Grafstal

Switzerland - Ground-source heat pump

 

Summary The purpose of the project was to build a low energy and low cost prefabricated house without compromising the comfort. The house is a 3-storey building with the third floor being a loft in regular use. Due to the overall reduced energy consumption in low energy houses the proportion of the heat required for DHW increases from 30% to 40%.
This sets new requirements for a heating system. In this residence a vertical ground-source heat pump provides heating from October to April and DHW all year round. The COP of the 4.9 kW heat pump is 4.6 at test conditions.

Building and design values

 

Building type:	Single-family house
Location:	Grafstal, Switzerland
Year of construction:	1998
Number of storeys:	3
Heated floor area (m2):	174
% of total floor area (%):	No data (cellar unheated at 13ºC)
Design outdoor temperature (ºC)	Heating:	-11	Cooling:	-
Design indoor temperature (ºC)	Heating:	20	Cooling:	-
Degree days	Heating:	3 308	Cooling:	-
Base temperature for degree days (ºC)	Heating:	12	Cooling:	-

Heating and cooling

 

Application:	Space heating and DHW
Heat pump type(s):	Brine-to-water (same heat pump for heating and DHW; but different operation regimes)
Heat pump installed capacity (kW)	Heating:	4.9 B0W351	Cooling:	-
Refrigerant:	R290
Heat source	Brine
Details:	Circulation pump 69 W (ground source), Pump 49 W (heat distribution)
a) Type of ground heat exchanger:	Vertical, 32 mm diameter double U-pipe
b) Borehole depth (m):	100
c) Pipe length (m):	4 x 100
d) Heat transfer fluid:	Ethylene glycol (25%)
e) Flow rate (l/h):	1 600
Distribution system(s):	Floor heating elements/water without thermostatic valves
Supply and return temperature (ºC)	Heating:	34/292	Cooling:	-
Auxiliary system:	None
Heat pump design:	Heating demand of the house (20ºC, air change rate=0.3/h) according to a SIA3 model was calculated as 215 MJ/m2a. The actual amount used was 208 MJ/m2year (22ºC indoor). The return temperature control is dependent on outdoor temperature. Under normal conditions, the pump is turned on/off up to 6 times. The minimum operation time is 1.5 hours. Up to three times per year (on the coldest days), the heat pump is on without interruption. DHW (38 l/person/day, 50ºC) is provided all year round.
Supplementary system:	Single room air heat recovery unit.
Heat pump system completion date:	1998

1 Brine at 0ºC, water at 35ºC.
2 With a utility cutoff period of 4 hours; continuous operation would allow 30/25
3 Huber Energietechnik, Ingenieur- und Planungsbüro SIA; model no. 380/1

Performance (NTH4)

 

Energy for heating, DHW and ventilation1	Heat pump	Aux. heating system	Auxiliaries2
Energy input (kWh/year):	2 649	-	522
Energy output (kWh/year):	12 625	-	2663
Energy cost (CHF/year)4:	498.81	-	-
Cost tariff (CHF/kWh)5:	0.16/0.10	-	-

1 The information is based on data collected during two heating seasons: 1998-1999 and 1999-2000.
2 Auxiliaries include circulation pumps for ground source and heat distribution
3 Assumption: 50% of auxiliaries’ energy input can be used for space&hot water heating
4 Calculated from values during low (38%) and high (62%) tariff period and basic annual (CHF 57.60) cost. Costs include auxiliaries!
5 High/low

Coefficient of performance (COP)
Heating:	4.6 (space heating)
Test conditions:	B0/W35
Cooling:	-
Test conditions:	-

Heat pump cost breakdown
Heat pump including control unit (CHF):	9 700
Installation, including transport (CHF):	3 400
Ground Source incl. Installation (CHF)	11 000
Capital cost (excl. heat pump, ground source and installation) (CHF):	19 350
Maintenance (CHF/year):	150

 

Comparison of annual costs
Options 1-6:

1. Oil-fired boiler (16 kW), electric backup for DHW; balanced ventilation with heat recovery

2. Air-to-water heat pump (4.6 kWth) and exhaust-air heat pump water heater

3. Gas-fired boiler (13 kW), electric backup for DHW; balanced ventilation with heat recovery

4. Wood-fired burner, electric boiler for DHW, balanced ventilation with heat recovery

5. Integrated heat pump system for space heating, DHW, ventilation and heat recovery, back-up electric boiler (for application in "passive house").

6. Ground-source heat pump (4.9 kWth), electric backup for DHW, balanced ventilation with heat recovery

Operational experience and other comments (NTH4)
This project has demonstrated that a low energy and low cost prefabricated house can be built without compromising the comfort. Similar 3-storey houses exist on this housing estate in an area about 556 m above the sea level. The average annual temperature is 9.5ºC.
Some heat pump-related improvement ideas were raised after completing the monitoring programme:

• The circulation pump for the ground source could be replaced by a type using only one third of the electricity (70W).

• The heat pump capacity should be lower than required rather than higher than required.

• Low temperature distribution systems (Tsup>25°C) are self-regulating. Mixing or thermostatic valves are not necessary.

• Overheating in fall and spring may be a problem. Shading devices are essential.

• The equipment should be in a heated space (to avoid heat losses from storage vessel).

It is estimated that the heat pump can be used for 15 years, the pipes and ground source etc. for 30 years.

Ordering information
Technical Handbook in German:
(NTH4): Th.Afjei, A.Bühring, M.Dürig, A.Huber, P.Keller, E.Shafai, P.Widmer, G.Zweifel: Kostengünstige Wärmepumpenheizung für Niedrigenergiehäuser: Technisches Handbuch - Grundlagen, Planungsvorgehen und Praxisbeispiele, Schlussbericht Phase 4, Bundesamt für Energie 2000 to be ordered for CHF 40.- at:

ENET, Egnacherstrasse 69, CH-9320 Arbon, Tel. +41-71-440 02 55, enet @ temas.ch

 

 

Last updated: 1 March 2004

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