Design Considerations for Borehole Thermal Energy Storage (BTES): A Review with Emphasis on Convective Heat Transfer

Table 1

Calculated steady-state heat losses from a cylindrical BTES array, with a temperature at the edge of the store of 50°C, an ambient undisturbed average rock temperature of 12°C, a rock thermal conductivity of 2.2 W m^-1 K^-1, and a rock volumetric heat capacity of 2.2 MJ m^-3 K^-1 with expanded polystyrene insulation (thermal conductivity 0.04 W m^-1 K^-1) of thickness 0.5 m extending over the top of the BTES and to a depth down the sides, using the mathematical technique of [17].

Radius	Depth		Heat losses				BTES heat capacity	Heat capacity/
			Insulated top	Insulated sides	Uninsulated portion	Total
m	m	m	kW	kW	kW	kW	MWh K^-1	MWh K^-1 kW^-1
20	60	6	3.8	1.1	38.8	43.8	46.1	1.1
30	60	6	8.6	1.7	53.2	63.5	103.7	1.6
45	60	6	19.3	2.6	74.3	96.2	233.3	2.4
60	60	6	34.4	3.4	95.5	133.3	414.7	3.1
80	60	6	61.1	4.6	123.7	189.4	737.2	3.9
20	60	12	3.8	2.3	34.1	40.2	46.1	1.1
30	60	12	8.6	3.4	46.2	58.2	103.7	1.8
45	60	12	19.3	5.2	63.8	88.3	233.3	2.6
60	60	12	34.4	6.9	81.5	122.8	414.7	3.4
80	60	12	61.1	9.2	105.1	175.4	737.2	4.2
20	100	6	3.8	1.1	48.9	53.9	76.8	1.4
30	100	6	8.6	1.7	65.0	75.3	172.8	2.3
45	100	6	19.3	2.6	89.1	111.0	388.8	3.5
60	100	6	34.4	3.4	112.9	150.8	691.2	4.6
80	100	6	61.1	4.6	144.6	210.3	1228.7	5.8