3.1  Borehole thermal energy storage

Project Leader

• M. Bernier

Description

Typically, boreholes are approximately 100 m deep with a 15 cm diameter (Fig. 9). They are equipped with a U-tube pipe to from a ground heat exchanger. This approach has been used for a number of years. However, as mentioned in a recent editorial on the state of GCHP systems, Spitler (2005) points out that more work is needed on the development of more cost-effective borehole heat exchangers. 

Sub-project 3.1a will look at improving existing borehole designs. First, a novel four-pipe borehole with two independent U-tube circuits (a solar recharging U-tube and a heat pump U-tube) will be studied. This borehole would be shorter with a relatively large diameter so as to include water-saturated sand. This arrangement would take advantage of the latent heat of fusion of water during peak heat pump operation where the saturated sand would be intentionally frozen. Then, injecting solar heat when available will melt ice. This four-pipe approach with ground freezing has the potential to be used as mid-term (weekly) storage. Integration of this new borehole in a system will be examined in 3.1b. When several boreholes are grouped together so as to form a bore field, borehole thermal interactions become important. At a recent international thermal storage conference results of an inter-model comparison of bore field models showed serious discrepancies among them which might be attributed to improper modeling of borehole interaction (Spitler et al., 2009b). Sub-project 3.1a will examine bore field heat transfer to reduce uncertainty associated with the use of borehole models.

Bore fields can also be used for seeasonal therrmal storagee.  Typically, inn this type off storage, solar energy is used in the summer to recharge the bore field. Then, this stored thermal energy is used during the next winter season for space heating purposes of a community. Some systems could also benefit from the use of large diurnal storage tanks surrounded with boreholes, which act as a thermal guard.  This technique is envisioned for future Okotoks-type projects. However, the scientific knowledge base for such a system needs to be expanded. Sub-project 3.1a, in conjunction with 3.2a, will address this. 

The concept of mid-term to long-terr thermal storage at the individual hhouse levell is not new but has not been thoroughly examined for modern housing and in the Figure 99. Borehole seasonal TESS linked Canadian context. Based upon a critical review of the to solar system.  Literature, Pinel and Beausoleil-Morrison (20099) proposed a storage concept that uses the soil patch beneath the house as a thermal store. With this, the soil layer is excavated, insulated and protected with a vapor barrier to retain moisture. The store is then filled with a moist storage material and the house foundation built on top of it.

Sub-Projects

• 3.1a  Borehole and borefield heat transfer
• 3.1b  Small-scale residential type storage
• 3.1c  Seasonal borehole storage temperature level