Solar Thermal Storage System - Zero Energy Home Challenge

I am updating this portion of the blog to give you the latest information regarding the Solar Heat Storage and Delivery System. One of the updates that may be of interest is the diagrammatic drawing of the system, created by Mike LeBeau, it most definitely helps to understand how we are building the system. The controls for the system will be added to the drawing in the near future.

Additionally, I applied for an Innovation Credit for this system through the LEED for Homes rating system program and have receive notification that it will be allowed for an additional credit. I had applied for several others, but have learned through this process that only extraordinary energy saving or environmental improvements are being accepted, it was communicated to me that of the 2000 + requests received as of March 2008 only 4 or 5 had been approved. This was one of the approved Innovative Credits.

Solar Heat Storage & Heat Delivery System – Innovation Credit Request Documentation:

There are 92 solar heat collector vacuum tubes planned for the roof that will be collecting an average of 172,500 BTU’s per day that that there is full sun. We have assumed that there will be at least 120 days worth of full sun available for producing heat available for long-term storage, outside of the times when it would go directly to either short-term storage or to an end use. This will mean that we can put around 20.7 million BTU’s into storage per year. We estimate that approximately 2 million BTU’s per year will be needed for heating of domestic hot water.

The energy model that we ran for this home indicates that we will need 9.7 million BTU’s annually for space heating. This is 4.6 kBTU/sq-ft. or 15 kWh/m2

We have 9,000 cubic feet of a sand and taconite mix (approximately 50:50) under the foot print of the main portion of the house which is insulated with 16 inches of expanded polystyrene (EPS) on all six sides and has cross-linked polyethylene (PEX) tubing running horizontally at every 9 inches vertically through the sand and taconite. The taconite bed is on the top and the sand bed on the bottom with 2 inches of extruded polystyrene (XPS) separating them. The 2 inches of XPS between the two beds was installed only for testing purposes so the heat storage performance of the taconite and sand beds could be evaluated more-or-less independently. Sensors will be installed at various locations in each bed to monitor performance and fine tune the control strategy.

Heat storage capacity for the space under conditioned space is estimated as follows:

Energy density of water = 4,190 kJ/m3 oC (www.engineeringtoolbox.com)
Energy density of taconite = 2,560 kJ/m3 oC (www.engineeringtoolbox.com)

Specific heat of sand = 830 J/kg oC (www.engineeringtoolbox.com)
Density of packed dry sand = 1,682 kg/ m3 (www.simetric.co.uk)
Energy density of packed dry sand = 1,682 kg/ m3 x 830 J/kg oC = 1,396 kJ/m3 oC

950 sq-ft x 3 feet= 2,850 cu-ft total volume of sand
950 sq-ft x 3 feet= 2,850 cu-ft total volume of taconite

2,850 cu-ft x 7.5 gal/cu-ft = 21,375 gal

Assuming that the energy density of sand is about 0.333 that of water:
21,375 gal x 0.333 = 7,118 gal of water storage gal equivalent

Assuming that the energy density of taconite is 0.611 that of water:
21,375 gal x 0.611 = 13,060 gal of water heat storage equivalent

Combined total heat storage = 20,178 gal water heat storage equivalents

20,178 gal water x 8.34 lb/gal = 168,285 lb water heat storage equivalent

Assuming that an 80 oF maximum usable temperature range is available, the total heat that can be stored in the sand and taconite beds combined = 13,462,762 BTU

A closed loop heat transfer circuit will transport the heat from the roof top solar collector to a flat plate heat exchangers and a pair of insulated water storage tanks in the utility room basement. A heat transfer fluid, consisting of propylene glycol and water, will be directed first through a heat exchanger coil in the bottom of an 80 gallon indirect fired hot water tank. During the non heating season, once the 80 gallon tank reaches a desired maximum temperature of 150-160 degrees the heat transfer fluid is redirected to a flat plate heat exchanger that will direct the excess heat into the sand/taconite thermal storage area. During the heating season the heat transfer fluid will be directed to a coil in a 400 gallon bulk thermal storage tank. When that larger tank reaches a desired operating temperature any surplus energy will be transferred from the tank and into the closed loops of PEX tubing going through the sand and taconite beds via a separate closed loop that is common with the space heating distribution system. Energy will move to the storage bed when there is a surplus and be removed from it when need for space or water heating. The 400 gallon tank will be used to create the desired temperature needed for space heating by calling for heat from the heat transfer fluid coming from either the sand/tanconite thermal storage area or directly from the heat transfer fluid coming from the solar collectors.

Since the heat will only flow from hot to cold, water in the PEX tubing in the floor slab will have to be somewhat above the desired slab temperature. If a slab temperature of 75 oF is desired, water at 80 oF or above should be sufficiently hot. To supply water at this temperature, the sand and taconite beds will need to be somewhat above this temperature, perhaps 90 oF at a minimum. Based on the heat capacity storage calculations, if the sand and taconite beds were to reach 170 oF by late fall, the two beds combined could release about 13.5 million BTU without their temperature not going below 90 oF. Assuming that one third of the approximately 21 million BTU of heat that is anticipated from the solar collector vacuum tubes is lost through the 16 inches of EPS around the sand and taconite beds and elsewhere in the system, there should still be about 14 million BTU available for space heating and domestic hot water, which is about 2 to 3 million BTU more than the energy models predicts for the total annual space heating and domestic hot water needs of the house. Should there be any excess heat, to prevent overheating of the sand and taconite beds, the heat can be diverted into the garage floor slab, which has PEX tubing in it to keep the garage warm and its floor ice-free in winter.

A small electric boiler will be integral with the solar heat collection system and will supplement and back-up the solar thermal system. The location will be next to the water storage tanks. This boiler will back up the domestic hot water tank, if ever needed, by heating the 80 gallon tank via a second heat exchanger coil suspended near the top. It will also back up the space heating system by working in parallel with energy from the solar thermal system either from the larger storage tank or from the sand and taconite long term storage bed. The electric boiler will be connected to a 8kw propane powered generator for the rare situation when the power is out for a long period of time and the system is not capable of keeping up with the electric loads for the ecohome. An electric boiler was selected because of the ability for electric boilers to be size in smaller increments than propane boilers, making it more appropriate for the small loads needed for this project.

The electric boiler will be controlled by the Sunny Island device to allow it to switch into operating mode should the battery backup or grid power go down.

The 400 gallon water solar storage tank is also important for heating the home during the shoulder heating seasons, spring and fall. Installing a system that allows the heat collected to go directly into the 400 gallon tank and then into the floor heating coils was important to allow quick access to the heat when the sand/taconite is likely to be loosing capacity during the spring and the sun is gaining its strength. And conversely, when the sun is loosing its strength in the fall and the taconite/sand thermal storage is at capacity, the solar heat is more efficiently sent right to the in-floor heating system.