CPD Programme
>
However, these are not final and may be moved up or down. Heat pumps have already proved their
worth as energy efficient cooling and heating systems for today’s buildings. With the latest development of ‘heating-only’ heat pumps designed to take the place of the heating-only gas boiler, or the biomass/ biogas system, there is now an alternative renewable technology to be considered. For the forseeable future it seems likely that all the renewable technologies will have specific applications that benefit most from the particular technology. For example, biomass may be best for projects that require large amounts of hot water, such as hospitals. In the size range 50 to 200kW heating capacity, heating-only heat pumps offer ease of design and installation, ease of operation and maintenance, and flexibility in heat source (ground or air). In a comparison between heat pumps and gas- fired boilers, the most efficient gas boiler provides 93 units of heat for 100 units of gas input, while a heat pump operating at a COP (see below) of 3.5 can produce the same 93 units of heat from 53 units of gas input. The principles of operation of a heat pump
have been well documented in previous CPD articles, the most recent being the February 2010 issue, ‘Design of air-source heat pumps for heating and hot Water’. The performance of a heat pump is defined by its COP (coefficient of performance), which is the ratio between useful heat output and the total energy input (normally electrical energy) to drive the system. A typical seasonal value of COP for an air-source heat pump providing hot water at 40C in an ambient of 0C would be 3.5 to 4.0. A typical value for a ground
Technology Solid biomass
Biodiesel (restricted use) Biogas on-site combustion Ground-source heat pumps Air-source heat pumps Solar thermal
Solid biomass
Biogas on-site combustion Ground-source heat pumps Air-source heat pumps Solar thermal
Solid biomass
Ground-source heat pumps Biomethane injection
Scale
Up to 45kW Up to 45kW Up to 45kW Up to 45kW Up to 45kW Up to 20kW
Medium installations 45kW–500kW 45kW–200kW 45kW–350kW 45kW–350kW 20kW–100kW
Large installations
500kW and above 350kW and above All scales
Figure 1: Proposed tariff levels for the Renewable Heat Incentive 76 CIBSE Journal September 2010
1.6 - 2.5 1.5 4
15 20 15
Source: RHI consultation document
source heat pump would be 4.5 to 5.0. This means that, for every kW of electrical energy to drive the heat pump, 3.5 to 5.0kW of heat is generated (ie, an overall ‘point of use’ efficiency of 350% to 500%).
In order to maximise COP it is necessary
to minimise the temperature difference between the heat source and the heating requirement. CIBSE Guide F: Energy Efficiency in Buildings, states that for every 1C increase in this temperature difference the heat pump requires 3% more energy to drive it. Design and specification for a heat pump system is therefore vital, as is commissioning and maintenance of the installed system. In his book, Sustainable Energy – Without
the Hot Air, Professor David MacKay maintains that heat pumps are superior in efficiency to condensing boilers, even if the heat pumps are powered by electricity from a power station burning natural gas. In the CIBSE Journal August 2010, Huw Blackwell of Hoare Lea compares the performance of heat pumps with combined heat and power, and drawing out some of the practical points about heat pumps that can adversely affect their efficiency, such as the need for defrosting the outside heat source coil when the ambient drops below about 5C. A further article in the same Journal issue, ‘Gaining Ground, Heat Pumps 2 Specification’, acknowledges the growth that has taken place in the heat pump market and again emphasises the need for good design at the building stage before selecting the appropriate heating technologies.
Comparing renewable technologies If we take a fairly typical office building of about 2,800 sq m and a design heat load
Tariffs (pence/kWh) Tariff lifetime (years) Small installations 9
6.5 5.5 7
7.5 18
6.5 5.5 5.5 2
17
15 15 10 23 18 20
15 10 20 20 20
A fairly typical office building may be able to make use of an air-to-water heat pump
200kW, to meet the heating demand, the following systems can be considered: • Air-to-water heat pump comprising an outdoor unit and two water heating units; • A biomass, wood pellet-burning boiler; and • A condensing gas boiler. The heat pump system is shown in Figure 2 and the biomass cycle in Figure 3. Biomass is a biological material such as wood, waste or biogas and is a renewable energy source. A biomass boiler is regarded as a zero carbon technology and the cycle between production and absorption of CO2 is short in comparison with other fuels. In this example, the costs for the biomass
system are based on the Biomass Heating Guide CTG012 produced by the Carbon Trust. The tariffs and emissions factors in Figure 4 are based on information in the SAP 2009 version 9.90(March 2010) document. Figures 5 to 8 speak for themselves with regard to comparative costs, efficiency and emissions. Note that the water flow temperatures achievable for the three options are: • Heat pump
55C
• Biomass boiler • Gas boiler
90C 85C
There are several issues that need to be addressed when using biomass boilers. In urban and inner city areas delivery and storage of the fuel is a problem, even prohibiting its application in some cases. There can also be a problem in rural areas where delivery involves long transport distances for the fuel, resulting in an overall increase in CO2 emissions for the application. Maintenance and testing of a biomass
boiler and associated equipment is generally greater in time and cost than heat pumps or gas boilers. Should biomass installations increase significantly in the future, there may come a point where the UK cannot meet the fuel demand and the fuel has to be imported. This again would increase the CO2 emissions.
www.cibsejournal.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88