Let's store some energy

Let's say, hypothetically, that you are in a place where thermal energy is abundant in the summer, but scarce in winter. I have no idea where the inspiration for this absolutely hypothetical situation comes from, but bear with me.

And let's further say that you know that your household uses about 3,000 liters of heating oil every year, which comes out to about 2,520 kg. Since heating oil has a gross heating value of about 43 MJ/kgSource: https://www.engineeringtoolbox.com/fuels-higher-calorific-values-d_169.html, that's about 108,360 MJ of heat energy for one year. Quite an awesome number, to be honest.

As a comparisonSource: https://web.de/magazine/gesundheit/grafik-kalorienaufnahme-weltweit-kalorien-landen-teller-31251562, a standard German man uses about 3,300 kcal/day, or 13.8 MJ/day. That comes out to about 5,043 MJ/year. So, you heat about 21 times more energy than you eatIn other words, you're consuming about 117 kg of heating oil per year as food. Well, energy-wise..

Anyway, let's say you want to get that heating energy from the sun. Problem is, you need to store it somewhere. Fortunately, one of the best heat-storage substancesSource: https://en.wikipedia.org/wiki/Table_of_specific_heat_capacities that exists is available cheaply and easily almost everywhere: Hydrogen, which has a specific heat of 14.30 kJ/kg·K. Unfortunately, hydrogen is... not easy to store. So you settle for something much more practicalNow I can hear you technologists out there going on about Magical Musk And His Amazing Powerwalls. Well, a new Powerwall 2 stores about 50 MJ of energy for a price of 6,900 € in a volume of 0.13 m³. So you'd need about 2200 powerwalls for a total price of 15,180,000 € and total volume of 280 m³, tightly packed. Not that great!: water, with a specific heat of 4.18 kJ/kg·K.

Let's say you get your water at 20°C and store it at close to 100 °C. You'll have it pressurised or something. For every kg of water, that gives you storage for 334 kJ.

Now it's really simple. You need to store 108,360 MJ, so you need 325,000 kg of water. That's not that much! Only 325 cubic meters! That easily fits into a cube of 7 m each side.

So, the plan is really simple now. Simply dig a basin 7m deep and 7m wide, put a water tank in there, some vacuum plates on the outside and boom, free heating energy! That's only 294 square meters of vacuum insulation, which will cost you about a meager 4-5000 US dollarsSource: https://www.made-in-china.com/products-search/hot-china-products/Vacuum_Insulated_Panel_Price.html.

Unfortunately, our vendor has only -1 of the vacuum plates we need in stock.

Ah, but wait

you say. Where do you actually get the hot water from? ⊕ I mean, apart from the fantastic zero-loss isolation you have around your water tank.

Well, solar collectors, obviously. A good, evacuated-panel solar collector can collect about 5 kWh/m²/day on a sunny day Source: https://en.wikipedia.org/wiki/Solar_thermal_collector, our about 18 MJ/m²/day. You really want to fill that up during the 90 hottest days of the year, so you'll need to collect 1204 MJ/day. Which you can do with only 67 square meters of solar collection. Unfortunately, that's a square of 8.2 m each side, so a bit more than your water tank. But with all the infrastructure you need around it, maybe not.

Sounds like an easy and cheap plan, right?