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Combined heating & cooling - Heat pump - Ground-source - Office / Germany
Case study assignment
A government agency in Germany is building a new headquarters and is interested in creating an environmentally responsible, low-energy office building. The architects have designed a building with a high performance envelope and plentiful use of daylighting. You are a consulting engineer who has been asked to investigate the financial viability of using a ground-source heat pump (GSHP) system to further reduce the proposed building's energy consumption.
Site information
The building will be located near Langen, close to the Frankfurt airport. The nearest suitable meteorological station is Stuttgart. Assume that the soil type is equivalent to light rock. The site is located inside a groundwater protection area. Geotechnical engineers have performed long-term ground temperature simulations, which indicated that the mean annual ground temperature fluctuates between 16°C and 5°C, with a mean earth temperature of 10.5°C.
The building footprint is approximately 9,600 m² located on a 15,000 m² plot. The 6-story building has 57,800 m² of floor space and 230,000 m³ in total volume. The building envelope is a high performance double façade with high levels of insulation and high performance windows. Daylighting, low lighting densities and energy efficient equipment will reduce electrical and cooling loads.
The location is near a natural gas-fired electric power generating station. Waste heat from this power plant is available through a district heating system and is a financially attractive heating energy source in the area. Assume a 50% heating system seasonal efficiency and an air-conditioner seasonal COP of 3 for the base case system.
For the greenhouse gas analysis, the electric power generation fuel mix is approximately as follows: 8.1% natural gas, 52.1% coal, 3.1% #6 oil, 32.1% nuclear and 4.6% small hydro.
Financial information
Typical financial figures are provided for your analysis: 80% of the project cost is to be paid for in debt, the interest on the debt will be 8% over a 20-year term, the discount rate used by the client is 8% and the inflation rate is 2.5%. Energy prices are expected to rise slightly below the inflation rate. The GSHP system is expected to last 25 years.
Energy from the district heating system can be delivered to the building at a rate of €40/MWh (or €0.207/m³ of natural gas) and electricity is available for €0.088/kWh. You assume that the maintenance costs will be similar for the two systems.
A conventional air-conditioning system (e.g. chiller and cooling tower) for the project is estimated to cost €270,000 and require significant parts replacement costs (€180,000) after 15 years. The heat pumps for the project are expected to cost €150/kW of cooling, with about 25% of the value of the system needing to be replaced after 15 years. Where required, supplemental heat rejection equipment (e.g. cooling tower) is estimated to cost €100/kW, with a parts replacement cost of approximately 75% of the original cost every 15 years.
Prepare a RETScreen study, documenting any assumptions that you are required to make, and report on the significant conclusions from this analysis.
Solution
The worked-out solution is the data file selected from within the RETScreen Project Database. The user automatically downloads the Project Database file while downloading the RETScreen software.
Teacher's notes
Real project
Results
When the German Air Traffic Control Authority (DFS) decided to build its new headquarters in Langen, Germany, they chose to make it a Low Energy Office (LEO) building. The target annual energy consumption was set at 100 kWh/m², or about 35% lower than conventional German office buildings.
The building has 6 storeys, 57,000 m² of floor space and is designed to house 1,200 workers. The envelope is highly insulated with a double façade, high thermal mass and high performance windows. The building utilizes a high-efficiency ground-source heat pump (GSHP) system, with supplemental heating provided by a local natural gas-fired district heating network.
System description
Two GSHP borehole fields are utilized to provide primary heating and cooling to the building. A total of 154 boreholes supply 340 kW of cooling capacity and 330 kW of heating capacity. The boreholes are 70 m deep with 5 m horizontal distance between them.
Due to the building's location in a groundwater protection area, only pure water is used in the borehole field. Therefore when operating in heating mode, care is taken to avoid temperatures below +4°C at the evaporator side of the heat pump so as to prevent freezing problems.
In heating mode, a high performance (COP 6.0) ammonia heat pump is used to supply 30 °C water to "Thermo-Active" ceilings (i.e. radiant ceilings) and ventilation air heating coils. Peak heating loads are projected to be 700 kW, so supplemental heat from the district heating system is utilized on the coldest days. Despite the somewhat under-sized heat pump, the district heating system will only need to meet 30% of the annual heating load.
In cooling mode, the cold water from the boreholes is used directly in the Thermo-Active ceilings and ventilation dehumidification coils, with no heat pump operation. Peak cooling loads are met using conventional chillers. Nearly 80% of the annual cooling load is met by the GSHP alone.
Lessons learned
The big picture
Ground-source heat pumps are a mature renewable energy technology that have the benefit of being marketed by main-stream Heating, Ventilation and Air Conditioning (HVAC) manufacturers and commercial networks. GSHP systems are durable, reliable and often cost competitive with conventional heating and cooling systems, particularly in new construction applications. To date, more than 800,000 GSHP systems have been installed in Europe alone. European objectives call for a doubling of the current installed GSHP capacity by 2010.
Photo
Office - Heat pump - Ground-source, Langen, Germany
References
Case study assignment
A government agency in Germany is building a new headquarters and is interested in creating an environmentally responsible, low-energy office building. The architects have designed a building with a high performance envelope and plentiful use of daylighting. You are a consulting engineer who has been asked to investigate the financial viability of using a ground-source heat pump (GSHP) system to further reduce the proposed building's energy consumption.
Site information
The building will be located near Langen, close to the Frankfurt airport. The nearest suitable meteorological station is Stuttgart. Assume that the soil type is equivalent to light rock. The site is located inside a groundwater protection area. Geotechnical engineers have performed long-term ground temperature simulations, which indicated that the mean annual ground temperature fluctuates between 16°C and 5°C, with a mean earth temperature of 10.5°C.
The building footprint is approximately 9,600 m² located on a 15,000 m² plot. The 6-story building has 57,800 m² of floor space and 230,000 m³ in total volume. The building envelope is a high performance double façade with high levels of insulation and high performance windows. Daylighting, low lighting densities and energy efficient equipment will reduce electrical and cooling loads.
The location is near a natural gas-fired electric power generating station. Waste heat from this power plant is available through a district heating system and is a financially attractive heating energy source in the area. Assume a 50% heating system seasonal efficiency and an air-conditioner seasonal COP of 3 for the base case system.
For the greenhouse gas analysis, the electric power generation fuel mix is approximately as follows: 8.1% natural gas, 52.1% coal, 3.1% #6 oil, 32.1% nuclear and 4.6% small hydro.
Financial information
Typical financial figures are provided for your analysis: 80% of the project cost is to be paid for in debt, the interest on the debt will be 8% over a 20-year term, the discount rate used by the client is 8% and the inflation rate is 2.5%. Energy prices are expected to rise slightly below the inflation rate. The GSHP system is expected to last 25 years.
Energy from the district heating system can be delivered to the building at a rate of €40/MWh (or €0.207/m³ of natural gas) and electricity is available for €0.088/kWh. You assume that the maintenance costs will be similar for the two systems.
A conventional air-conditioning system (e.g. chiller and cooling tower) for the project is estimated to cost €270,000 and require significant parts replacement costs (€180,000) after 15 years. The heat pumps for the project are expected to cost €150/kW of cooling, with about 25% of the value of the system needing to be replaced after 15 years. Where required, supplemental heat rejection equipment (e.g. cooling tower) is estimated to cost €100/kW, with a parts replacement cost of approximately 75% of the original cost every 15 years.
Prepare a RETScreen study, documenting any assumptions that you are required to make, and report on the significant conclusions from this analysis.
Solution
The worked-out solution is the data file selected from within the RETScreen Project Database. The user automatically downloads the Project Database file while downloading the RETScreen software.
Teacher's notes
- The 5,400 m² of land area that is not utilized by the building footprint is not enough area for a horizontal ground heat exchanger; therefore a vertical loop system is required.
- In Europe, warmer summer-time indoor temperatures and larger temperature swings throughout the day are typically more acceptable to building occupants, as compared with the situation in North America. Therefore cooling systems are often undersized relative to peak cooling loads. This practice, coupled with a cool climate, suggests that the ground-source heat pump (GSHP) system would likely be designed to meet the heating load.
- The base case heating fuel is defined in order to calculate the GHG emissions impacts of the project. It is a natural gas co-generation plant that powers the district heating system and therefore natural gas is assumed to be the base case fuel for GHG calculations. The client however is charged on the basis of MWh of thermal energy delivered (€40/MWh). It is assumed that the overall efficiency of the district heating system (including combustion, distribution and interconnection) is about 50%. This means that the client in effect pays €20/MWh for the gas itself. Given a heating content of 10.33 kWh/m³ for natural gas, the effective cost of gas to the client is estimated as €20/MWh x 0.01033 MWh/m³ = €0.207/m³.
- Since base case (and supplemental) heating is provided by the district heating system, no boiler is needed and thus no corresponding credits are included in the GSHP cost analysis.
- At the time of this analysis, US$1 = €1.14 and CAN$1 = €0.72.
- As explained in the Real Project section, the GSHP system is not sized to meet either the peak heating or peak cooling loads. At peak loads, conventional systems are used to provide supplemental heating and cooling. The real GSHP actually built is therefore significantly smaller than the one modelled in the RETScreen analysis (which is sized for the peak heating load). This results in a more conservative financial analysis.
Real project
Results
When the German Air Traffic Control Authority (DFS) decided to build its new headquarters in Langen, Germany, they chose to make it a Low Energy Office (LEO) building. The target annual energy consumption was set at 100 kWh/m², or about 35% lower than conventional German office buildings.
The building has 6 storeys, 57,000 m² of floor space and is designed to house 1,200 workers. The envelope is highly insulated with a double façade, high thermal mass and high performance windows. The building utilizes a high-efficiency ground-source heat pump (GSHP) system, with supplemental heating provided by a local natural gas-fired district heating network.
System description
Two GSHP borehole fields are utilized to provide primary heating and cooling to the building. A total of 154 boreholes supply 340 kW of cooling capacity and 330 kW of heating capacity. The boreholes are 70 m deep with 5 m horizontal distance between them.
Due to the building's location in a groundwater protection area, only pure water is used in the borehole field. Therefore when operating in heating mode, care is taken to avoid temperatures below +4°C at the evaporator side of the heat pump so as to prevent freezing problems.
In heating mode, a high performance (COP 6.0) ammonia heat pump is used to supply 30 °C water to "Thermo-Active" ceilings (i.e. radiant ceilings) and ventilation air heating coils. Peak heating loads are projected to be 700 kW, so supplemental heat from the district heating system is utilized on the coldest days. Despite the somewhat under-sized heat pump, the district heating system will only need to meet 30% of the annual heating load.
In cooling mode, the cold water from the boreholes is used directly in the Thermo-Active ceilings and ventilation dehumidification coils, with no heat pump operation. Peak cooling loads are met using conventional chillers. Nearly 80% of the annual cooling load is met by the GSHP alone.
Lessons learned
- The high performance of ground-source heat pumps, and their ability to address both heating and cooling needs, makes them financially viable even where inexpensive energy supply options such as district heating are available.
- Ground-source heat pumps can be safely implemented in sensitive areas such as near protected groundwater aquifers.
- The use of water in the ground heat exchanger required that the return temperature not be allowed to drop below 4°C. This limitation was partially compensated by using high conductivity grout to optimize heat transfer.
The big picture
Ground-source heat pumps are a mature renewable energy technology that have the benefit of being marketed by main-stream Heating, Ventilation and Air Conditioning (HVAC) manufacturers and commercial networks. GSHP systems are durable, reliable and often cost competitive with conventional heating and cooling systems, particularly in new construction applications. To date, more than 800,000 GSHP systems have been installed in Europe alone. European objectives call for a doubling of the current installed GSHP capacity by 2010.
Photo
Office - Heat pump - Ground-source, Langen, Germany
References
- Kemp, Steve, "Personal communication," Enermodal Engineering Ltd., 2002.
- Sanner, Burkhard and Kohlsch, Oliver, Examples of Ground-Source Heat Pumps (GSHP) in Germany, International Summer School on Direct Application of Geothermal Energy, 2001.
- UBeG, The Low-Energy-Office of Deutche Flugsicherung (German Air Traffic Control) in Langen, with Geothermal Heat and Cold Storage, Wetzlar, Germany, 2001.
