We need to creat hot lava using Sun to power our homes
My 1.3 peak kW photovoltaic powerplant reduced my energy bills by half. Could Sun be the solution for our energy needs? What about winter?
The recent energy crisis and rising energy prices caused many people to consider local electric production. Mainly like photovoltaic (PV) solar power plants combined with battery storage. European Committee is even considering making roof solar pannel mandatory for new buildings. Could the sun solve our energy issues? Perhaps, but we need something else than batteries.
My 1.3 kW Photovoltaic powerplant
A few years back, I decided to extend my house with a winter garden with built-in semi-transparent photovoltaic panels. I do like to combine the pleasant with the useful and this seems like a good idea. The cost of the roof made of solar panels was actually slightly lower than if made of durable glass.
The power output of 8 panels is just 1.3 peak kW (under ideal conditions). So I installed a 1 kW three-phase micro-inventor, which is on sale just for 250 EUR. So the initial investment was low. How much energy savings do I get?
The power consumption was significantly reduced from 9.5kWh to 5.6kWh (as of Jun). The system doesn’t have any batteries. So about 2.7 kWh of the excess power is supplied back into the grid. However, as it is subtracted from my energy bill. So I only pay the power distribution fees. This virtual battery works effectively almost as effectively as a local battery. The current daily savings is about 1 EUR for most of the year. Hence the payback period is 2–3 years. So is solar the solution for our energy needs? Could it reduce the energy needed all year?
Solar works only during the summer
As we see can see in the following graph, we need about 2–3 times more power for our factories, trams and other assets than during the night. As photovoltaic (PV) works only during daylight, it seems reasonable that PV could provide the majority of our electricity needs.
We can compensate for cloudy days with some large battery storage. However, this is an annual average. What about, if we look at how much energy is required and produced in the different seasons? A typical PV plant makes a fraction of energy during the winter in Europe. This is caused by the short daylight and also a suboptimal angle of the solar panels towards the sun. On the other hand, the energy consumption increases especially due to heating.
Let’s check some numbers of modern, passive energy standard family houses equipped with 6pkW solar panels [source] and a heat pump.
As illustrated above, if the house would have sufficient battery storage capacity, it could be self-efficient between March and October. The amount of energy produced daily is actually 2x–3x higher than what is needed. The excess energy will be either wasted or surged into the power grid.
On the other hand, the energy required during the winter is many times higher than PV can be produced. And we need to expect that there will be a period of cloudy days when zero power would be generated. Overall, the battery would not be even charged during daylight and the house is dependent on the electricity from the power grid.
Hence we need some electricity to power our home appliances and another way to keep the house warm and to create hot water, especially from November to February.
How to heat the house efficiently during the winter
We could simply just use electricity, and use our heat pump to power the house. However, that would not solve our energy independence. The simplest alternative and the most common and available technology is to use pellet stoves. Trees and bushes store the sun's energy for us as they grow.
The pellets stove doesn’t take much space, cost up to 10.000 EUR, and allows fast regulation of the required heat. Also burning wood is CO2 neutral and efficient. We will need about 2000 kg of pellets as 1kg provides 4.9 kWh of heat. The cost of 1000kg of pellets is about 250 EUR. That is a very cheap source of heat considering today's prices. However, tens of bags with pellets will need to be stored and the stove storage re-filled every week. And unfortunately, there is not enough biomass to cover all our heating needs.
As we have exceeded energy during the summer, the question is how we could store that energy. Obviously, we could not use a typical electric battery. That would be too expensive as we would need to store GWh of power.
Is there some other way how to store the heat?
What about storing heat in a water tank or some other material? Just heat it up during the summer and release the heat during the winter.
If you review more the specific heat capacity of the commonly available materials, water is the winner. It has one of the highest thermal capacities, which is 4.18 kJ per kg per K. So how much water do we need?
If we would heat 10.1 liters of the water by 80C, we would store 1kWh of energy (1 kWh = 3 600 kJ = 4.18 kJ * 80 * 10.1). To cover the heat needed for the house, we should store about 8,000 kWh of energy. That is 86m3 of water. That is a cube storage tank of 4x4x4 meters or a cylinder of diameter 9m and 1m in height.
The storage tank should be well insulated, so we don’t waste the energy in the ground, however, the size doesn’t seem unrealistic. It could be placed under the house so leverage even the residual heat. We could well isolate some bladder water tanks or build that storage directly under the house.
Could we reduce the volume?
The second factor to consider the required volume is the temperature difference. For example, Iron has almost the same volumetric heat capacity (3.537) as water (4.1796). We could heat it to several hundred degrees instead of 90° C. If we heat iron just to 480° C, we would decrease the volume by 4.25x. On the other hand, the thermal radiation increase with the fourth power of temperature (T⁴). That is 16x higher energy loss. So some fire-proof insulation like ceramic fiber will need to be used to slow down the heat exchange.
So, we would need about 20 m3 of iron or 24 m3 of dry sand/granite. That is a cube with 3x3x3 m3 or a sphere of a 1.8m radius (to minimize the heat loss). The volume could be even smaller if we heat up the material to higher temperatures. Sand is available everywhere, cheap at the price of less than 100 USD per cubic meter.
Could this idea be realized?
Sounds like it is not just a theory, but such systems are being built already. Close to Berlin is just to be opened 56 million liters water tank available to store about 200 MWh of heat. Innovative startups like Ecovat are looking to provide a smaller local system.
And as I was writing this article, the news bring information about the startup from Findland Polar Night Energy, which introduce to the World the first sand-based heat storage. Eventually, the sand can be heated up to 1000 °C. That would allow generating electricity by creating steam & turbine or by a highly efficient Stirling engine.
So it seems physically feasible to use solar and a small battery during the summer to cover house energy needs. The excess energy could be stored in the form of heat in either water or sand battery for the winter season.
Perhaps, we could reconsider the old pre-verb not to build our houses on the sand. And construct sand basement heat storage under each new building.
