Often talk is about storing solar energy. Usually they mean electrical energy. By using "annualized geo solar" heat storage with insulation reducing heat loss summer heat can be stored in the ground under a building. Invented by Don Stephens and his engineer, this takes attic or other summer heat into the ground under a building, using insulation to reduce escaping. This duo even has a finite element analysis program to work out proper design for a new building before any of it is even started. This can be a retrofit also.
Think of the effect on my heat bills if the soil under and around the house goes from 48 deg.F to 78! I've already gotten a small but noticeable improvement just by blowing summer air through the basement with thermostat controlled fans. Soil temperature is up three degrees from last March, and more from the starting temperature. No frozen pipes this winter, although the lowest outdoor temperature was 11 DegF lower than the winter last year.
In my own case, I'm currently digging trenches for circulating heat transfer tubes into the soil under the house. After the PEX tubes are in and soil replaced and packed down, foam will be placed above each trench, with a few feet to each side. This will be under the floor. This foam prevents August heat from returning to the house in August - it needs time to travel to the bare strips between the foam strips. At first, likely a circulation pump will bring attic heat using water down from the attic and warm the soil (also prewarm domestic hot water). Assuming the soil gets warm enough, a heat pump will be added to extend the harvest season. "Wing insulation" outside the house will reduce the escape through the soil.
Although this retrofit is an active system, much can be learned from the passive solar site: www.thenaturalhome.com. Among other things, one branch site discusses wing insulation in some detail.
The primary cost for this is the labor - I'm not in a hurry, and am too lazy for calesthenics, so that isn't a problem! ;-)) Once installed the system can last a long time. Operating costs are covered by the decrease in air conditioning costs. Calculate how much heat can be stored by raising 500 tons of dirt to a 30 degF higher temperature! And you only dig a portion of the dirt, rather than needing to remove all of it.
Later the south roof can be modified with Conserval's Solarwall (tm) system to increase heat collection. This in return can later be up-graded with their PV modification. Once all this is in place, collection efficiency reaches about 70%. Storage efficiency is lower. However by eliminating much digging cost, price of storage is kept low.