Gitanos y la intervención del tercer sector.

more closely and hence improve energy efficiency. These may comprise an integrated package of modules or independent boilers, including CHP. As the load increases, individual modules are progressively switched on. Since

each boiler performs close to its individual design duty, efficiency is maintained. The overall plant can therefore provide an improved part load efficiency characteristic, as shown in Figure 10.4. (Note that Figure 10.4 is based on older, less efficient boilers but the principle still applies.) Careful sequence control is fundamental to this approach (see 10.3).

Within practical limits, and where the dilution effect of parallel connected boilers is not significant, the greater the number of stages of sequence control, the better the efficiency. It is normally preferable to use a greater number of smaller boilers to provide more stages of control, than create more stages by switching combina- tions of different sized boilers. Different sized boilers are often used where one acts as the summer boiler for hot water services. However, complete segregation of hot water services is normally more efficient. Different sized boilers also require more complex sequence control systems when being controlled from return water temper- ature as they could have different response times. Whilst these problems can be overcome by careful engineering and commissioning, boiler sequence control systems are normally best kept to simple principles for long term efficient operation. Identical boilers also reduce potential maintenance and spares problems.

In some instances, particularly for low temperature systems, it can be economic to specify all the boilers in a multiple arrangement as condensing. However, in most instances it is more economic to specify the lead boiler(s) as condensing, with high efficiency boiler(s) to top-up. This minimises capital cost while still keeping overall plant efficiency high(18)_.

Figure 10.5 provides a means of estimating the overall seasonal efficiency when combining condensing and high efficiency boiler plant. Moving from left to right increases the proportion that is condensing and hence there is a rise in overall efficiency. Whilst there is only a relatively small drop in overall efficiency in moving from 100% to two thirds condensing, this will reduce significantly the capital cost of the plant. It is common to find that 50–75% condensing often provides the shortest payback periods. Condensing and high efficiency boilers can also be combined with CHPplant, as shown in Figure 10.6. The most efficient plant should take the base load, i.e. the CHP plant first and the condensing boilers second. In the past, CHPplant was specifically sized to meet only the base load. However, the most cost-effective solution often involves

100 200 300 400 500 600 700 800 900 1 x 900 kW boiler 1 x 180 kW plus 2 x 360 kW boilers 3 x 300 kW boilers 90 80 70 60 50 40 30 20 System efficiency / % System load / kW

10-6 Part A: Designing the building

sizing CHPat a greater capacity than the base load with some CHP modulating capacity and/or heat dumping capacity to cope with periods of low heat demand (sum- mer) (see the dotted line in Figure 10.6). A full option appraisal should consider all the available possibilities and gradually focus on the most efficient, economic and practical combination, as shown in Good Practice Guide GPG 187(25)_{. See Section 5.2 for further details on}

CHP.

10.1.6

Heat pumps

Heat pumps can produce high coefficients of performance (CoP) when operating at low temperature differentials, as shown in Figure 10.7. Heat pumps have found wide use in applications where low grade heat is available, e.g. where low grade process heating is being dumped, or for ventilation extract heat recovery such as in swimming pools and supermarkets.

Heat pumps are available in a number of different forms and exploit different sources of low grade heat. Air source heat pumps may be used to extract heat either from outside air or from ventilation exhaust air. When outside air is used as a heat source, the CoPtends to decline as the air temperature drops. Problems with the heat exchanger icing can be experienced where outside air humidity is high, which is frequently the case in the UK. This requires periodic defrosting, often achieved by temporar- ily reversing the heat pump, reducing the CoP. Air-to-air heat pumps supplying heating only, using outside air as a

source in a typical UK climate, can therefore have relatively low CoP.

When used to provide heating only, the CoP of heat pumps does not usually compensate for the increased financial and environmental cost of using electricity. Where the need for cool- ing has been established, e.g. in retail outlets, reversible heat pumps

can be an effective way of providing both cooling and heating. Small split-unit heat pumps are common in small shops and office and these require good interlinked controls to ensure that units providing heating do not fight nearby units supplying cooling. The coefficients of performance for heat pumps in the heating cycle should not generally be less than shown in Table 10.8.

Ground or water source heat pumps extract heat from the ground, bodies of water at ambient temperature, or from the outflow of waste heat. These heat sources have greater specific heat than air and, provided it has sufficient mass, vary much less with outside temperature. Small ground source heat pumps can therefore have a seasonal CoPof around 2.8 in a typical UK climate(26)_.

100% condensing boilers 50/50 100% conventional boilers Condensing boiler A: Underfloor heating system

B: Standard sized radiators C: Standard sized radiators

High efficiency conventional boiler Atmospheric conventional boiler A B C Condensing boiler 90 85 80 75

Estimated combined seasonal efficiency / %

Mix of boiler types

Figure 10.5 Seasonal efficiency of mixed boiler systems

Figure 10.6 Combining CHPand boiler plant (adapted from Good

Practice Guide GPG 176(23)₎ J F M A M J J A S O N D High efficiency boiler Condensing boiler Example of CHP

sized above base load (see 9.1.4) 1200 1000 800 600 400 200 0 Heat load / kW

Month of the year CHP

Figure 10.7 Effect of temperature range on heat pump performance

–10 –5 0 Flow temperature 35 _°C Flow temperature 45 _°C Flow temperature 55 _°C 5 10 15 5·0 4·5 4·0 3·5 3·0 2·5 2·0 1·5 Coefficient of performance ( COP )

Outside air temperature / _°C

Table 10.8Minimum coefficients of performance for heat pumps in heating cycle Heating capacity / kW CoP Up to 20 2.2 21 to 60 2.4 61 to 120 2.5 Over 120 2.6

Heating and hot water design 10-7

The CoPs given above are for electrically driven vapour compression cycle heat pumps. Absorption cycle heat pumps have a much lower CoPbut have the advantage that they can be powered directly by gas. When used for heating, the CoPobtainable in practice (of around 1.4) still offers a considerable advantage over a boiler.