Can You Run A Heat Pump On Solar?

How Many Solar Panels Are Needed To Run A Heat Pump?

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In an effort to be environmentally friendly, and to save money, many homeowners are opting for a heat pump heating system. What if you could combine your solar panels with this in order to provide the power needed?

Solar energy is one of the most efficient ways to produce electricity and it makes complete sense to become more energy-independent, both from the point of view of saving fossil fuel resources and also trimming the household budget.

A combination of solar and heat pump would allow users not only to reduce consumption levels in high-demand winter months but also cut down dependency from utility companies – one of our biggest goals for the future is to be eco-friendly!

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How Many Solar Panels Are Needed To Run A Heat Pump?

As a general rule you would need 1500 watts of solar power (5 x 300 watt solar panels) for every 1 Ton of AC heat pump rating. Energy storage in the form of deep cycle batteries and a suitably sized inverter may also be required.

Can you run a heat pump on solar panels?

How much solar power would a heat pump need?

Can you run a heat pump on solar at all? It might seem like an odd question, but it is one that has been asked by many homeowners in the past.

The answer to this question depends on which type of heat pump is being used and how much power it requires.

Solar panel heat pump combination

In order to determine how much energy a particular type of heat pump will need, we first need to know what type of system they are running off of: air-to-air or ground source.

Once we know what type of system the homeowner has installed, then we can figure out which wattage rating needs to be put into play for their solar panels.

This is an important question for anyone who is considering installing a solar panel system to power their home. The answer will be dependent on several factors including the size of your solar panels:

  • the size and type of heat pump you have installed in your house
  • how efficient the heat pump is (the more efficient it is, the less energy it will need)
  • what other forms of heating you use in your house

How Do Solar Heat Pumps Work?

The heat pump has been around for some time now but its implementation is still not perfect. A true solar heat pump uses solar thermal collectors to collect the sun’s energy, rather than PV electric panels that just harvest power and store in batteries or other energy storage devices.

The Thermodynamics Solar System combines both these technologies by joining two incomplete technologies together: a heat-pump and a solar thermal collector. After this stage, the liquid passes  through to an an exchanger to complete the heat energy transfer.

Solar assisted heat pump

Diagram – components of a solar assisted heat pump  Courtesy Sciencedirect.com

This kind of solar heat pump is not the subject of this post – our concern is investigating the possibility of running a conventional heat pump with solar electric PV panels.

  • Is it possible to run a heat pump with solar?
  • How many solar panels would be need to run a heat pump on solar?
  • What other equipment would be needed to run a heat pump with solar panels?

In this post we’ll take a look at how much power and energy each type of heat pump needs to function, how much energy solar panels can generate for any location and what other equipment is need to run it.

First of all, let’s look at the basic operation of an ordinary heat pump.

How does a heat pump work?

A heat pump is a device that can transfer thermal energy from one space to another with reduced input of external power. Typically they can generate 400% more heat or cooling energy than the input energy required to operate them.

They are mainly used in heating and cooling buildings by transferring the opposite direction as would happen naturally, using refrigeration cycles which use electric or gas powered compressors.

The key takeaway here is that, compared to other forms of heating or cooling, the electric power required to operate them, is much reduced – this makes operating them by solar panels particularly interesting!

How many types of heat pump are there?

Three different types of heat pumps exist: air-to-air, water source, and geothermal (ground source). They collect heat from outside your home (the air) or the ground below it to keep you nice and warm inside during winter months.

The most common type is the air-source heat pump which transfers this natural warmth between your house and its surroundings for a cosy interior environment – even if it’s snowing outside!

Ground source heat pumps extract heat from the ground, either from a lattice of pipes embedded a metre underground, or from a single borehole that is drilled much deeper.

Heat pump efficiency is measured by its coefficient of performance (CoP), a value between 2.5 to 4, which is the ratio of energy input in kW to heating energy output in kW.

Ground source heat pump run on solar?

Loops of underground pipes exchange heat – ground source heat pump.

The temperatures underground are relatively constant throughout the year. At the center of the Earth it’s 10,800°F, but we don’t have to go that far to see some significant temperature change!

The temperature at 2 meters (12 feet) underground is fairly constant all year round at about 55°F, depending on location and horizontal loops of piping are laid  in trenches over a wide area.

If space is limited, one or more deep boreholes can be drilled up to 300 feet (100 metres) deep. In general, the temperature increase by 1°F for every 40 feet of depth.

Water source heat pumps are not generally used for in domestic situations, as the body of water needs to be of a substantial volume to absorb the heat differential created.

We’ll consider ground source heat pumps for running with solar panels.

How much power does a heat pump use?

Before tackling the question of how many solar panels we need to run a ground source heat pump (GSHP), we need to work out how much energy an average pump takes.

Let’s use a 3 Ton GSHP for our example. We’re not really interested in the complete design cycle for the exchanger loops, etc, but we do need to know how many watts it pulls when starting and running – yes, these two values are different!

It gets complicated because heat pumps, like refrigerators, freezers and anything else with a compressor motor, runs in cycles. Sometimes it will be off, then it will start and run for a while until target temperature is reached.

The start current is known as the surge current and is typically 2 to 3 times bigger than running current!

Larger units have soft-start electronics added to ramp up the voltage slowly to reduce this start load.

Because of the unknowns related to heat pump cycle times and currents (and also Power Factor) it’s much easier to think in terms of energy consumed over time measured in kilowatt-hours (kWh).

1 ton = 3517 watts (approximately) so out hypothetical 3 ton GSHP unit is rated at about 10.55 kilowatts (kW).

Typical energy consumption is 5500 kWh/year.

How much does it cost to run a heat pump?

Assuming the average cost of electricity in the US to be $0.15, the cost of running a 3 ton heat pump would be around:

5500 kWh x 0.15 = $825 per year

This represents about half the cost of an equivalent size furnace/air system.

Can solar panels power a heat pump?

Of the many factors affect solar panel efficiency, orientation, title angle and irradiance at your location are the most important.

Normally, the first two can be optimized according to the building roof design, but the irradiance is completely beyond our control – it depends on where you are!

In the USA, the variation in how much sunshine you get over the year can be substantial, depending on which state you’re in.

How much sunshine falls on a solar installation is measure in kilowatt-hours per meter squared per day (kWh/m2/day).

I’ll choose a location at random to illustrate – Jackson, Mississippi and use a site called Global Solar Atlas to find the right values for this location:

The image below shows the irradiance value for Jackson:

Irradiance (insolation) for Jackson, Miss,USA

How much sunshine does Jackson, Miss get?

The value of 4.716 kWh/m2/day is also known as Peak-sun-hours and is used to calculate how many solar panels are needed for any particular location’s energy needs.

This is how we work out the number of panels to run a 3 ton ground source heat pump:

Pump energy requirements per year = 5500 kWh

1 x 285 watt solar panel generates 0.285 x 1721 peak-sun-hours = 490 kWh/year

Number of 285 watt solar panels required = 5500/490 = 11.2 solar panels (12)

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Note: The above assumes optimum orientation, tilt angle and partial shading optimized by micro-inverters.

There is a slight problem with the above calculation – it uses energy usage and production figures over the whole year.

However, if the heat pump is used mostly for space heating, with a small proportion for water heating, then most of the energy will be used in the winter months!

The table below shows the irradiation from the sun for each month of the year for Jackson, Mississippi.

Insolation by month for Houston, Texas by month (kWh/m2/day - also are known as Peak-Sun-Hours)


JAN

FEB

MAR

Apr

May

Jun

Jul

Aug

Sep

OCT

NOV

DEC

2.52

3.24

4.24

5.34

5.89

5.98

5.80

5.36

4.79

3.99

2.94

2.39

In the summer months the energy available from the sun in 6 months of summer is 33.16 kWh/m2/day (or Peak Sun Hours) while in the winter period it is just 19.36, so the energy available from the sun in winter is a whopping 42% less than in summer!

Is this a problem? It depends on what kind of solar system you have installed, or are intending to install.

How Grid-Tie Solar Installation Works

Can you run a heat pump from solar panels - grid tie system

Net metering measures solar power in and out of the grid

The vast majority of home-owners will have a grid-tie solar system. Basically, net-metering keeps track of power going in and out of the grid.

In the summer months there’s likely to be over-production, so power goes into the grid and the utility company stores credits for this. In the winter months it’s likely that there will be solar under-production and power is drawn from the grid – the summer credits will now be used.

With a grid-tie solar installation variations between summer/winter solar production don’t really matter, as it all gets ironed out by the credits tracked by the net-meter.

However, this is not the case with an off-grid solar system.

How does a basic off-grid solar system work?

Can I run a heat pump with an off-grid solar system?

Batteries are need to run a heat pump with an off-grid system

By definition, an off-grid solar installation has no grid connection, so everything relies on how much power can be generated and the size of the battery bank used for home energy storage.

A big enough set of batteries to power the home for 1 to 2 days is typical, but account needs to be taken of the difference in production efficiency across the seasons.

We know for Jackson, Mississippi that winter solar generation is 42% less than in summer, so that needs to be factored in when calculating solar panel wattage. In this case the calculation becomes:

Number of 285 watt solar panels required = 5500/490 x 100/42 = 26.72 solar panels (27)

As you can see, quite a difference!

In addition, the batteries must be capable of handling the associated compressor surge current, unless a soft-start device is installed.

Summary – Can you run a heat pump on solar?

A  medium rated heat pump can be run on solar and integrated into an existing grid-tie residential solar system by simply adding more solar panels and up-grading the inverter. Variations in seasonal solar production will be smoothed out by the net-metering credits system.

In the case of an off-grid solar system considerably more solar panels are need to meet the demand in winter and an adequately sized battery bank for energy storage.

How Many Solar Panels Are Needed To Run A Heat Pump?

As a general rule you would need 1500 watts of solar power (5 x 300 watt solar panels) for every 1 Ton of AC heat pump rating. Energy storage in the form of deep cycle batteries and a suitably sized inverter may also be required.

People also ask

Do heat pumps use a lot of electricity?

A heat pump may sound like an expensive investment, but the benefits outweigh the costs. A typical one-to-one (single unit) will raise your electricity bill by around $75 per month – depending on where you live and how many units are in use at any given time. However, using a single unit of air conditioning could cost up to five or six times that amount!

Is it cheaper to leave heat pump on all day?

With colder temperatures arriving, it is important to make sure you are using your home’s heat pump efficiently. Leaving the machine on when not in use can be expensive and wasteful. The best way to ensure that happens? Plan ahead for times where you know there will be a low need for heating as well as high demand periods so both extremes have time to recover before switching back again!

Should I turn off my heat pump at night?

Even when it’s cold, a heat pump can keep you warm. But if your house doesn’t have good insulation and the temperature drops quickly at night, then turning off the heat pump during that time is better than allowing all of its energy to go out into nothingness through outside air vents or cracks in ceilings or walls where there are no windows. If possible, use thick covers on top of mattresses for extra warmth!


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