt angle of 45° |
| 2 | Nominal Heating Load | 23 457 Btu/hr (6.875 KW) - 9.77 Btu/hr.ft² (30.83 Watts/m²) |
| 3 | Roof R value | 75 |
| 4 | Walls R Value | 38 |
| 5 | Slab R Value | 75 |
| 6 | Windows Thermal Properties | U value 0.16 (IP) and SHGC 0.21 |
| 7 | Space Heating Type | Low Temperature Hot Water Hydronic Infloor Heating (Water Supply Temperature 95°F (35°C)) |
There was multiple holistic design reviews with project's architect and design team to optimize the thermal properties of the envelope, in the off the grid context. Since envelope has a longer life span of any HVAC or electrical system, priority was given to investing in more energy efficient and durable envelope.
Base Scenario
Base scenario serves as a benchmark for comparing the Off-The-Grid deesign with a conventional HVAC design. Assuming that the above property will be heated by hydronic infloor heating and that hot water (whether for space or domestic hot water (DHW) heating) is produced by an electric boiler, Annual/Monthly heating demand will be as follows:
Total Annual space heating demand will be 12 805 KWh (57.42 KWh/Year.m²), with peaks in December, January and February each year.
For annual DHW consumption, we assumed two adults and two kids. Result was an annual demand of 3 492 KWh. The Total combined demand for space heating and DHW heating is 16 297 KWh/year (73 KWh/Year.m²).
Propsed Off the Grid Design:
The propsed Off the grid design incorporates Solar Thermal, Solar Hybrid (Thermal and Photovoltaic), Solar Photovoltaic (for the plug loads only) and an Air to Water Heat Pump. We have also included a boiler as an emergency heat.
Vacuum Tube Solar Collectors
Evacuated Tube Solar Collectors are installed at the end of the row of solar thermal panels. This type of panels have great thermal performance in Canadian winter and can produce medium temperature hot water regardless of the outdoor temperature.
Hybrid PVT Solar Panels
Hybrid PVT (Photovoltaic and Thermal) are installed at the begining of the row because this type of panel have a greater tthermal performance in the summer and lower thermal performance in winter. Also their thermal performance diminishes with the entering fluid temperature to the panel. This configuration allows the vacuum tube to snow melt flat plate PVT panels after a heavy snow storm. PVT Panels can be a great heat source for a swimming pool in the summer.
PVT panels electricty generation is higher than conventional PV due to the fact that fluid circulating in the thermal module helps cool down PV cells which increase their electrical efficiency.
Air to Water Heat Pump
ATW heat pump acts as a secondary heat source. In the abscense of solar energy production, ATW heat pump uses electricty stored in batteries to produce Hot Water for both Space Heating and DHW heating.
Thermal Energy Storage
Hot Water or Hot Water/Glycol Mixture is stored in buffer tanks to provide heating demand at night when the sun is absent. During the day, we overproduce thermal energy to instantaneously heat the house and supply it with DHW and to also store heat for the night.
Electrical Energy Storage
DC power produced by Hybrid PVT Panels and PV Panels is stored in electrical batteries. Batteries were sized for three days peak winter range. This means that batteries are able to supply energy required for plug loads, heating and DHW heating for 3 consecutive days without solar energy production.
Solar Simulation Results and Quantity of Panels
The primary heat source for Heat Transfer Fluid for space heating will be the combination of Vacuum Tubes and Hybrdid PVT Panels.
After multiple iterations to optimize the quantity of solar thermal and hybrid PVT Panels, the final optimial scenario, has 4 x 30 Evacuated Tube Panels and 10 x Hybrid PVT Panels.
Evacuated Tubes Annual/Monthly energy output is as follows:
Total annual energy production for the evacuated tube panels is 7 094 KWh for an annual demand of 16 297 KWh. This represents 43.5% of the annual space heating and DHW heating demand.
Hybrid PVT solar Panels annual/monthly thermal and electrical energy production is as follows:
Thermal Performance
Electrical Performance
Total annual thermal energy production for the Hybrid PVT panels is 3 543 KWh. Most of the production happens in the summer and the shoulder season. The only useful heat is the one produced between January and May which is 1 310.8 KWh. This represents only 8% of the annual thermal energy demand.
Hybrid PVT solar Panels annual electricty production is 4 151 KWh. Annual Air to Water Heat Pump annual consumption will be 3816 while the backup emergency boiler will consume 103 KWh/year.
Air to Water Heat Pump
Backup Boiler
Solar Thermal & ATW Heat Pump design summary
Annual Solar thermal energy production represents 51.53% of the annual thermal energy demand. The remaining 48.47% of thermal energy are produced primarily by the ATW Heat Pump which consumes 3 816 KWh, produced by the hybrid PVT panels, to produce 7789 KWh.
| Type of Demand | Value of Demand | Vacuum Tube Solar Collectors Generation | Hybrid PVT Thermal Energy Generation | Hybrid PVT Electrical Energy Generation | ATW Heat Pump Electrcial Consumption | ATW Heat PumpThermal Energy Production | Boiler Power Consumption / Generation |
|---|---|---|---|---|---|---|---|
| Space Heating (KWh) | 12 805 | 4 650 (36%) | |||||
| Domestic Hot Water Heating (KWh) | 3 492 | 2 444 (70%) | |||||
| Total Demand (KWh) | 16 297 (100%) | 7 094 (43. 53%) | 1 310.8 (8%) | 4 151 (25%) | 3 816 | 7 789.2 (COP=2.04 - 47.8%) | 103 (0.63%) |
Electrical Plug Loads are primaly produced by PV panels since 94.4% of the electrical energy produced by Hybrid PVT panels is used to power the air to water heat pump and backup boiler.
PV Array Annual electricity generation
PV Array - 18 Panels (625W each)
The Annual electrcity generation is 13 383 KWh. The gree graph shows monthly electrical production. Despite the fact that peak production happens in July, having the panels installed at 45 degree tilt angle not only reduces the risk of snow accumulation on the panels but increases the monthly winter electricity generation and reduces the deficit in production during the winter months with peak demands.
Conclusion
The cost of the above Solar / Heat Pump System is 110 KCAD$ (Materials + Installation). A house the same size as this one, will cost around 7 KCAD$/year in utility bills. Customer can get around 20K CAD$ Federal + Provincial Grants. The pay back for the above system will be (110-20) KCAD$ / 7 KCAD$/Year = 12.8 Years.
The life span of a similar system is 25 Years. that means half of the life span will be spent on paying back the investment. The local Utility Bill price to hook up the new house to the existing grid was 225-250 KCAD$.