Report : Ground source heat pump

MSc Advanced Sustainable Design : Building Capacity

Ground Source Heat Pumps

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Introduction Ground source heat pumps (GSHP) are heat generating systems that take heat which stored in the surface of the earth for use in space and water heating systems. Heat pumps can be used in combination with radiators or underfloor heating systems and can provide domestic hot water. The heat is transferred from the ground to the heating system by circulating a fluid, usually a mixture of water and antifreeze, around a loop of pipe buried in the ground.

Figure1 : A horizontal ground loop Source : Ecoliving

Ground-source heat pumps with horizontal collectors are the most common form of heat pump for low-rise housing in the UK. It offers a lot of benefits, including lower carbon emissions, reduced fuel bills, a cost-effective off-gas grid alternative to oil, solid fuel or electric radiators.

Heat pumps powered by a renewable energy source are completely renewable and are a good source of renewable heat.

A basic rule for the design of heat pump systems is therefore to minimise the temperature difference between the heat sink and the heat source to achieve maximum efficiency, for example for a heating application use the warmest available heat source and lowest possible distribution temperature.

The ratio between the heat provided to the sink and the energy required is known as the coefficient of performance (COP). Electrically driven heat pumps used for space heating applications in moderate climates usually have a COP of a least 3路5 at design conditions. This means that 3路5 kWh of heat is output for 1 kWh electricity used to drive the process. The coefficient of performance, cop is the key measure of heat pump performance. It is defined as: COP = Rate of heat delivered (kW) Power input (kW) This figure illustrates the typical cop reduction to be expected if the output temperature is raised from 35 to 55 掳C. When applied in the building, additional heat losses and power requirements for other auxiliaries may need to be accounted for.

Figure2 : Typical cop versus heat output temperature for heat input temperature of 0 掳C Source : Ground Source Heat Pumps 2013

MSc Advanced Sustainable Design : Building Capacity Table 1 : Description of ground collection systems Source : Ground Source Heat Pumps 2013

Type

Keyfact Vertical closed loop  Commonly use for medium sized systems in the 100 to 1000 kW range in the UK  Depths are between 50-200m below ground level.  Typically, 125–160 mm boreholes are drilled and boreholes are laid out in a suitable pattern typically with spacing of 4-15 m between the holes Closed loop in building piles  This can be used in any piled building and with any common piling method.  Typically cover about 50% of the building load.  An important consideration is to minimise the impact of the heat exchanger pipes on the pile design and installation program.  This gives a low additional cost over conventional equipment and the lowest possible extra piling cost.

Horizontal closed loop.  0.8-2m deep trenches are required for install a horizontal closed loop.  Various trenching methods and pipe geometries are used.  Horizontal systems are generally cheaper to install than vertical loops but do require a large area of land.  Systems up to 150 kW are most common although where land is plentiful, larger systems can be considered and have been installed.

Pond closed loop  This is a particularly efficient method of ground sourcing and can cover practically any size of load; systems of many MW are common.  They offer excellent installation costs but still or moving surface water is required. A constant water minimum depth of 2 to 3 m is required.  9 m2 of surface area is required per kW of system capacity.

Surface water system  where surface water is available, this can be used as a source by taking water through a similar system of heat exchangers to that outlined above.  Filtration is the main issue with these systems and can constitute a significant maintenance requirement.  A surface water system is used for cooling of the London Underground at Victoria station.

Table 2 : Recommended maximum length for different tube sizes.

Source : Heat Pumps : A guidance document for designers

Typically the pipe diameter is between 25 mm and 40 mm. Each size provides differrent maximum length. It can be seen from the table.

MSc Advanced Sustainable Design : Building Capacity

How does it work

1. Cold water antifreeze is pumped through the ground within a series of energyabsorbing pipes which known as ground naturally rise heat flows from warmer to cooler places. In this process the antifreeze makes circulating around the array is constantly warmed by the grounds low-grade heat.

2. Having increased in temperature the antifreeze mixture is fed into a heat exchanger which called the evaporator. Within the secondary seal side of the evaporator, heat exchanger is a refrigerant which acts as a heat transfer fluid. Then, the energy absorbed from the ground is transferred into the refrigerant which begins to boil and turn into a gas.

3. The refrigerant never physically mixes with the water antifreeze mixture. They are separated like sandwich layers by the plates of the heat exchanger which permits the heat transfer. This gas is then fed into a compressor. The pressure of the refrigerant gas is increasing then the compressor makes the gas temperature rise.

4. The hot refrigerant gas then flows into a second heat exchanger called the condenser which features an identical set up heat transfer plates. The condenser delivers water hot enough to heating distribution to serve the space heating system and if required the properties hot water needs.

5. Having transferred its heat the refrigerant gas reverts to a liquid. This liquid is then passed through an expansion valve at the end of the cycle to reduce pressure and temperature; ready to commence the cycle all over again.

MSc Advanced Sustainable Design : Building Capacity

Calculating Heat Pumps Size for Single House Why should architects or designers know about it?

Because it is very important to install the right size heat pump because a pump that is too small will never keep up. A heat pump that is too large for the space likewise will result in wasted energy and money.

Here is the formula for calculating: 1) Heating demands multiple house area = x1.

2) 30% added is the reason for covering high heating demand in winter = x2.

3) x1 plus x2 = kW that heat pumps needed.

Heating Demand = 0.2 kW/m2 Heating Demand = 0.1 kW/m2 Heating Demand = 0.02 kW/m2 For different quality of house, heating demand has been considered in different kW/m2. It can be seen from above pictures. For instance, the quality of house can be differentiated by ventilation, insulation, thermal mass, structure system, material use, etc.

More information about the methods for calculation of system energy efficiencies for specific heat pump systems in buildings are given in BS EN 15316-4-2 (BSI, 2008).

MSc Advanced Sustainable Design : Building Capacity

Worked Examples 1) Poor quality 2-storey house has area 150 m2 and locates on site dimension 10x12m.

What is the appropriate size of heat pump and which type is reasonable for this house?

0.2 x 150 = 30 kW (demand x area) = (x1) 30 x = 9 kW (x1 x 30%) = (x2) 30 + 9 = 39 kW (x1 + x2) = (Heat Pump Size / kW)

Another question is to consider about type of heat pump, because of the small land area and site is not near water resource. Thus, we can only consider vertical closed loop or closed loop in building piles, but looking at key facts of vertical closed loop (from page 2), it is commonly used for medium sized systems in the 100 to 1000 kW range and also have to consider about depth of land, whereas closed loop in building piles gives a low additional cost over conventional equipment and the lowest possible extra piling cost. Therefore, closed loop in building piles is the most appropriate for this house.

Answer = Heat Pump Size 39 kW / closed loop in building piles 2) High quality 2-storey house has area 450 m2 and locates on site dimension 25x40m.

Interestingly, back of this house has very large lake, the depth is 5m.

What is the appropriate size of heat pump and which type is reasonable for this house?

0.02 x 450 = 9 kW (demand x area) = (x1) 9 x = 2.7 kW (x1 x 30%) = (x2) 9 + 2.7 = 11.7 kW (x1 + x2) = (Heat Pump Size / kW)

Another question is to consider about type of heat pump, because of the large land area and site is near water resource. Thus, pond closed loop and surface water system can be considered. Looking at key facts of the latter (from page 2); filtration is the main issue with these systems and can constitute a significant maintenance requirement, whereas the former is an efficient method of ground sourcing and can cover practically any size of load; the minimum depth of water is 2-3m but the depth is 5m so it is clearly enough. However, horizontal closed loop also should be considered due to the fact that land is plentyful. This is the most common form of heat pump for low-rise housing in the UK and generally cheaper to install than vertical. Therefore, pond closed loop or horizontal closed loop is the most appropriate for this house.

Answer = Heat Pump Size 11.7 kW / pond closed loop or horizontal closed loop

MSc Advanced Sustainable Design : Building Capacity

Peer Review : Exercise Medium quality 2-storey house has area 225 m2 and locates on site dimension 25x35m.

What is the appropriate size of heat pump and which type is reasonable for this house?